fix: support modern protocol in multi-server clients (#4893)

* Support modern multi-server clients

🤖 Generated with Codex

* Preserve multi-server proxy protocol mode

🤖 Generated with Codex

* Layer proxy transport options

🤖 Generated with Codex

* Resolve aggregate protocol mode on connect

🤖 Generated with Codex

.claude/skills/review-security-report/SKILL.md

@@ -0,0 +1,95 @@
+---
+name: review-security-report
+description: Review FastMCP vulnerability reports before accepting, rejecting, patching, scoring, or publishing them. Use for security advisories, bug-bounty submissions, OAuth or MCP vulnerability claims, and proposed security fixes.
+---
+
+# Review a FastMCP security report
+
+A working proof of concept establishes behavior, not ownership or classification. Identify the
+component that violates a promised security boundary before changing code or advisory state.
+
+## Preserve the evidence
+
+Before editing an advisory, export the report, comments, proof of concept, configuration, and
+claimed affected versions. Record the reproduced commit and derive affected releases from history.
+
+Keep the investigation read-only until classification. Do not mutate the advisory or prepare a fix
+merely because the proof of concept works.
+
+## Reproduce the claim
+
+Use the smallest end-to-end reproduction against a supported release. Record:
+
+- defaults, non-default arguments, environment, and deployment assumptions;
+- what the attacker controls, what the victim does, and what authority the attacker gains;
+- the check or trust boundary allegedly bypassed; and
+- whether the attacker's prerequisites already provide equal or greater authority than the claimed
+  impact.
+
+A non-default configuration may still be vulnerable; a default may intentionally delegate a
+security decision elsewhere.
+
+## Identify the responsible layer
+
+- **FastMCP vulnerability:** FastMCP violates a promised boundary in a supported configuration.
+- **FastMCP bug:** FastMCP behaves incorrectly without creating attacker capability.
+- **Upstream vulnerability:** The defect belongs to a standard, dependency, identity provider,
+  client, proxy, or platform.
+- **Insecure deployment:** The operator omits, disables, or delegates a required control without
+  supplying the replacement required by its contract.
+- **Expected behavior:** The result follows the documented API contract or protocol.
+- **Documentation gap:** The implementation follows its intended contract, but the guidance creates
+  a reasonable expectation of protection.
+
+Attribute the violated boundary to its owner. Neither a dangerous configuration nor an available
+external mitigation decides ownership by itself. Distinguish failure of a primary control from
+failure of defense-in-depth.
+
+## Establish FastMCP's contract
+
+Read the code, released documentation, tests, and history. Determine:
+
+1. What FastMCP promises for the exact configuration, supported API, and affected releases.
+2. Whether the proof of concept bypasses, disables, or delegates that protection.
+3. Whether the behavior follows the implemented protocol despite any stricter FastMCP promise.
+
+If the intended contract remains unclear, ask the subsystem maintainer before accepting the report
+or proposing a fix. Current code alone does not define supported product behavior.
+
+## Separate OAuth proxy boundaries
+
+For OAuth proxy reports, check each layer independently:
+
+- Open DCR lets unknown clients register redirects; a matching attacker callback is not a redirect
+  validation bypass.
+- FastMCP consent binds the downstream client; ordinary upstream consent binds FastMCP's shared app.
+- `require_authorization_consent=False` removes consent; `"external"` delegates equivalent consent
+  and transaction binding outside FastMCP.
+- PKCE protects the verifier, not a code issued for a malicious client's own challenge.
+- Redirect allowlists are optional policy and can break hosted clients or open DCR.
+
+## Review the proposed fix independently
+
+A patch can block the proof of concept and still be wrong. Verify that it:
+
+- fixes the violated boundary at its source and enforces the claimed property;
+- does not silently change an intentional default or opt-in contract; and
+- preserves supported workflows without assuming one replacement control is the only valid design.
+
+Treat a change to the supported product contract as an enhancement requiring maintainer agreement,
+independently of the security report.
+
+## Classification checkpoint
+
+Before any GitHub mutation, state the decisive configuration and affected versions, boundary owner,
+realistic impact, and one disposition:
+
+- accept a draft advisory and prepare a private patch;
+- request specific missing evidence;
+- route as a normal FastMCP bug or upstream issue;
+- close as expected behavior or an insecure deployment;
+- ask the relevant maintainer to decide the contract.
+
+Separately recommend any documentation or warning clarification warranted by the investigation.
+
+When rejecting a report, acknowledge any valid reproduction and explain the decisive precondition.

.github/actions/setup-uv/action.yml

@@ -15,7 +15,7 @@ runs:
   using: "composite"
   steps:
     - name: Install uv
-      uses: astral-sh/setup-uv@v7
+      uses: astral-sh/setup-uv@v9.0.0
       with:
         enable-cache: true
         cache-dependency-glob: "uv.lock"

.github/workflows/auto-close-duplicates.yml

@@ -26,7 +26,7 @@ jobs:
           private-key: ${{ secrets.MARVIN_APP_PRIVATE_KEY }}
 
       - name: Install uv
-        uses: astral-sh/setup-uv@v7
+        uses: astral-sh/setup-uv@v9.0.0
 
       - name: Auto-close duplicate issues
         run: uv run scripts/auto_close_duplicates.py

.github/workflows/auto-close-needs-mre.yml

@@ -26,7 +26,7 @@ jobs:
           private-key: ${{ secrets.MARVIN_APP_PRIVATE_KEY }}
 
       - name: Install uv
-        uses: astral-sh/setup-uv@v7
+        uses: astral-sh/setup-uv@v9.0.0
 
       - name: Auto-close needs MRE issues
         run: uv run scripts/auto_close_needs_mre.py

.github/workflows/marvin-comment-on-issue.yml

@@ -28,7 +28,7 @@ jobs:
         uses: actions/checkout@v7
 
       - name: Install UV
-        uses: astral-sh/setup-uv@v7
+        uses: astral-sh/setup-uv@v9.0.0
         with:
           enable-cache: true
           cache-dependency-glob: "uv.lock"

.github/workflows/marvin-comment-on-pr.yml

@@ -30,7 +30,7 @@ jobs:
           fetch-depth: 0
 
       - name: Install UV
-        uses: astral-sh/setup-uv@v7
+        uses: astral-sh/setup-uv@v9.0.0
         with:
           enable-cache: true
           cache-dependency-glob: "uv.lock"

.github/workflows/marvin-dedupe-issues.yml

@@ -36,6 +36,12 @@ jobs:
         with:
           app-id: ${{ secrets.MARVIN_APP_ID }}
           private-key: ${{ secrets.MARVIN_APP_PRIVATE_KEY }}
+          # Match the job's `permissions:` block above. Unscoped, the token
+          # inherits the App installation's full set — which includes
+          # contents: write and actions: write, neither of which this job
+          # declares and both of which end up in the model's shell as GH_TOKEN.
+          permission-contents: read
+          permission-issues: write
 
       - name: Set dedupe prompt
         id: dedupe-prompt
@@ -114,8 +120,12 @@ jobs:
           prompt: ${{ steps.dedupe-prompt.outputs.PROMPT }}
           anthropic_api_key: ${{ secrets.ANTHROPIC_API_KEY_FOR_CI }}
           allowed_non_write_users: "*"
+          # No `Bash(gh api:*)`: it reaches every endpoint the token can, which
+          # is the reason marvin-label-triage routes its one write through
+          # .github/scripts/triage-label.sh instead. Dedupe searches, reads and
+          # comments — the four verbs below cover that.
           claude_args: |
-            --allowedTools "Bash(gh issue view:*)","Bash(gh search:*)","Bash(gh issue list:*)","Bash(gh api:*)","Bash(gh issue comment:*)",Task
+            --allowedTools "Bash(gh issue view:*)","Bash(gh search:*)","Bash(gh issue list:*)","Bash(gh issue comment:*)",Task
           settings: |
             {
               "model": "claude-sonnet-5",

.github/workflows/marvin-label-triage.yml

@@ -56,7 +56,14 @@ jobs:
         with:
           app-id: ${{ secrets.MARVIN_APP_ID }}
           private-key: ${{ secrets.MARVIN_APP_PRIVATE_KEY }}
-          owner: PrefectHQ
+          # No `owner:` — with it set and `repositories:` empty the token is
+          # scoped to every repo in the PrefectHQ installation. Triage only
+          # ever touches this one. The permissions below match the job's
+          # `permissions:` block; unscoped the token would also carry
+          # contents: write and actions: write from the App installation.
+          permission-contents: read
+          permission-issues: write
+          permission-pull-requests: write
 
       - name: Set triage prompt
         id: triage-prompt

.github/workflows/marvin-test-failure.yml

@@ -41,7 +41,7 @@ jobs:
 
       # Install UV package manager
       - name: Install UV
-        uses: astral-sh/setup-uv@v7
+        uses: astral-sh/setup-uv@v9.0.0
 
       # Install dependencies
       - name: Install dependencies

.github/workflows/publish-fastmcp-remote.yml

@@ -24,7 +24,7 @@ jobs:
           ref: ${{ github.event.workflow_run.head_sha || github.sha }}
 
       - name: Install uv
-        uses: astral-sh/setup-uv@v7
+        uses: astral-sh/setup-uv@v9.0.0
 
       - name: Build fastmcp-remote
         run: uv build --package fastmcp-remote

.github/workflows/publish-fastmcp-slim.yml

@@ -21,7 +21,7 @@ jobs:
           fetch-depth: 0
 
       - name: Install uv
-        uses: astral-sh/setup-uv@v7
+        uses: astral-sh/setup-uv@v9.0.0
 
       - name: Build fastmcp-slim
         run: uv build --package fastmcp-slim

.github/workflows/publish-fastmcp-tasks.yml

@@ -38,7 +38,7 @@ jobs:
           fi
 
       - name: Install uv
-        uses: astral-sh/setup-uv@v7
+        uses: astral-sh/setup-uv@v9.0.0
 
       - name: Build fastmcp-tasks
         if: steps.package_present.outputs.present == 'true'

.github/workflows/publish-fastmcp.yml

@@ -27,7 +27,7 @@ jobs:
           ref: ${{ github.event.workflow_run.head_sha || github.sha }}
 
       - name: Install uv
-        uses: astral-sh/setup-uv@v7
+        uses: astral-sh/setup-uv@v9.0.0
 
       - name: Build fastmcp
         run: uv build --package fastmcp

.github/workflows/run-schema-crash-test.yml

@@ -37,7 +37,7 @@ jobs:
       - uses: actions/checkout@v7
 
       - name: Install uv
-        uses: astral-sh/setup-uv@v7
+        uses: astral-sh/setup-uv@v9.0.0
 
       - name: Set up Python
         run: uv python install 3.12

.github/workflows/run-static.yml

@@ -37,6 +37,6 @@ jobs:
           resolution: locked
 
       - name: Run prek
-        uses: j178/prek-action@v2
+        uses: j178/prek-action@v3.0.0
         env:
           SKIP: no-commit-to-branch

.github/workflows/run-tests.yml

@@ -90,7 +90,7 @@ jobs:
       - name: Setup Node.js
         uses: actions/setup-node@v7
         with:
-          node-version: "22"
+          node-version: "24"
 
       - name: Run conformance tests
         uses: ./.github/actions/run-pytest

.github/workflows/run-upgrade-checks.yml

@@ -38,7 +38,7 @@ jobs:
           resolution: upgrade
 
       - name: Run prek
-        uses: j178/prek-action@v2
+        uses: j178/prek-action@v3.0.0
         env:
           SKIP: no-commit-to-branch
 

.github/workflows/update-config-schema.yml

@@ -33,7 +33,7 @@ jobs:
           token: ${{ steps.marvin-token.outputs.token }}
 
       - name: Install uv
-        uses: astral-sh/setup-uv@v7
+        uses: astral-sh/setup-uv@v9.0.0
         with:
           enable-cache: true
           cache-dependency-glob: "uv.lock"

.github/workflows/update-sdk-docs.yml

@@ -33,7 +33,7 @@ jobs:
           token: ${{ steps.marvin-token.outputs.token }}
 
       - name: Install uv
-        uses: astral-sh/setup-uv@v7
+        uses: astral-sh/setup-uv@v9.0.0
         with:
           enable-cache: true
           cache-dependency-glob: "uv.lock"

docs/cli/overview.mdx

@@ -28,6 +28,9 @@ fastmcp --help
 | [`generate-cli`](/cli/generate-cli) | Scaffold a standalone typed CLI from a server's tool schemas |
 | [`project prepare`](/cli/running#pre-building-environments) | Pre-install dependencies into a reusable uv project |
 | [`auth cimd`](/cli/auth) | Create and validate CIMD documents for OAuth |
+| `login` | Sign in to Prefect Horizon with a browser device flow |
+| `whoami` | Show the current Horizon account |
+| `logout` | Revoke the current Horizon key and remove the local credential |
 | `version` | Print version info (`--copy` to copy to clipboard) |
 
 ## Server Targets
@@ -81,6 +84,38 @@ Run [`fastmcp discover`](/cli/client#discovering-configured-servers) to see what
 
 ## Authentication
 
+### Prefect Horizon Account
+
+Use the top-level account commands to manage the credential for Prefect Horizon.
+
+```bash
+fastmcp login
+fastmcp whoami
+fastmcp logout
+```
+
+`fastmcp login` first uses `HORIZON_API_KEY` or a valid stored key when one is available.
+When login needs a new key, it shows a verification URL and code.
+It opens a browser when the terminal supports it.
+If the browser does not open, use the shown URL and code on another device.
+To switch accounts, run `fastmcp logout` before you run `fastmcp login` again.
+
+Login stores only the personal Horizon API key.
+It does not select or store a deployment organization.
+`fastmcp whoami` gets the current user from Horizon.
+`fastmcp logout` attempts to revoke the stored key and always removes its local credential.
+
+Set `HORIZON_API_KEY` to use an environment credential instead.
+The CLI gives that value first precedence and never stores it.
+When this variable controls the session, logout does not revoke or remove any credential.
+Remove the variable from your environment to sign out.
+
+Use `--json` for stable command results.
+During JSON login, the verification challenge goes to stderr and the final result goes to stdout.
+JSON mode does not open a browser or ask a question.
+
+### MCP Server Authentication
+
 When targeting an HTTP URL, the CLI enables OAuth authentication by default. If the server requires it, you'll be guided through the flow (typically opening a browser). If it doesn't, the setup is a silent no-op.
 
 To skip authentication entirely — useful for local development servers — pass `--auth none`:

docs/clients/client.mdx

@@ -212,7 +212,7 @@ async with Client("https://example.com/mcp", mode="auto") as client:
 <Note>
 `mode="auto"` is the default as of FastMCP 4.0. Earlier versions defaulted to `"legacy"`. If a server behaves unexpectedly under discovery, or you depend on the legacy `initialize` result, pin the old behavior with `Client(..., mode="legacy")`.
 
-The SSE transport is legacy-only — it cannot carry the sessionless modern era — so a client connecting over SSE always negotiates the legacy handshake, even under `mode="auto"`. A multi-server config (`MCPConfigTransport` with more than one server) is likewise legacy-only, because it mounts each backend behind a legacy-era proxy; a single-server config mirrors its one backend transport's era.
+The SSE transport is legacy-only — it cannot carry the sessionless modern era — so a client connecting over SSE always negotiates the legacy handshake, even under `mode="auto"`. A multi-server config (`MCPConfigTransport` with more than one server) negotiates the best era shared by every connected backend: it stays modern when every backend is modern-capable and reconnects every leg under the handshake era when any backend requires legacy. Pinning `mode="legacy"` or a modern version applies that mode to every backend. All-modern configurations follow normal modern semantics; applications that rely on handshake-era server-initiated sampling or elicitation should pin `mode="legacy"`.
 </Note>
 
 ## Response caching

docs/deployment/prefect-horizon.mdx

@@ -13,6 +13,39 @@ Horizon includes a **free personal tier for FastMCP users**, making it the faste
 Horizon is free for personal projects. Enterprise governance features are available for teams deploying to thousands of users.
 </Info>
 
+## FastMCP CLI Account
+
+Sign in to Horizon from the FastMCP CLI with the device authorization flow.
+
+```bash
+fastmcp login
+```
+
+The command uses an environment key or a valid stored key when one is available.
+When login needs a new key, it shows a verification URL and code before it opens the browser.
+If the browser cannot open, visit the shown URL and enter the code.
+To switch accounts, run `fastmcp logout` before you run `fastmcp login` again.
+New users can register and create their first Horizon organization in the browser.
+
+Check the active account after login.
+
+```bash
+fastmcp whoami
+```
+
+Remove the local credential and revoke the active personal API key when possible.
+
+```bash
+fastmcp logout
+```
+
+Login does not select or store a deployment organization.
+
+For an agent or a CI process, set `HORIZON_API_KEY` instead of storing a key.
+The CLI never writes the environment value to its credential file.
+When this variable controls the session, logout does not revoke or remove any credential.
+Remove the variable from the environment to sign out.
+
 ## The Platform
 
 Horizon is organized into four integrated pillars:

docs/more/faq.mdx

@@ -16,7 +16,7 @@ One change is silent, so go looking for it: an `except httpx.ConnectError:` arou
 
 `fastmcp.Client` defaults to `mode="auto"` as of FastMCP 4, so it negotiates the newest era both sides speak rather than pinning the handshake. Over streamable HTTP or stdio to a FastMCP server that means the sessionless `2026-07-28` protocol, where FastMCP 3 connected at `2025-11-25`.
 
-Two transports are exceptions: SSE predates the sessionless era and cannot carry it, and a multi-server `MCPConfigTransport` mounts each backend behind a legacy-era composite. Under `mode="auto"` the client recognizes both and settles on the handshake without probing, so seeing `2025-11-25` there is correct rather than a negotiation failure. Pinning a modern version explicitly on either skips that substitution and asks the transport for something it cannot serve, so leave them on auto or legacy.
+SSE is the transport exception: it predates the sessionless era and cannot carry it. Under `mode="auto"` the client recognizes SSE and settles on the handshake without probing, so seeing `2025-11-25` there is correct rather than a negotiation failure. A multi-server `MCPConfigTransport` resolves one era for the entire aggregate: all-modern backends negotiate `2026-07-28`, while any legacy backend moves the composite and every backend leg to the handshake era. Pinning a mode applies it to every backend in the configuration. All-modern configurations follow normal modern semantics; applications that rely on handshake-era server-initiated sampling or elicitation should pin `mode="legacy"`.
 
 The client probes `server/discover` and adopts the modern protocol when the server answers, falling back to the `initialize` handshake for anything that is not positive evidence of a modern peer — so a mixed fleet of servers still connects. Pin the old behavior per client with `Client(url, mode="legacy")`. See [Protocol negotiation](/clients/client#protocol-negotiation).
 

docs/python-sdk/fastmcp-server-dependencies.mdx

@@ -193,16 +193,16 @@ Extract headers from the current HTTP request if available.
 Never raises an exception, even if there is no active HTTP request (in which case
 an empty dict is returned).
 
-By default, strips problematic headers like `content-length` and `authorization`
-that cause issues if forwarded to downstream services. If `include_all` is True,
-all headers are returned.
+By default, strips problematic headers like `content-length`, and credential
+headers like `authorization` and `cookie`, that cause issues if forwarded to
+downstream services. If `include_all` is True, all headers are returned.
 
 The `include` parameter allows specific headers to be included even if they would
 normally be excluded. This is useful for proxy transports that need to forward
 authorization headers to upstream MCP servers.
 
 
-### `get_access_token` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L605" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+### `get_access_token` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L606" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 get_access_token() -> AccessToken | None
@@ -220,7 +220,7 @@ request is available.
 - The access token if an authenticated user is available, None otherwise.
 
 
-### `without_injected_parameters` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L664" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+### `without_injected_parameters` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L665" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 without_injected_parameters(fn: Callable[..., Any]) -> Callable[..., Any]
@@ -249,7 +249,7 @@ thread-affinity libraries (e.g. Windows COM). Ignored for async fns.
 - Async wrapper function without injected parameters
 
 
-### `resolve_dependencies` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L828" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+### `resolve_dependencies` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L829" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 resolve_dependencies(fn: Callable[..., Any], arguments: dict[str, Any]) -> AsyncGenerator[dict[str, Any], None]
@@ -278,7 +278,7 @@ time, so all injection goes through the unified DI system.
       which will be filtered out)
 
 
-### `CurrentContext` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L956" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+### `CurrentContext` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L957" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 CurrentContext() -> Context
@@ -297,7 +297,7 @@ current MCP operation (tool/resource/prompt call).
 - `RuntimeError`: If no active context found (during resolution)
 
 
-### `OptionalCurrentContext` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L981" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+### `OptionalCurrentContext` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L982" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 OptionalCurrentContext() -> Context | None
@@ -307,7 +307,7 @@ OptionalCurrentContext() -> Context | None
 Get the current FastMCP Context, or None when no context is active.
 
 
-### `CurrentFastMCP` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1001" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+### `CurrentFastMCP` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1002" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 CurrentFastMCP() -> FastMCP
@@ -325,7 +325,7 @@ This dependency provides access to the active FastMCP server.
 - `RuntimeError`: If no server in context (during resolution)
 
 
-### `CurrentRequest` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1041" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+### `CurrentRequest` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1042" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 CurrentRequest() -> Request
@@ -345,7 +345,7 @@ current HTTP request. Only available when running over HTTP transports
 - `RuntimeError`: If no HTTP request in context (e.g., STDIO transport)
 
 
-### `CurrentHeaders` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1082" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+### `CurrentHeaders` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1086" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 CurrentHeaders() -> dict[str, str]
@@ -355,15 +355,15 @@ CurrentHeaders() -> dict[str, str]
 Get the current HTTP request headers.
 
 This dependency provides access to the HTTP headers for the current request,
-including the authorization header. Returns an empty dictionary when no HTTP
-request is available, making it safe to use in code that might run over any
-transport.
+including the `authorization` and `cookie` headers, which `get_http_headers()`
+withholds by default. Returns an empty dictionary when no HTTP request is
+available, making it safe to use in code that might run over any transport.
 
 **Returns:**
 - A dependency that resolves to a dictionary of header name -> value
 
 
-### `CurrentAccessToken` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1300" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+### `CurrentAccessToken` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1304" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 CurrentAccessToken() -> AccessToken
@@ -382,7 +382,7 @@ authenticated request. Raises an error if no authentication is present.
 - `RuntimeError`: If no authenticated user (use get_access_token() for optional)
 
 
-### `TokenClaim` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1357" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+### `TokenClaim` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1361" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 TokenClaim(name: str) -> str
@@ -425,7 +425,7 @@ distributed-trace parent. Those live in the raw params dict under ``_meta``,
 which this wrapper lifts once so downstream consumers have a stable surface.
 
 
-### `ProgressLike` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1110" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+### `ProgressLike` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1114" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 
 Protocol for progress tracking interface.
@@ -436,7 +436,7 @@ and Docket's Progress (worker context).
 
 **Methods:**
 
-#### `current` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1118" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+#### `current` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1122" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 current(self) -> int | None
@@ -445,7 +445,7 @@ current(self) -> int | None
 Current progress value.
 
 
-#### `total` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1123" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+#### `total` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1127" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 total(self) -> int
@@ -454,7 +454,7 @@ total(self) -> int
 Total/target progress value.
 
 
-#### `message` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1128" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+#### `message` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1132" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 message(self) -> str | None
@@ -463,7 +463,7 @@ message(self) -> str | None
 Current progress message.
 
 
-#### `set_total` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1132" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+#### `set_total` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1136" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 set_total(self, total: int) -> None
@@ -472,7 +472,7 @@ set_total(self, total: int) -> None
 Set the total/target value for progress tracking.
 
 
-#### `increment` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1136" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+#### `increment` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1140" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 increment(self, amount: int = 1) -> None
@@ -481,7 +481,7 @@ increment(self, amount: int = 1) -> None
 Atomically increment the current progress value.
 
 
-#### `set_message` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1140" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+#### `set_message` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1144" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 set_message(self, message: str | None) -> None
@@ -490,7 +490,7 @@ set_message(self, message: str | None) -> None
 Update the progress status message.
 
 
-### `InMemoryProgress` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1145" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+### `InMemoryProgress` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1149" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 
 In-memory progress tracker for immediate tool execution.
@@ -502,25 +502,25 @@ progress doesn't need to be observable across processes.
 
 **Methods:**
 
-#### `current` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1170" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+#### `current` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1174" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 current(self) -> int | None
 ```
 
-#### `total` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1174" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+#### `total` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1178" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 total(self) -> int
 ```
 
-#### `message` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1178" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+#### `message` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1182" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 message(self) -> str | None
 ```
 
-#### `set_total` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1181" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+#### `set_total` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1185" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 set_total(self, total: int) -> None
@@ -529,7 +529,7 @@ set_total(self, total: int) -> None
 Set the total/target value for progress tracking.
 
 
-#### `increment` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1187" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+#### `increment` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1191" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 increment(self, amount: int = 1) -> None
@@ -538,7 +538,7 @@ increment(self, amount: int = 1) -> None
 Atomically increment the current progress value.
 
 
-#### `set_message` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1196" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+#### `set_message` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1200" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 set_message(self, message: str | None) -> None
@@ -547,7 +547,7 @@ set_message(self, message: str | None) -> None
 Update the progress status message.
 
 
-### `Progress` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1201" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+### `Progress` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1205" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 
 Progress dependency that works in both server and worker contexts.
@@ -562,7 +562,7 @@ share mutable state.
 
 **Methods:**
 
-#### `current` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1242" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+#### `current` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1246" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 current(self) -> int | None
@@ -571,7 +571,7 @@ current(self) -> int | None
 Current progress value.
 
 
-#### `total` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1248" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+#### `total` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1252" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 total(self) -> int
@@ -580,7 +580,7 @@ total(self) -> int
 Total/target progress value.
 
 
-#### `message` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1254" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+#### `message` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1258" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 message(self) -> str | None
@@ -589,7 +589,7 @@ message(self) -> str | None
 Current progress message.
 
 
-#### `set_total` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1259" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+#### `set_total` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1263" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 set_total(self, total: int) -> None
@@ -598,7 +598,7 @@ set_total(self, total: int) -> None
 Set the total/target value for progress tracking.
 
 
-#### `increment` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1264" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+#### `increment` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1268" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 increment(self, amount: int = 1) -> None
@@ -607,7 +607,7 @@ increment(self, amount: int = 1) -> None
 Atomically increment the current progress value.
 
 
-#### `set_message` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1269" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+#### `set_message` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/dependencies.py#L1273" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 set_message(self, message: str | None) -> None

docs/python-sdk/fastmcp-utilities-inspect.mdx

@@ -86,7 +86,7 @@ Uses Client to get the standard MCP protocol format with camelCase fields.
 Includes version metadata at the top level.
 
 
-### `format_info` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/utilities/inspect.py#L502" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
+### `format_info` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/utilities/inspect.py#L503" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
 
 ```python
 format_info(mcp: FastMCP[Any] | SDKServer, format: InspectFormat | Literal['fastmcp', 'mcp'], info: FastMCPInfo | None = None) -> bytes

docs/servers/dependency-injection.mdx

@@ -199,7 +199,15 @@ def get_user_agent() -> str:
     return headers.get("user-agent", "Unknown")
 ```
 
-By default, problematic headers like `host` and `content-length` are excluded. Use `get_http_headers(include_all=True)` to include all headers.
+By default, problematic headers like `host` and `content-length` are excluded, along with the credential headers `authorization` and `cookie`. Credentials are withheld because most callers forward whatever they receive, and a session cookie scoped to your MCP host should not reach a separate backend origin.
+
+To read a credential header, ask for it by name:
+
+```python
+headers = get_http_headers(include={"cookie"})
+```
+
+`CurrentHeaders()` already includes both credential headers, since it exposes the current request to your handler rather than forwarding it. Use `get_http_headers(include_all=True)` to include every header.
 
 ### Access Token
 

examples/testing_demo/uv.lock

@@ -15,13 +15,12 @@ exclude-newer-span = "P1W"
 [options.exclude-newer-package]
 mcp-types = false
 prefab-ui = false
-truststore = false
-fastmcp-slim = false
+pydocket = false
+uncalled-for = false
 fastmcp = false
 mcp = false
-httpcore2 = false
 fastmcp-remote = false
-httpx2 = false
+fastmcp-slim = false
 
 [[package]]
 name = "aiofile"
@@ -259,59 +258,59 @@ wheels = [
 
 [[package]]
 name = "cryptography"
-version = "49.0.0"
+version = "50.0.0"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
     { name = "cffi", marker = "platform_python_implementation != 'PyPy'" },
     { name = "typing-extensions", marker = "python_full_version < '3.11'" },
 ]
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 ]
 
 [[package]]

fastmcp_slim/fastmcp/cli/cli.py

@@ -21,6 +21,7 @@ import fastmcp
 from fastmcp.cli import run as run_module
 from fastmcp.cli.auth import auth_app
 from fastmcp.cli.client import call_command, discover_command, list_command
+from fastmcp.cli.deploy.command import login, logout, whoami
 from fastmcp.cli.generate import generate_cli_command
 from fastmcp.cli.install import install_app
 from fastmcp.utilities.cli import is_already_in_uv_subprocess, load_and_merge_config
@@ -1134,6 +1135,11 @@ app.command(generate_cli_command, name="generate-cli")
 # Add auth subcommand group (includes CIMD commands)
 app.command(auth_app)
 
+# Add Prefect Horizon account commands
+app.command(login)
+app.command(logout)
+app.command(whoami)
+
 
 if __name__ == "__main__":
     app()

fastmcp_slim/fastmcp/cli/deploy/command.py

@@ -0,0 +1,398 @@
+"""Public Prefect Horizon authentication commands."""
+
+from __future__ import annotations
+
+import os
+import platform
+import sys
+import webbrowser
+from typing import Annotated, NoReturn
+
+from cyclopts import Parameter
+from pydantic import SecretStr
+from rich.status import Status
+
+import fastmcp
+from fastmcp.cli.deploy.authentication import (
+    DeviceAuthorizationDeniedError,
+    DeviceAuthorizationError,
+    DeviceAuthorizationExpiredError,
+    authorize_device,
+)
+from fastmcp.cli.deploy.configuration import (
+    ConfigurationStore,
+    HorizonConfiguration,
+)
+from fastmcp.cli.deploy.credentials import (
+    AuthenticationRequiredError,
+    CredentialStore,
+    ResolvedCredential,
+)
+from fastmcp.cli.deploy.horizon_client import (
+    DeviceAuthorization,
+    DeviceMetadata,
+    HorizonClient,
+    HorizonResponseError,
+    HorizonUnauthorizedError,
+    HorizonUnavailableError,
+    HorizonUser,
+)
+from fastmcp.cli.deploy.output import (
+    CommandName,
+    ErrorCategory,
+    emit_device_challenge,
+    emit_environment_logout,
+    emit_error,
+    emit_identity,
+    emit_logout,
+    start_device_approval_status,
+    stop_device_approval_status,
+)
+from fastmcp.cli.deploy.state import StateFileError, state_lock
+
+JsonOption = Annotated[
+    bool,
+    Parameter(
+        name="--json",
+        help="Write one final JSON result to stdout",
+        negative=(),
+    ),
+]
+HostOption = Annotated[
+    str | None,
+    Parameter(
+        name="--host",
+        help="Use and save a different Horizon host URL",
+    ),
+]
+
+
+def _can_open_browser() -> bool:
+    return sys.stdin.isatty() and sys.stdout.isatty()
+
+
+def _device_metadata() -> DeviceMetadata:
+    return DeviceMetadata(
+        device_name=platform.node() or None,
+        platform=platform.system().lower() or None,
+        architecture=platform.machine().lower() or None,
+        client_version=fastmcp.__version__,
+    )
+
+
+def _load_session_snapshot(
+    credentials: CredentialStore,
+) -> tuple[HorizonConfiguration, ResolvedCredential | None]:
+    with state_lock(credentials.path.parent):
+        configuration = ConfigurationStore(credentials.path.parent).load()
+        environment_key = os.environ.get("HORIZON_API_KEY")
+        if environment_key:
+            credential = ResolvedCredential(
+                api_key=SecretStr(environment_key),
+                source="environment",
+            )
+        else:
+            stored_key = credentials.load()
+            credential = (
+                ResolvedCredential(api_key=stored_key, source="stored")
+                if stored_key is not None
+                else None
+            )
+    return configuration, credential
+
+
+def _fail(
+    command: CommandName,
+    category: ErrorCategory,
+    message: str,
+    *,
+    json_output: bool,
+    details: dict[str, object] | None = None,
+) -> NoReturn:
+    emit_error(
+        command,
+        category,
+        message,
+        json_output=json_output,
+        details=details,
+    )
+    raise SystemExit(1)
+
+
+def _fail_for_expected_error(
+    command: CommandName,
+    error: Exception,
+    *,
+    json_output: bool,
+) -> NoReturn:
+    if isinstance(error, AuthenticationRequiredError):
+        _fail(
+            command,
+            "authentication_required",
+            "Run `fastmcp login` to sign in to Prefect Horizon.",
+            json_output=json_output,
+        )
+    if isinstance(error, HorizonUnauthorizedError):
+        _fail(
+            command,
+            "authentication_invalid",
+            "The Horizon credential is not valid. Run `fastmcp login` again.",
+            json_output=json_output,
+        )
+    if isinstance(error, DeviceAuthorizationDeniedError):
+        _fail(
+            command,
+            "authorization_denied",
+            "The device authorization request was denied.",
+            json_output=json_output,
+        )
+    if isinstance(error, DeviceAuthorizationExpiredError):
+        _fail(
+            command,
+            "authorization_expired",
+            "The device authorization request expired. Run the command again.",
+            json_output=json_output,
+        )
+    if isinstance(error, DeviceAuthorizationError):
+        _fail(
+            command,
+            "authorization_failed",
+            "The device authorization request failed. Run the command again.",
+            json_output=json_output,
+        )
+    if isinstance(error, HorizonUnavailableError):
+        _fail(
+            command,
+            "horizon_unavailable",
+            "The Horizon API is unavailable. Try again later.",
+            json_output=json_output,
+        )
+    if isinstance(error, HorizonResponseError):
+        _fail(
+            command,
+            "horizon_error",
+            "Horizon returned an unexpected response. Try again later.",
+            json_output=json_output,
+        )
+    if isinstance(error, StateFileError):
+        _fail(
+            command,
+            "state_error",
+            "The local Horizon state is invalid.",
+            json_output=json_output,
+        )
+    raise error
+
+
+async def _get_user(
+    api_origin: str,
+    credential: ResolvedCredential,
+) -> HorizonUser:
+    async with HorizonClient(api_origin, api_key=credential.api_key) as client:
+        return await client.get_current_user()
+
+
+async def login(
+    *,
+    host: HostOption = None,
+    json_output: JsonOption = False,
+) -> None:
+    """Sign in to Prefect Horizon."""
+    credentials = CredentialStore()
+
+    try:
+        configuration_store = ConfigurationStore()
+        requested_configuration: HorizonConfiguration | None = None
+        if host is not None:
+            try:
+                requested_configuration = configuration_store.set_api_origin(
+                    host,
+                    credentials=credentials,
+                )
+            except ValueError:
+                _fail(
+                    "login",
+                    "invalid_host",
+                    "The Horizon host must be an HTTP origin.",
+                    json_output=json_output,
+                )
+
+        configuration, credential = _load_session_snapshot(credentials)
+        if (
+            requested_configuration is not None
+            and configuration.api_origin != requested_configuration.api_origin
+        ):
+            raise StateFileError("The Horizon host changed during login")
+
+        async def device_authorization():
+            approval_status: Status | None = None
+
+            def show_challenge(challenge: DeviceAuthorization) -> None:
+                nonlocal approval_status
+                emit_device_challenge(challenge, json_output=json_output)
+                approval_status = start_device_approval_status(json_output=json_output)
+
+            try:
+                async with HorizonClient(configuration.api_origin) as client:
+                    return await authorize_device(
+                        client,
+                        metadata=_device_metadata(),
+                        on_challenge=show_challenge,
+                        open_browser=not json_output and _can_open_browser(),
+                        browser_opener=webbrowser.open,
+                    )
+            finally:
+                stop_device_approval_status(approval_status)
+
+        async def interactive_credential() -> ResolvedCredential:
+            api_key = await device_authorization()
+            credentials.save_for_origin(
+                api_key,
+                expected_api_origin=configuration.api_origin,
+            )
+            return ResolvedCredential(api_key=api_key, source="interactive")
+
+        if credential is None:
+            credential = await interactive_credential()
+
+        try:
+            user = await _get_user(
+                configuration.api_origin,
+                credential,
+            )
+        except HorizonUnauthorizedError:
+            if credential.source == "environment":
+                raise
+
+            credentials.clear_if_matches(
+                credential.api_key,
+                expected_api_origin=configuration.api_origin,
+            )
+            if credential.source == "interactive":
+                raise
+
+            credential = await interactive_credential()
+            try:
+                user = await _get_user(
+                    configuration.api_origin,
+                    credential,
+                )
+            except HorizonUnauthorizedError:
+                credentials.clear_if_matches(
+                    credential.api_key,
+                    expected_api_origin=configuration.api_origin,
+                )
+                raise
+    except (
+        AuthenticationRequiredError,
+        DeviceAuthorizationError,
+        HorizonResponseError,
+        HorizonUnauthorizedError,
+        HorizonUnavailableError,
+        StateFileError,
+    ) as error:
+        _fail_for_expected_error("login", error, json_output=json_output)
+
+    emit_identity(
+        "login",
+        user,
+        json_output=json_output,
+    )
+
+
+async def whoami(
+    *,
+    json_output: JsonOption = False,
+) -> None:
+    """Show the current Prefect Horizon user."""
+    credentials = CredentialStore()
+    configuration: HorizonConfiguration | None = None
+    credential: ResolvedCredential | None = None
+
+    try:
+        configuration, credential = _load_session_snapshot(credentials)
+        if credential is None:
+            raise AuthenticationRequiredError("Horizon authentication is required")
+        user = await _get_user(
+            configuration.api_origin,
+            credential,
+        )
+    except HorizonUnauthorizedError as error:
+        if (
+            configuration is not None
+            and credential is not None
+            and credential.source == "stored"
+        ):
+            try:
+                credentials.clear_if_matches(
+                    credential.api_key,
+                    expected_api_origin=configuration.api_origin,
+                )
+            except StateFileError as cleanup_error:
+                _fail_for_expected_error(
+                    "whoami",
+                    cleanup_error,
+                    json_output=json_output,
+                )
+        _fail_for_expected_error("whoami", error, json_output=json_output)
+    except (
+        AuthenticationRequiredError,
+        HorizonResponseError,
+        HorizonUnavailableError,
+        StateFileError,
+    ) as error:
+        _fail_for_expected_error("whoami", error, json_output=json_output)
+
+    emit_identity(
+        "whoami",
+        user,
+        json_output=json_output,
+    )
+
+
+async def logout(
+    *,
+    json_output: JsonOption = False,
+) -> None:
+    """Revoke the current Horizon key and remove the local credential."""
+    credentials = CredentialStore()
+
+    if os.environ.get("HORIZON_API_KEY"):
+        emit_environment_logout(json_output=json_output)
+        return
+
+    try:
+        configuration, credential = _load_session_snapshot(credentials)
+        if credential is None:
+            emit_logout(remote_revoked=False, json_output=json_output)
+            return
+
+        async with HorizonClient(
+            configuration.api_origin,
+            api_key=credential.api_key,
+        ) as client:
+            try:
+                await client.revoke_current_api_key()
+            finally:
+                credentials.clear_if_matches(
+                    credential.api_key,
+                    expected_api_origin=configuration.api_origin,
+                )
+    except HorizonUnauthorizedError:
+        emit_logout(remote_revoked=False, json_output=json_output)
+        return
+    except (HorizonResponseError, HorizonUnavailableError):
+        _fail(
+            "logout",
+            "remote_revocation_failed",
+            "The local credential was removed, but the remote key can remain active.",
+            json_output=json_output,
+            details={
+                "localCredentialRemoved": True,
+                "remoteCredentialMayRemain": True,
+            },
+        )
+    except StateFileError as error:
+        _fail_for_expected_error("logout", error, json_output=json_output)
+
+    emit_logout(remote_revoked=True, json_output=json_output)

fastmcp_slim/fastmcp/cli/deploy/credentials.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import os
+import secrets
 from collections.abc import Awaitable, Callable, Mapping
 from dataclasses import dataclass
 from pathlib import Path
@@ -96,6 +97,32 @@ class CredentialStore:
                 raise StateFileError("The Horizon host changed during login")
             self.save(api_key)
 
+    def clear_if_matches(
+        self,
+        api_key: SecretStr | str,
+        *,
+        expected_api_origin: str,
+    ) -> None:
+        """Clear a key only while its Horizon origin and value are active."""
+        from fastmcp.cli.deploy.configuration import ConfigurationStore
+
+        expected_api_origin = normalize_api_origin(expected_api_origin)
+        expected_api_key = (
+            api_key.get_secret_value() if isinstance(api_key, SecretStr) else api_key
+        )
+        with state_lock(self.path.parent):
+            active_api_origin = ConfigurationStore(self.path.parent).load().api_origin
+            active_api_key = self.load()
+            if (
+                active_api_origin == expected_api_origin
+                and active_api_key is not None
+                and secrets.compare_digest(
+                    active_api_key.get_secret_value(),
+                    expected_api_key,
+                )
+            ):
+                self.clear()
+
     def clear(self) -> None:
         remove_state(self.path)
 

fastmcp_slim/fastmcp/cli/deploy/output.py

@@ -0,0 +1,297 @@
+"""Stable terminal and JSON output for Horizon CLI commands."""
+
+from __future__ import annotations
+
+import json
+import sys
+from typing import Literal
+
+from rich import box
+from rich.align import Align
+from rich.console import Console, Group
+from rich.padding import Padding
+from rich.panel import Panel
+from rich.status import Status
+from rich.table import Table
+from rich.text import Text
+
+from fastmcp.cli.deploy.horizon_client import DeviceAuthorization, HorizonUser
+
+CommandName = Literal["login", "logout", "whoami"]
+ErrorCategory = Literal[
+    "authentication_invalid",
+    "authentication_required",
+    "authorization_denied",
+    "authorization_expired",
+    "authorization_failed",
+    "horizon_error",
+    "horizon_unavailable",
+    "invalid_host",
+    "remote_revocation_failed",
+    "state_error",
+]
+
+console = Console()
+error_console = Console(stderr=True)
+
+
+def _write_json(payload: object, *, stderr: bool = False) -> None:
+    stream = sys.stderr if stderr else sys.stdout
+    print(json.dumps(payload, separators=(",", ":")), file=stream, flush=True)
+
+
+def _banner(title: str, *, style: str) -> Panel:
+    return Panel(
+        Align.center(Text(title, style=f"bold {style}")),
+        box=box.ROUNDED,
+        border_style=style,
+        padding=(0, 1),
+        width=52,
+    )
+
+
+def _account_panel(
+    user: HorizonUser,
+    *,
+    title: str,
+    message: str,
+) -> Panel:
+    name = Text(user.name or user.email, style="bold")
+    details: list[Text] = [name]
+    if user.name:
+        details.append(Text(user.email, style="cyan"))
+    details.extend([Text(), Text(message, style="green")])
+    return Panel(
+        Group(*details),
+        title=Text(title, style="bold green"),
+        title_align="left",
+        box=box.ROUNDED,
+        border_style="green",
+        padding=(1, 2),
+        width=52,
+    )
+
+
+def _format_duration(seconds: int) -> str:
+    if seconds % 60 == 0:
+        minutes = seconds // 60
+        unit = "minute" if minutes == 1 else "minutes"
+        return f"{minutes} {unit}"
+    unit = "second" if seconds == 1 else "seconds"
+    return f"{seconds} {unit}"
+
+
+def emit_device_challenge(
+    authorization: DeviceAuthorization,
+    *,
+    json_output: bool,
+) -> None:
+    """Show a device challenge before polling starts."""
+    if json_output:
+        _write_json(
+            {
+                "event": "device_authorization",
+                "verificationUrl": authorization.verification_uri,
+                "verificationUrlComplete": authorization.verification_uri_complete,
+                "userCode": authorization.user_code,
+            },
+            stderr=True,
+        )
+        return
+
+    console.print()
+    console.print(_banner("Deploy FastMCP on Horizon", style="magenta"))
+    console.print()
+    console.print(Text("✓ Device authorization started", style="bold green"))
+    console.print()
+    console.print("  Open this URL in your browser:")
+    console.print()
+    console.print(
+        Padding(
+            Text(authorization.verification_uri_complete, style="cyan underline"),
+            (0, 2),
+        )
+    )
+    console.print()
+    console.print("  Confirm this code:")
+    console.print()
+    code = Table.grid()
+    code.add_column(justify="center", width=52)
+    code.add_row(Text(authorization.user_code, style="bold"))
+    console.print(code)
+    console.print()
+    expires_in = _format_duration(authorization.expires_in)
+    console.print(Text(f"The request expires in {expires_in}.", style="dim"))
+    console.print(Text("Press Ctrl-C to cancel.", style="dim"))
+    console.print()
+
+
+def start_device_approval_status(*, json_output: bool) -> Status | None:
+    """Start the terminal spinner while the browser approval is pending."""
+    if json_output:
+        return None
+    status = console.status(
+        "[cyan]Waiting for approval in your browser[/cyan]",
+        spinner="dots",
+        spinner_style="cyan",
+    )
+    status.start()
+    return status
+
+
+def stop_device_approval_status(status: Status | None) -> None:
+    """Stop a device approval spinner when one is active."""
+    if status is not None:
+        status.stop()
+
+
+def emit_identity(
+    command: Literal["login", "whoami"],
+    user: HorizonUser,
+    *,
+    json_output: bool,
+) -> None:
+    """Show the authenticated user."""
+    if json_output:
+        _write_json(
+            {
+                "ok": True,
+                "command": command,
+                "user": user.model_dump(mode="json"),
+            }
+        )
+        return
+
+    console.print()
+    if command == "login":
+        panel = _account_panel(
+            user,
+            title="Logged into Horizon",
+            message="You are signed in to FastMCP.",
+        )
+    else:
+        panel = _account_panel(
+            user,
+            title="Horizon Account",
+            message="● Signed in",
+        )
+    console.print(panel)
+    console.print()
+
+
+def emit_environment_logout(*, json_output: bool) -> None:
+    """Explain why logout cannot change an environment credential."""
+    if json_output:
+        _write_json(
+            {
+                "ok": True,
+                "command": "logout",
+                "credentialSource": "environment",
+                "localCredentialRemoved": False,
+                "remoteRevoked": False,
+            }
+        )
+        return
+
+    message = Group(
+        Text("This session uses HORIZON_API_KEY.", style="bold"),
+        Text("Remove it from your environment to sign out."),
+        Text("No credential was revoked or removed.", style="dim"),
+    )
+    console.print()
+    console.print(
+        Panel(
+            message,
+            title=Text("Horizon Account", style="bold cyan"),
+            title_align="left",
+            box=box.ROUNDED,
+            border_style="cyan",
+            padding=(1, 2),
+            width=60,
+        )
+    )
+    console.print()
+
+
+def emit_logout(
+    *,
+    remote_revoked: bool,
+    json_output: bool,
+) -> None:
+    """Show a successful local logout result."""
+    if json_output:
+        _write_json(
+            {
+                "ok": True,
+                "command": "logout",
+                "localCredentialRemoved": True,
+                "remoteRevoked": remote_revoked,
+            }
+        )
+        return
+
+    if remote_revoked:
+        title = "Logged out of Horizon"
+        message = "The Horizon credential was revoked and removed from this device."
+        style = "green"
+    else:
+        title = "Horizon Account"
+        message = "No active Horizon credential remains on this device."
+        style = "cyan"
+
+    console.print()
+    console.print(
+        Panel(
+            Text(message),
+            title=Text(title, style=f"bold {style}"),
+            title_align="left",
+            box=box.ROUNDED,
+            border_style=style,
+            padding=(1, 2),
+            width=60,
+        )
+    )
+    console.print()
+
+
+def emit_error(
+    command: CommandName,
+    category: ErrorCategory,
+    message: str,
+    *,
+    json_output: bool,
+    details: dict[str, object] | None = None,
+) -> None:
+    """Show a stable expected command failure."""
+    if json_output:
+        payload: dict[str, object] = {
+            "ok": False,
+            "command": command,
+            "error": {
+                "category": category,
+                "message": message,
+            },
+        }
+        if details:
+            payload.update(details)
+        _write_json(payload)
+        return
+
+    titles = {
+        "login": "✗ Sign in failed",
+        "logout": "✗ Sign out failed",
+        "whoami": "✗ Account lookup failed",
+    }
+    error_console.print()
+    error_console.print(
+        Panel(
+            Text(message),
+            title=Text(titles[command], style="bold red"),
+            title_align="left",
+            box=box.ROUNDED,
+            border_style="red",
+            padding=(1, 2),
+            width=60,
+        )
+    )
+    error_console.print()

fastmcp_slim/fastmcp/client/client.py

@@ -9,7 +9,7 @@ import ssl
 import uuid
 from collections.abc import AsyncIterator, Callable, Coroutine, Mapping, Sequence
 from contextlib import AsyncExitStack, asynccontextmanager, suppress
-from dataclasses import dataclass, field
+from dataclasses import dataclass, field, replace
 from pathlib import Path
 from typing import TYPE_CHECKING, Any, Generic, Literal, TypeVar, cast, overload
 
@@ -800,11 +800,23 @@ class Client(
 
     @asynccontextmanager
     async def _context_manager(self):
+        transport_options = self._transport_options
+        if (
+            isinstance(self.transport, MCPConfigTransport)
+            and len(self.transport.config.mcpServers) > 1
+        ):
+            # Resolve mutable connection policy here rather than at construction:
+            # both `Client.mode` and an MCPConfig's server set may change before
+            # the client connects. Preserve options contributed by other layers.
+            transport_options = replace(
+                transport_options or TransportOptions(), backend_mode=self.mode
+            )
+
         # Only passed when this client actually wants non-default settings, so an
         # ordinary client never sends an argument a transport might not accept.
-        if self._transport_options is not None:
+        if transport_options is not None:
             connection = self.transport.connect_session(
-                transport_options=self._transport_options, **self._session_kwargs
+                transport_options=transport_options, **self._session_kwargs
             )
         else:
             connection = self.transport.connect_session(**self._session_kwargs)
@@ -847,9 +859,10 @@ class Client(
         else:
             timeout = normalize_timeout_to_seconds(timeout)
 
-        # A legacy-only transport (SSE, a multi-server proxy config) cannot serve
-        # the modern era; treat "auto" as "legacy" there rather than probing
-        # server/discover, which some such servers answer but then cannot serve.
+        # A legacy-only transport cannot serve the modern era; treat "auto" as
+        # "legacy" there rather than probing server/discover. Multi-server config
+        # transports resolve this flag from their connected backend set before
+        # negotiation reaches this point.
         effective_mode = self.mode
         if effective_mode == "auto" and self.transport.legacy_only:
             effective_mode = "legacy"

fastmcp_slim/fastmcp/client/dependencies.py

@@ -1,6 +1,21 @@
 """Client-side dependency helpers."""
 
 
+def _get_forwardable_http_headers() -> dict[str, str]:
+    """Return ambient headers safe to copy onto a new MCP connection.
+
+    MCP transport and routing headers describe one HTTP hop and must be
+    regenerated for the new connection. `Last-Event-ID` likewise belongs to
+    the inbound connection's event stream. Other headers, including
+    authorization and custom proxy headers, are preserved.
+    """
+    return {
+        name: value
+        for name, value in get_http_headers(include={"authorization"}).items()
+        if not name.startswith("mcp-") and name != "last-event-id"
+    }
+
+
 def get_http_headers(
     include_all: bool = False,
     include: set[str] | None = None,

fastmcp_slim/fastmcp/client/transports/base.py

@@ -44,22 +44,27 @@ class TransportOptions:
 
     These belong to the client rather than to the transport, so a transport
     shared between clients doesn't leak one client's settings to another.
+    Different client layers may own different fields; a layer that adds its
+    settings must preserve the existing options rather than replace the bundle.
 
     Attributes:
         session_class: The ClientSession class to instantiate. Proxies supply a
             session that skips output-schema validation, since they relay
             results rather than consume them.
-        forward_incoming_headers: Whether to forward the inbound request's
-            authorization header upstream. Only appropriate for proxies, where
-            the caller's credentials are meant to be propagated. Honored by the
-            HTTP and SSE transports; ignored by the others.
+        forward_incoming_headers: Whether to forward eligible inbound HTTP
+            headers upstream, including authorization. Hop-specific HTTP headers
+            and MCP transport, routing, and event-stream state are excluded
+            because each backend connection owns that state. Only appropriate
+            for proxies; honored by the HTTP and SSE transports and ignored by
+            the others.
         backend_mode: The connect `mode` to give backend clients that a wrapping
             transport builds on this client's behalf, so a chain of connections
             speaks one protocol era end to end. `None` leaves each backend
             client at its own default. Honored by `MCPConfigTransport`, whose
-            multi-server form mounts a proxy per configured server; ignored by
-            transports that connect to a single backend directly, since those
-            carry the connecting client's own session and era.
+            multi-server form mounts a proxy per configured server and resolves
+            one shared era for the aggregate; ignored by transports that connect
+            to a single backend directly, since those carry the connecting
+            client's own session and era.
     """
 
     session_class: type[ClientSession] = ClientSession

fastmcp_slim/fastmcp/client/transports/config.py

@@ -3,6 +3,7 @@ from collections.abc import AsyncIterator
 from typing import TYPE_CHECKING, Any
 
 from mcp import ClientSession
+from mcp_types.version import MODERN_PROTOCOL_VERSIONS
 from typing_extensions import Unpack
 
 from fastmcp import _install_hints
@@ -85,6 +86,7 @@ class MCPConfigTransport(ClientTransport):
         self.name_as_prefix = name_as_prefix
         self._transports: list[ClientTransport] = []
         self._request_state_security: RequestStateSecurity | None = None
+        self._resolved_legacy_only = False
 
         if not self.config.mcpServers:
             raise ValueError("No MCP servers defined in the config")
@@ -112,18 +114,17 @@ class MCPConfigTransport(ClientTransport):
 
     @property
     def legacy_only(self) -> bool:
-        """Whether this config can only carry the legacy protocol era.
+        """Whether the connected composite resolved to the legacy protocol era.
 
         A single-server config delegates directly to the underlying transport
-        (no proxy), so it inherits that transport's era capability — a modern
-        Streamable HTTP backend must stay modern-capable under `mode="auto"`.
-        A multi-server config mounts each backend behind a legacy-era
-        `ProxyClient` on a composite server, so the composite it exposes is
-        legacy-era and `mode="auto"` should negotiate the handshake.
+        (no proxy), so it inherits that transport's era capability. A
+        multi-server config resolves this value while connecting its backends:
+        all-modern backends leave the composite modern-capable, while any
+        legacy backend moves every leg to the handshake era.
         """
         if len(self.config.mcpServers) == 1:
             return self.transport.legacy_only
-        return True
+        return self._resolved_legacy_only
 
     @contextlib.asynccontextmanager
     async def connect_session(
@@ -152,49 +153,96 @@ class MCPConfigTransport(ClientTransport):
             ) from exc
 
         timeout = session_kwargs.get("read_timeout_seconds")
-        composite = FastMCP[Any](
-            name="MCPRouter", request_state_security=self._request_state_security
+        requested_mode = (
+            transport_options.backend_mode
+            if transport_options is not None
+            and transport_options.backend_mode is not None
+            else "legacy"
         )
 
-        # The composite is only a router: every real backend is reached through
-        # one of the mounted proxies below, so the era the connecting client
-        # negotiates with the composite means nothing unless those backend legs
-        # negotiate it too. `backend_mode` carries the connecting client's era
-        # down to them, keeping the whole chain on one era end to end.
-        backend_mode = (
-            transport_options.backend_mode if transport_options is not None else None
-        )
+        # Close transports retained from a previous connection before replacing
+        # them. The active connection's exit stack owns their normal cleanup.
+        for transport in self._transports:
+            await transport.close()
+        self._transports = []
 
-        async with contextlib.AsyncExitStack() as stack:
-            # Close any previous transports from prior connections to avoid leaking
-            for t in self._transports:
-                await t.close()
-            self._transports = []
+        stack = contextlib.AsyncExitStack()
+        await stack.__aenter__()
+        try:
+            composite, backend_versions = await self._build_composite(
+                FastMCP, timeout, stack, requested_mode
+            )
 
-            for name, server_config in self.config.mcpServers.items():
-                try:
-                    transport, _client, proxy = await self._create_proxy(
-                        name, server_config, timeout, stack, backend_mode
-                    )
-                except Exception:  # Broad catch is intentional: failure modes
-                    # are diverse (OSError, TimeoutError, RuntimeError, etc.)
-                    # and the whole point is to skip any server that can't connect.
-                    logger.warning(
-                        "Failed to connect to MCP server %r, skipping",
-                        name,
-                        exc_info=True,
-                    )
-                    continue
-                self._transports.append(transport)
-                composite.mount(proxy, namespace=name if self.name_as_prefix else None)
-
-            if not self._transports:
-                raise ConnectionError("All MCP servers failed to connect")
+            # `auto` is an aggregate negotiation: the composite can only expose
+            # one era to its caller, so a single legacy backend makes legacy the
+            # best mutual era. Reconnect the modern backends under that era too;
+            # otherwise push- and result-based interactions would meet halfway
+            # through the proxy chain and fail despite the frontend fallback.
+            legacy_backends = [
+                version not in MODERN_PROTOCOL_VERSIONS for version in backend_versions
+            ]
+            if (
+                requested_mode == "auto"
+                and any(legacy_backends)
+                and not all(legacy_backends)
+            ):
+                await stack.aclose()
+                stack = contextlib.AsyncExitStack()
+                await stack.__aenter__()
+                composite, _ = await self._build_composite(
+                    FastMCP, timeout, stack, "legacy"
+                )
+                self._resolved_legacy_only = True
+            else:
+                self._resolved_legacy_only = requested_mode == "legacy" or all(
+                    legacy_backends
+                )
 
             async with FastMCPTransport(mcp=composite).connect_session(
                 transport_options=transport_options, **session_kwargs
             ) as session:
                 yield session
+        finally:
+            await stack.aclose()
+            self._resolved_legacy_only = False
+
+    async def _build_composite(
+        self,
+        fastmcp_type: type["FastMCP[Any]"],
+        timeout: float | None,
+        stack: contextlib.AsyncExitStack,
+        backend_mode: str,
+    ) -> tuple["FastMCP[Any]", list[str]]:
+        """Connect configured backends and mount their proxies on one router."""
+        composite = fastmcp_type(
+            name="MCPRouter", request_state_security=self._request_state_security
+        )
+        self._transports = []
+        backend_versions: list[str] = []
+
+        for name, server_config in self.config.mcpServers.items():
+            try:
+                transport, client, proxy = await self._create_proxy(
+                    name, server_config, timeout, stack, backend_mode
+                )
+            except Exception:  # Broad catch is intentional: failure modes
+                # are diverse (OSError, TimeoutError, RuntimeError, etc.) and
+                # one unavailable server must not take down healthy siblings.
+                logger.warning(
+                    "Failed to connect to MCP server %r, skipping",
+                    name,
+                    exc_info=True,
+                )
+                continue
+            self._transports.append(transport)
+            assert client.protocol_version is not None
+            backend_versions.append(client.protocol_version)
+            composite.mount(proxy, namespace=name if self.name_as_prefix else None)
+
+        if not self._transports:
+            raise ConnectionError("All MCP servers failed to connect")
+
+        return composite, backend_versions
 
     async def _create_proxy(
         self,

fastmcp_slim/fastmcp/client/transports/http.py

@@ -20,7 +20,7 @@ from fastmcp.client.auth.client_credentials import (
     PrivateKeyJWTOAuthProvider,
 )
 from fastmcp.client.auth.oauth import OAuth
-from fastmcp.client.dependencies import get_http_headers
+from fastmcp.client.dependencies import _get_forwardable_http_headers
 from fastmcp.client.transports.base import (
     ClientTransport,
     SessionKwargs,
@@ -161,12 +161,12 @@ class StreamableHttpTransport(ClientTransport):
     ) -> AsyncIterator[ClientSession]:
         options = transport_options or TransportOptions()
 
-        # When used in a proxy, forward the inbound request's authorization
-        # header to the upstream server. This is off by default so that a
-        # plain Client used inside a server tool handler doesn't accidentally
-        # leak the caller's credentials to an unrelated remote server.
+        # Proxies preserve eligible inbound headers while starting a distinct
+        # MCP connection with its own transport state.
+        # This is off by default so a plain Client used inside a server tool
+        # handler cannot leak caller headers to an unrelated remote server.
         if options.forward_incoming_headers:
-            headers = get_http_headers(include={"authorization"}) | self.headers
+            headers = _get_forwardable_http_headers() | self.headers
         else:
             headers = dict(self.headers)
 

fastmcp_slim/fastmcp/client/transports/sse.py

@@ -21,7 +21,7 @@ from fastmcp.client.auth.client_credentials import (
     PrivateKeyJWTOAuthProvider,
 )
 from fastmcp.client.auth.oauth import OAuth
-from fastmcp.client.dependencies import get_http_headers
+from fastmcp.client.dependencies import _get_forwardable_http_headers
 from fastmcp.client.transports.base import (
     ClientTransport,
     SessionKwargs,
@@ -138,14 +138,12 @@ class SSETransport(ClientTransport):
         options = transport_options or TransportOptions()
         client_kwargs: dict[str, Any] = {}
 
-        # When used in a proxy, forward the inbound request's authorization
-        # header to the upstream server. This is off by default so that a
-        # plain Client used inside a server tool handler doesn't accidentally
-        # leak the caller's credentials to an unrelated remote server.
+        # Proxies preserve eligible inbound headers while starting a distinct
+        # MCP connection with its own transport state.
+        # This is off by default so a plain Client used inside a server tool
+        # handler cannot leak caller headers to an unrelated remote server.
         if options.forward_incoming_headers:
-            client_kwargs["headers"] = (
-                get_http_headers(include={"authorization"}) | self.headers
-            )
+            client_kwargs["headers"] = _get_forwardable_http_headers() | self.headers
         else:
             client_kwargs["headers"] = dict(self.headers)
 

fastmcp_slim/fastmcp/server/dependencies.py

@@ -547,9 +547,9 @@ def get_http_headers(
     Never raises an exception, even if there is no active HTTP request (in which case
     an empty dict is returned).
 
-    By default, strips problematic headers like `content-length` and `authorization`
-    that cause issues if forwarded to downstream services. If `include_all` is True,
-    all headers are returned.
+    By default, strips problematic headers like `content-length`, and credential
+    headers like `authorization` and `cookie`, that cause issues if forwarded to
+    downstream services. If `include_all` is True, all headers are returned.
 
     The `include` parameter allows specific headers to be included even if they would
     normally be excluded. This is useful for proxy transports that need to forward
@@ -570,6 +570,7 @@ def get_http_headers(
             "expect",
             "accept",
             "authorization",
+            "cookie",
             # Proxy-related headers
             "proxy-authenticate",
             "proxy-authorization",
@@ -1068,7 +1069,10 @@ class _CurrentHeaders(Dependency[dict[str, str]]):
     """Async context manager for HTTP Headers dependency."""
 
     async def __aenter__(self) -> dict[str, str]:
-        return get_http_headers(include={"authorization"})
+        # Credential headers are denied by default because most callers forward
+        # what they get. This dependency only exposes the current request to the
+        # handler, so it opts them back in.
+        return get_http_headers(include={"authorization", "cookie"})
 
     async def __aexit__(
         self,
@@ -1083,9 +1087,9 @@ def CurrentHeaders() -> dict[str, str]:
     """Get the current HTTP request headers.
 
     This dependency provides access to the HTTP headers for the current request,
-    including the authorization header. Returns an empty dictionary when no HTTP
-    request is available, making it safe to use in code that might run over any
-    transport.
+    including the `authorization` and `cookie` headers, which `get_http_headers()`
+    withholds by default. Returns an empty dictionary when no HTTP request is
+    available, making it safe to use in code that might run over any transport.
 
     Returns:
         A dependency that resolves to a dictionary of header name -> value

fastmcp_slim/fastmcp/server/providers/proxy.py

@@ -96,14 +96,25 @@ class _ForwardingClientSession(ClientSession):
 
 
 # Settings every proxy-backend connection uses: relay results without policing
-# the backend's output schema, and forward the caller's authorization header
-# upstream (appropriate for a proxy, where credentials are meant to propagate).
+# the backend's output schema, and forward eligible caller headers upstream
+# without inheriting frontend-owned MCP transport state.
 PROXY_TRANSPORT_OPTIONS = TransportOptions(
     session_class=_ForwardingClientSession,
     forward_incoming_headers=True,
 )
 
 
+def _with_proxy_transport_options(
+    options: TransportOptions | None,
+) -> TransportOptions:
+    """Layer proxy-owned settings onto options supplied by another client layer."""
+    return replace(
+        options or TransportOptions(),
+        session_class=PROXY_TRANSPORT_OPTIONS.session_class,
+        forward_incoming_headers=PROXY_TRANSPORT_OPTIONS.forward_incoming_headers,
+    )
+
+
 #: Transport-level failures that can escape a backend connection attempt.
 #: `Client._connect` wraps most connect failures in a ``RuntimeError("Client
 #: failed to connect: ...")``, but a transport can also surface an httpx or
@@ -1353,7 +1364,8 @@ def _create_client_factory(
             # stopping at the composite router (see
             # `TransportOptions.backend_mode`).
             fresh._transport_options = replace(
-                PROXY_TRANSPORT_OPTIONS, backend_mode=fresh.mode
+                _with_proxy_transport_options(fresh._transport_options),
+                backend_mode=fresh.mode,
             )
             return fresh
 
@@ -1423,7 +1435,8 @@ def _create_client_factory(
                 # moment a client is built for this request — so it tracks the
                 # front era rather than whatever was true at construction.
                 fresh._transport_options = replace(
-                    PROXY_TRANSPORT_OPTIONS, backend_mode=backend_mode
+                    _with_proxy_transport_options(fresh._transport_options),
+                    backend_mode=backend_mode,
                 )
             return fresh
 
@@ -1744,7 +1757,7 @@ class ProxyClient(Client[ClientTransportT]):
                 self._proxy_restoring_handler_keys.add(key)
         super().__init__(transport=transport, **kwargs)  # ty: ignore[no-matching-overload]
 
-        self._transport_options = PROXY_TRANSPORT_OPTIONS
+        self._transport_options = _with_proxy_transport_options(self._transport_options)
 
     def _bind_restoring_handlers(self) -> None:
         if "roots" in self._proxy_restoring_handler_keys:

fastmcp_slim/fastmcp/server/transforms/search/bm25.py

@@ -3,6 +3,7 @@
 import hashlib
 import math
 import re
+import unicodedata
 from collections.abc import Sequence
 from typing import Annotated, Any
 
@@ -16,8 +17,9 @@ from fastmcp.tools.base import Tool
 
 
 def _tokenize(text: str) -> list[str]:
-    """Lowercase, split on non-alphanumeric, filter short tokens."""
-    return [t for t in re.split(r"[^a-z0-9]+", text.lower()) if len(t) > 1]
+    """Normalize and extract Unicode alphanumeric tokens."""
+    normalized = unicodedata.normalize("NFKC", text).casefold()
+    return re.findall(r"[^\W_]{2,}", normalized)
 
 
 class _BM25Index:

fastmcp_slim/fastmcp/utilities/inspect.py

@@ -489,6 +489,7 @@ async def format_mcp_info(mcp: FastMCP[Any] | SDKServer) -> bytes:
                 "mcp": importlib.metadata.version("mcp"),  # MCP protocol version
             },
             "serverInfo": server_info,
+            "instructions": client.initialize_result.instructions,
             "capabilities": {},  # MCP format doesn't include capabilities at top level
             "tools": tools_result.tools,
             "prompts": prompts_result.prompts,

tests/cli/deploy/test_command.py

@@ -0,0 +1,462 @@
+import json
+from collections.abc import Callable, Iterator
+from contextlib import contextmanager
+from pathlib import Path
+from unittest.mock import Mock
+from urllib.parse import parse_qs
+
+import httpx2
+import pytest
+from pydantic import SecretStr
+
+import fastmcp
+import fastmcp.cli.deploy.authentication as authentication_module
+import fastmcp.cli.deploy.command as command_module
+from fastmcp.cli.deploy.command import login, logout, whoami
+from fastmcp.cli.deploy.configuration import ConfigurationStore
+from fastmcp.cli.deploy.credentials import CredentialStore
+from fastmcp.cli.deploy.horizon_client import HorizonClient
+from fastmcp.cli.deploy.state import StateFileError
+
+
+class HorizonAuthAPI:
+    def __init__(
+        self,
+        *,
+        token_error: str | None = None,
+        revoke_status: int = 204,
+        invalid_api_key: str | None = None,
+        on_request: Callable[[httpx2.Request], None] | None = None,
+    ) -> None:
+        self.token_error = token_error
+        self.revoke_status = revoke_status
+        self.invalid_api_key = invalid_api_key
+        self.on_request = on_request
+        self.requests: list[httpx2.Request] = []
+
+    def __call__(self, request: httpx2.Request) -> httpx2.Response:
+        self.requests.append(request)
+        if self.on_request is not None:
+            self.on_request(request)
+        path = request.url.path
+        if path == "/api/v0/oauth/device/authorization":
+            return httpx2.Response(
+                200,
+                json={
+                    "device_code": "device-secret",
+                    "user_code": "ABCD-EFGH",
+                    "verification_uri": "https://horizon.prefect.io/oauth/device",
+                    "verification_uri_complete": (
+                        "https://horizon.prefect.io/oauth/device?user_code=ABCD-EFGH"
+                    ),
+                    "expires_in": 600,
+                    "interval": 1,
+                },
+            )
+        if path == "/api/v0/oauth/device/token":
+            if self.token_error is not None:
+                return httpx2.Response(400, json={"error": self.token_error})
+            return httpx2.Response(
+                200,
+                json={"access_token": "fmcp_device_key", "token_type": "Bearer"},
+            )
+        if path == "/api/v0/me":
+            if request.headers.get("Authorization") == (
+                f"Bearer {self.invalid_api_key}"
+            ):
+                return httpx2.Response(401)
+            return httpx2.Response(
+                200,
+                json={
+                    "user": {
+                        "id": "user-1",
+                        "email": "ada@example.com",
+                        "name": "Ada",
+                    }
+                },
+            )
+        if path == "/api/v0/me/api-key":
+            return httpx2.Response(self.revoke_status)
+        raise AssertionError(f"Unexpected request: {request.method} {path}")
+
+
+@pytest.fixture
+def use_horizon_api(
+    monkeypatch: pytest.MonkeyPatch,
+) -> Callable[[HorizonAuthAPI], None]:
+    def use(api: HorizonAuthAPI) -> None:
+        transport = httpx2.MockTransport(api)
+
+        def client(
+            api_origin: str,
+            *,
+            api_key: SecretStr | str | None = None,
+        ) -> HorizonClient:
+            return HorizonClient(
+                api_origin,
+                api_key=api_key,
+                transport=transport,
+            )
+
+        monkeypatch.setattr(command_module, "HorizonClient", client)
+
+    return use
+
+
+def test_session_snapshot_reads_host_and_credential_under_one_lock(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    events: list[str] = []
+    configuration_load = ConfigurationStore.load
+    credential_load = CredentialStore.load
+
+    @contextmanager
+    def lock(directory: Path) -> Iterator[None]:
+        events.append("lock")
+        yield
+        events.append("unlock")
+
+    def load_configuration(store: ConfigurationStore):
+        events.append("configuration")
+        return configuration_load(store)
+
+    def load_credential(store: CredentialStore):
+        events.append("credential")
+        return credential_load(store)
+
+    monkeypatch.setattr(command_module, "state_lock", lock)
+    monkeypatch.setattr(ConfigurationStore, "load", load_configuration)
+    monkeypatch.setattr(CredentialStore, "load", load_credential)
+
+    configuration, credential = command_module._load_session_snapshot(CredentialStore())
+
+    assert configuration.api_origin == "https://horizon.prefect.io"
+    assert credential is None
+    assert events == ["lock", "configuration", "credential", "unlock"]
+
+
+@pytest.fixture(autouse=True)
+def no_device_poll_delay(monkeypatch: pytest.MonkeyPatch) -> None:
+    async def sleep(_: float) -> None:
+        return None
+
+    monkeypatch.setattr(authentication_module.asyncio, "sleep", sleep)
+
+
+async def test_json_login_writes_one_result_and_challenge_to_stderr(
+    use_horizon_api: Callable[[HorizonAuthAPI], None],
+    capsys: pytest.CaptureFixture[str],
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    api = HorizonAuthAPI()
+    use_horizon_api(api)
+    browser_open = Mock()
+    monkeypatch.setattr(command_module.webbrowser, "open", browser_open)
+    monkeypatch.setattr(command_module.platform, "node", lambda: "Avery's laptop")
+    monkeypatch.setattr(command_module.platform, "system", lambda: "Darwin")
+    monkeypatch.setattr(command_module.platform, "machine", lambda: "arm64")
+    monkeypatch.setattr(command_module.fastmcp, "__version__", "4.0.0")
+
+    await login(json_output=True)
+
+    captured = capsys.readouterr()
+    stdout_lines = captured.out.strip().splitlines()
+    assert len(stdout_lines) == 1
+    assert json.loads(stdout_lines[0]) == {
+        "ok": True,
+        "command": "login",
+        "user": {
+            "id": "user-1",
+            "email": "ada@example.com",
+            "name": "Ada",
+        },
+    }
+    assert json.loads(captured.err) == {
+        "event": "device_authorization",
+        "verificationUrl": "https://horizon.prefect.io/oauth/device",
+        "verificationUrlComplete": (
+            "https://horizon.prefect.io/oauth/device?user_code=ABCD-EFGH"
+        ),
+        "userCode": "ABCD-EFGH",
+    }
+    browser_open.assert_not_called()
+    authorization_request = next(
+        request
+        for request in api.requests
+        if request.url.path == "/api/v0/oauth/device/authorization"
+    )
+    assert parse_qs(authorization_request.content.decode()) == {
+        "client_id": ["fastmcp-cli"],
+        "device_name": ["Avery's laptop"],
+        "platform": ["darwin"],
+        "architecture": ["arm64"],
+        "client_version": ["4.0.0"],
+    }
+
+    state = json.loads(CredentialStore().path.read_text())
+    assert state == {"schemaVersion": 1, "apiKey": "fmcp_device_key"}
+    assert not (fastmcp.settings.home / "cli" / "config.json").exists()
+
+
+async def test_login_host_is_saved_before_device_authorization(
+    use_horizon_api: Callable[[HorizonAuthAPI], None],
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    api = HorizonAuthAPI()
+    use_horizon_api(api)
+
+    await login(host="https://dev.horizon.prefect.io/", json_output=True)
+
+    assert json.loads(capsys.readouterr().out)["ok"] is True
+    configuration_path = fastmcp.settings.home / "cli" / "config.json"
+    assert json.loads(configuration_path.read_text()) == {
+        "schemaVersion": 1,
+        "apiOrigin": "https://dev.horizon.prefect.io",
+    }
+    assert {request.url.host for request in api.requests} == {"dev.horizon.prefect.io"}
+
+
+async def test_login_rejects_an_invalid_host(
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    with pytest.raises(SystemExit, match="1"):
+        await login(host="https://horizon.prefect.io/path", json_output=True)
+
+    result = json.loads(capsys.readouterr().out)
+    assert result["error"]["category"] == "invalid_host"
+    assert CredentialStore().path.exists() is False
+
+
+async def test_tty_login_survives_browser_open_failure(
+    use_horizon_api: Callable[[HorizonAuthAPI], None],
+    capsys: pytest.CaptureFixture[str],
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    use_horizon_api(HorizonAuthAPI())
+    browser_open = Mock(side_effect=OSError("No browser"))
+    monkeypatch.setattr(command_module, "_can_open_browser", lambda: True)
+    monkeypatch.setattr(command_module.webbrowser, "open", browser_open)
+
+    await login()
+
+    output = capsys.readouterr().out
+    assert "https://horizon.prefect.io/oauth/device" in output
+    assert "ABCD-EFGH" in output
+    assert "Logged into Horizon" in output
+    assert "Ada" in output
+    assert "ada@example.com" in output
+    assert "Organization" not in output
+    browser_open.assert_called_once()
+
+
+async def test_whoami_uses_the_stored_key_after_a_restart(
+    use_horizon_api: Callable[[HorizonAuthAPI], None],
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    api = HorizonAuthAPI()
+    use_horizon_api(api)
+    await login(json_output=True)
+    capsys.readouterr()
+
+    await whoami(json_output=True)
+
+    result = json.loads(capsys.readouterr().out)
+    assert result["command"] == "whoami"
+    assert result["user"]["email"] == "ada@example.com"
+    assert [request.url.path for request in api.requests].count("/api/v0/me") == 2
+
+
+async def test_login_replaces_an_invalid_stored_key(
+    use_horizon_api: Callable[[HorizonAuthAPI], None],
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    use_horizon_api(HorizonAuthAPI(invalid_api_key="fmcp_stale_key"))
+    CredentialStore().save("fmcp_stale_key")
+
+    await login(json_output=True)
+
+    captured = capsys.readouterr()
+    assert json.loads(captured.out)["ok"] is True
+    assert json.loads(captured.err)["event"] == "device_authorization"
+    stored_key = CredentialStore().load()
+    assert stored_key is not None
+    assert stored_key.get_secret_value() == "fmcp_device_key"
+
+
+async def test_login_never_persists_an_environment_key(
+    use_horizon_api: Callable[[HorizonAuthAPI], None],
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    api = HorizonAuthAPI()
+    use_horizon_api(api)
+    monkeypatch.setenv("HORIZON_API_KEY", "fmcp_environment_key")
+
+    await login(json_output=True)
+
+    assert CredentialStore().path.exists() is False
+    assert not any(
+        request.url.path.startswith("/api/v0/oauth/device") for request in api.requests
+    )
+
+
+async def test_json_whoami_reports_a_failed_rejected_key_cleanup(
+    use_horizon_api: Callable[[HorizonAuthAPI], None],
+    capsys: pytest.CaptureFixture[str],
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    use_horizon_api(HorizonAuthAPI(invalid_api_key="fmcp_stale_key"))
+    CredentialStore().save("fmcp_stale_key")
+
+    def fail_clear(store: CredentialStore) -> None:
+        raise StateFileError("cleanup failed")
+
+    monkeypatch.setattr(CredentialStore, "clear", fail_clear)
+
+    with pytest.raises(SystemExit, match="1"):
+        await whoami(json_output=True)
+
+    result = json.loads(capsys.readouterr().out)
+    assert result["error"]["category"] == "state_error"
+
+
+async def test_whoami_does_not_clear_a_newer_host_credential(
+    use_horizon_api: Callable[[HorizonAuthAPI], None],
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    credentials = CredentialStore()
+    credentials.save("fmcp_stale_key")
+    switched = False
+
+    def switch_host(request: httpx2.Request) -> None:
+        nonlocal switched
+        if request.url.path != "/api/v0/me" or switched:
+            return
+        switched = True
+        ConfigurationStore().set_api_origin(
+            "https://dev.horizon.prefect.io",
+            credentials=credentials,
+        )
+        credentials.save_for_origin(
+            "fmcp_new_key",
+            expected_api_origin="https://dev.horizon.prefect.io",
+        )
+
+    api = HorizonAuthAPI(
+        invalid_api_key="fmcp_stale_key",
+        on_request=switch_host,
+    )
+    use_horizon_api(api)
+
+    with pytest.raises(SystemExit, match="1"):
+        await whoami(json_output=True)
+
+    result = json.loads(capsys.readouterr().out)
+    assert result["error"]["category"] == "authentication_invalid"
+    assert ConfigurationStore().load().api_origin == ("https://dev.horizon.prefect.io")
+    stored_key = credentials.load()
+    assert stored_key is not None
+    assert stored_key.get_secret_value() == "fmcp_new_key"
+    assert api.requests[0].url.host == "horizon.prefect.io"
+
+
+async def test_json_whoami_does_not_start_device_authorization(
+    capsys: pytest.CaptureFixture[str],
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    browser_open = Mock()
+    monkeypatch.setattr(command_module.webbrowser, "open", browser_open)
+
+    with pytest.raises(SystemExit, match="1"):
+        await whoami(json_output=True)
+
+    result = json.loads(capsys.readouterr().out)
+    assert result["error"]["category"] == "authentication_required"
+    browser_open.assert_not_called()
+
+
+@pytest.mark.parametrize(
+    ("token_error", "category"),
+    [
+        ("access_denied", "authorization_denied"),
+        ("expired_token", "authorization_expired"),
+    ],
+)
+async def test_json_login_reports_stable_device_failures(
+    token_error: str,
+    category: str,
+    use_horizon_api: Callable[[HorizonAuthAPI], None],
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    use_horizon_api(HorizonAuthAPI(token_error=token_error))
+
+    with pytest.raises(SystemExit, match="1"):
+        await login(json_output=True)
+
+    result = json.loads(capsys.readouterr().out)
+    assert result["error"]["category"] == category
+    assert CredentialStore().path.exists() is False
+
+
+async def test_logout_does_not_modify_environment_or_stored_credentials(
+    use_horizon_api: Callable[[HorizonAuthAPI], None],
+    capsys: pytest.CaptureFixture[str],
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    api = HorizonAuthAPI()
+    use_horizon_api(api)
+    monkeypatch.setenv("HORIZON_API_KEY", "fmcp_environment_key")
+    CredentialStore().save("fmcp_stored_key")
+
+    await logout(json_output=True)
+
+    assert json.loads(capsys.readouterr().out) == {
+        "ok": True,
+        "command": "logout",
+        "credentialSource": "environment",
+        "localCredentialRemoved": False,
+        "remoteRevoked": False,
+    }
+    stored_key = CredentialStore().load()
+    assert stored_key is not None
+    assert stored_key.get_secret_value() == "fmcp_stored_key"
+    assert api.requests == []
+
+
+async def test_logout_revokes_the_remote_key_and_clears_local_state(
+    use_horizon_api: Callable[[HorizonAuthAPI], None],
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    api = HorizonAuthAPI()
+    use_horizon_api(api)
+    CredentialStore().save("fmcp_stored_key")
+
+    await logout(json_output=True)
+
+    assert json.loads(capsys.readouterr().out) == {
+        "ok": True,
+        "command": "logout",
+        "localCredentialRemoved": True,
+        "remoteRevoked": True,
+    }
+    assert CredentialStore().path.exists() is False
+    assert any(
+        request.method == "DELETE" and request.url.path == "/api/v0/me/api-key"
+        for request in api.requests
+    )
+
+
+async def test_logout_clears_local_state_when_remote_revocation_fails(
+    use_horizon_api: Callable[[HorizonAuthAPI], None],
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    use_horizon_api(HorizonAuthAPI(revoke_status=503))
+    CredentialStore().save("fmcp_stored_key")
+
+    with pytest.raises(SystemExit, match="1"):
+        await logout(json_output=True)
+
+    result = json.loads(capsys.readouterr().out)
+    assert result["error"]["category"] == "remote_revocation_failed"
+    assert result["localCredentialRemoved"] is True
+    assert result["remoteCredentialMayRemain"] is True
+    assert CredentialStore().path.exists() is False

tests/cli/deploy/test_credentials.py

@@ -183,6 +183,29 @@ async def test_interactive_credential_rejects_an_origin_change(
     assert store.load() is None
 
 
+def test_conditional_clear_preserves_newer_state(tmp_path: Path) -> None:
+    store = CredentialStore(tmp_path)
+    store.save("fmcp_current")
+
+    store.clear_if_matches(
+        "fmcp_different",
+        expected_api_origin="https://horizon.prefect.io",
+    )
+    store.clear_if_matches(
+        "fmcp_current",
+        expected_api_origin="https://dev.horizon.prefect.io",
+    )
+
+    assert load_secret(store).get_secret_value() == "fmcp_current"
+
+    store.clear_if_matches(
+        "fmcp_current",
+        expected_api_origin="https://horizon.prefect.io",
+    )
+
+    assert store.load() is None
+
+
 async def test_missing_noninteractive_credential_is_explicit(tmp_path: Path) -> None:
     with pytest.raises(AuthenticationRequiredError):
         await resolve_credential(CredentialStore(tmp_path), environ={})

tests/cli/deploy/test_output.py

@@ -0,0 +1,167 @@
+import json
+
+import pytest
+
+from fastmcp.cli.deploy.horizon_client import DeviceAuthorization, HorizonUser
+from fastmcp.cli.deploy.output import (
+    emit_device_challenge,
+    emit_environment_logout,
+    emit_error,
+    emit_identity,
+    emit_logout,
+)
+
+
+def authorization() -> DeviceAuthorization:
+    return DeviceAuthorization(
+        device_code="device-secret",
+        user_code="ABCD-EFGH",
+        verification_uri="https://horizon.prefect.io/oauth/device",
+        verification_uri_complete=(
+            "https://horizon.prefect.io/oauth/device?user_code=ABCD-EFGH"
+        ),
+        expires_in=600,
+        interval=5,
+    )
+
+
+def user() -> HorizonUser:
+    return HorizonUser(id="user-1", email="ada@example.com", name="Ada")
+
+
+def test_json_device_challenge_uses_only_stderr(
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    emit_device_challenge(authorization(), json_output=True)
+
+    captured = capsys.readouterr()
+    assert captured.out == ""
+    assert json.loads(captured.err) == {
+        "event": "device_authorization",
+        "verificationUrl": "https://horizon.prefect.io/oauth/device",
+        "verificationUrlComplete": (
+            "https://horizon.prefect.io/oauth/device?user_code=ABCD-EFGH"
+        ),
+        "userCode": "ABCD-EFGH",
+    }
+
+
+def test_tty_device_challenge_uses_the_sign_in_layout(
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    emit_device_challenge(authorization(), json_output=False)
+
+    output = capsys.readouterr().out
+    assert "│" in output
+    assert "Deploy FastMCP on Horizon" in output
+    assert "✓ Device authorization started" in output
+    assert "https://horizon.prefect.io/oauth/device?user_code=ABCD-EFGH" in output
+    assert "ABCD-EFGH" in output
+    assert "The request expires in 10 minutes." in output
+
+
+def test_json_identity_has_stable_fields(
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    emit_identity("login", user(), json_output=True)
+
+    result = json.loads(capsys.readouterr().out)
+    assert result == {
+        "ok": True,
+        "command": "login",
+        "user": {
+            "id": "user-1",
+            "email": "ada@example.com",
+            "name": "Ada",
+        },
+    }
+
+
+def test_tty_identity_uses_an_account_panel(
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    emit_identity("whoami", user(), json_output=False)
+
+    output = capsys.readouterr().out
+    assert "│" in output
+    assert "Horizon Account" in output
+    assert "Ada" in output
+    assert "ada@example.com" in output
+    assert "● Signed in" in output
+    assert "Organization" not in output
+
+
+def test_json_error_has_stable_fields(
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    emit_error(
+        "logout",
+        "remote_revocation_failed",
+        "The remote key can remain active.",
+        json_output=True,
+        details={
+            "localCredentialRemoved": True,
+            "remoteCredentialMayRemain": True,
+        },
+    )
+
+    result = json.loads(capsys.readouterr().out)
+    assert result == {
+        "ok": False,
+        "command": "logout",
+        "error": {
+            "category": "remote_revocation_failed",
+            "message": "The remote key can remain active.",
+        },
+        "localCredentialRemoved": True,
+        "remoteCredentialMayRemain": True,
+    }
+
+
+def test_tty_environment_logout_explains_that_no_action_was_taken(
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    emit_environment_logout(json_output=False)
+
+    output = capsys.readouterr().out
+    assert "Horizon Account" in output
+    assert "This session uses HORIZON_API_KEY." in output
+    assert "Remove it from your environment to sign out." in output
+    assert "No credential was revoked or removed." in output
+
+
+def test_json_environment_logout_has_stable_fields(
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    emit_environment_logout(json_output=True)
+
+    assert json.loads(capsys.readouterr().out) == {
+        "ok": True,
+        "command": "logout",
+        "credentialSource": "environment",
+        "localCredentialRemoved": False,
+        "remoteRevoked": False,
+    }
+
+
+def test_tty_logout_uses_the_horizon_header(
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    emit_logout(remote_revoked=True, json_output=False)
+
+    output = capsys.readouterr().out
+    assert "Logged out of Horizon" in output
+    assert "│" in output
+
+
+def test_json_logout_has_stable_fields(
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    emit_logout(remote_revoked=True, json_output=True)
+
+    assert json.loads(capsys.readouterr().out) == {
+        "ok": True,
+        "command": "logout",
+        "localCredentialRemoved": True,
+        "remoteRevoked": True,
+    }

tests/cli/test_cli.py

@@ -35,6 +35,20 @@ class TestMainCLI:
         assert isinstance(exc_info.value, SystemExit)
         assert exc_info.value.code == 1
 
+    @pytest.mark.parametrize("name", ["login", "logout", "whoami"])
+    def test_horizon_account_commands_are_top_level(self, name: str):
+        command, bound, _ = app.parse_args([name, "--json"])
+
+        assert command.__name__ == name  # type: ignore[attr-defined]  # ty:ignore[unresolved-attribute]
+        assert bound.arguments == {"json_output": True}
+
+    def test_login_accepts_a_horizon_host(self):
+        _, bound, _ = app.parse_args(
+            ["login", "--host", "https://dev.horizon.prefect.io"]
+        )
+
+        assert bound.arguments == {"host": "https://dev.horizon.prefect.io"}
+
 
 class TestVersionCommand:
     """Test the version command."""

tests/server/http/test_http_dependencies.py

@@ -177,6 +177,66 @@ async def test_get_http_headers_excludes_content_type(sse_server: ASGIServer):
             assert headers["x-custom-header"] == "should-be-included"
 
 
+async def test_get_http_headers_excludes_cookie(sse_server: ASGIServer):
+    """get_http_headers() must not leak the caller's Cookie to a backend.
+
+    The OpenAPI provider forwards this mapping to the upstream named in the
+    spec, so a session cookie scoped to the MCP host would otherwise reach a
+    separate origin on every tool call. Callers that genuinely need it can ask
+    for it back with `include={"cookie"}`, the same escape hatch authorization
+    uses.
+    """
+    from fastmcp.server.dependencies import get_http_headers
+
+    server = FastMCP()
+
+    @server.tool
+    def default_headers() -> dict[str, str]:
+        return get_http_headers()
+
+    @server.tool
+    def opted_in_headers() -> dict[str, str]:
+        return get_http_headers(include={"cookie"})
+
+    async with asgi_server(server, transport="sse") as running_server:
+        async with running_server.client(
+            headers={"Cookie": "session=alice-secret", "X-Keep": "yes"}
+        ) as client:
+            default = (await client.call_tool("default_headers")).data
+            assert "cookie" not in default
+            assert default["x-keep"] == "yes"
+
+            opted_in = (await client.call_tool("opted_in_headers")).data
+            assert opted_in["cookie"] == "session=alice-secret"
+
+
+async def test_current_headers_still_exposes_cookie(sse_server: ASGIServer):
+    """CurrentHeaders() reads the request, so credentials stay visible.
+
+    The default denylist protects call sites that forward headers upstream.
+    A handler inspecting its own request needs the cookie, the same way it
+    already needs authorization.
+    """
+    from fastmcp.server.dependencies import CurrentHeaders
+
+    server = FastMCP()
+
+    @server.tool
+    def read_request(headers: dict = CurrentHeaders()) -> dict[str, str]:
+        return headers
+
+    async with asgi_server(server, transport="sse") as running_server:
+        async with running_server.client(
+            headers={
+                "Cookie": "session=alice-secret",
+                "Authorization": "Bearer alice-token",
+            }
+        ) as client:
+            headers = (await client.call_tool("read_request")).data
+            assert headers["cookie"] == "session=alice-secret"
+            assert headers["authorization"] == "Bearer alice-token"
+
+
 def _worker_snapshot_headers() -> dict[str, str]:
     """Read the HTTP headers snapshotted at task submission from inside a worker."""
     task_info = get_task_context()

tests/server/providers/proxy/test_proxy_headers.py

@@ -0,0 +1,107 @@
+"""Header forwarding across ProxyProvider HTTP hops."""
+
+import json
+from typing import Any
+
+import httpx2
+from mcp import MCPError
+from mcp_types import METHOD_NOT_FOUND
+from mcp_types.version import LATEST_HANDSHAKE_VERSION, LATEST_MODERN_VERSION
+
+from fastmcp import FastMCP
+from fastmcp.server.middleware import Middleware
+from fastmcp.server.providers.proxy import ProxyClient, ProxyProvider
+from fastmcp.utilities.tests import asgi_server
+
+
+async def test_proxy_does_not_forward_frontend_mcp_headers_to_legacy_backend():
+    """A modern frontend's transport state does not contaminate a legacy backend."""
+    captured_requests: list[httpx2.Request] = []
+
+    class RejectDiscovery(Middleware):
+        async def on_discover(self, context, call_next):
+            raise MCPError(code=METHOD_NOT_FOUND, message="Method not found")
+
+    backend = FastMCP("Legacy Backend", middleware=[RejectDiscovery()])
+
+    @backend.tool
+    def legacy_ping() -> str:
+        return "pong"
+
+    async with asgi_server(backend) as running_backend:
+
+        async def capture_request(request: httpx2.Request) -> None:
+            captured_requests.append(request)
+
+        def backend_http_client(
+            headers: dict[str, str] | None = None,
+            timeout: httpx2.Timeout | None = None,
+            auth: httpx2.Auth | None = None,
+            **kwargs: Any,
+        ) -> httpx2.AsyncClient:
+            return running_backend.http_client(
+                headers=headers,
+                timeout=timeout,
+                auth=auth,
+                event_hooks={"request": [capture_request]},
+                **kwargs,
+            )
+
+        backend_transport = running_backend.transport(
+            httpx_client_factory=backend_http_client
+        )
+        proxy = FastMCP(
+            "Proxy",
+            providers=[
+                ProxyProvider(lambda: ProxyClient(backend_transport, mode="auto"))
+            ],
+        )
+
+        async with asgi_server(proxy) as running_proxy:
+            async with running_proxy.client(
+                mode="auto",
+                headers={
+                    "Authorization": "Bearer frontend-token",
+                    "X-Proxy-Custom": "preserved",
+                    "Mcp-Name": "frontend-name",
+                    "Mcp-Param-Tenant": "frontend-tenant",
+                    "Mcp-Session-Id": "frontend-session",
+                    "Last-Event-ID": "frontend-event",
+                },
+            ) as client:
+                assert client.protocol_version == LATEST_MODERN_VERSION
+                tools = await client.list_tools()
+
+    assert [tool.name for tool in tools] == ["legacy_ping"]
+
+    def request_for(method: str) -> httpx2.Request:
+        return next(
+            request
+            for request in captured_requests
+            if request.method == "POST"
+            and json.loads(request.content).get("method") == method
+        )
+
+    discover = request_for("server/discover")
+    initialize = request_for("initialize")
+    list_tools = request_for("tools/list")
+
+    assert discover.headers["mcp-protocol-version"] == LATEST_MODERN_VERSION
+    assert discover.headers["mcp-method"] == "server/discover"
+
+    assert "mcp-protocol-version" not in initialize.headers
+    assert "mcp-method" not in initialize.headers
+    initialize_body = json.loads(initialize.content)
+    assert initialize_body["params"]["protocolVersion"] == LATEST_HANDSHAKE_VERSION
+
+    assert list_tools.headers["mcp-protocol-version"] == LATEST_HANDSHAKE_VERSION
+    assert "mcp-method" not in list_tools.headers
+    assert list_tools.headers["mcp-session-id"] != "frontend-session"
+
+    for request in (discover, initialize, list_tools):
+        assert request.headers["authorization"] == "Bearer frontend-token"
+        assert request.headers["x-proxy-custom"] == "preserved"
+        assert "mcp-name" not in request.headers
+        assert "mcp-param-tenant" not in request.headers
+        assert request.headers.get("mcp-session-id") != "frontend-session"
+        assert "last-event-id" not in request.headers

tests/server/test_mrtr_guards.py

@@ -636,6 +636,22 @@ class TestMultiServerConfigEraMirroring:
         """Two entries so the transport takes its multi-server composite path."""
         return {"mcpServers": {"a": {"url": url}, "b": {"url": url}}}
 
+    async def test_direct_multi_server_client_drives_modern_guard_round_trips(self):
+        """The config-based client itself stays modern through every backend leg."""
+        async with run_server_async(_era_reporting_backend()) as url:
+            asked: list[str] = []
+            async with Client(
+                self._config(url),
+                elicitation_handler=_two_answer_handler(asked),
+            ) as client:
+                era = await client.call_tool("a_backend_era", {})
+                result = await client.call_tool("a_book_flight", {})
+
+        assert client.protocol_version is None
+        assert era.data == "2026-07-28"
+        assert result.data == "Booked Paris on 2026-08-01"
+        assert len(asked) == 2
+
     async def test_modern_front_reaches_modern_backends(self):
         """A modern front reaches each real backend on a modern session, and a
         backend guard tool round-trips end to end across both proxy hops."""

tests/server/transforms/test_search.py

@@ -436,6 +436,18 @@ class TestBM25Index:
         index.build(["alpha beta gamma"])
         assert index.query("zzz", 5) == []
 
+    def test_unicode_tokens(self):
+        index = _BM25Index()
+        index.build(["show portfolio", "показать портфель"])
+        assert index.query("портфель", 5) == [1]
+
+    def test_unicode_normalization(self):
+        index = _BM25Index()
+        index.build(["Straße ＰＯＲＴＦＯＬＩＯ cafe\u0301"])
+        assert index.query("STRASSE", 5) == [0]
+        assert index.query("portfolio", 5) == [0]
+        assert index.query("café", 5) == [0]
+
 
 # ---------------------------------------------------------------------------
 # call_tool self-reference guard

tests/test_mcp_config.py

@@ -15,6 +15,7 @@ from unittest.mock import AsyncMock, patch
 import psutil
 import pytest
 from mcp_types import TextContent
+from mcp_types.version import MODERN_PROTOCOL_VERSIONS
 from pydantic import ConfigDict
 
 from fastmcp import Context, FastMCP
@@ -39,6 +40,7 @@ from fastmcp.mcp_config import (
     TransformingStdioMCPServer,
 )
 from fastmcp.server.elicitation import AcceptedElicitation
+from fastmcp.server.providers.proxy import ProxyClient
 from fastmcp.tools.base import Tool as FastMCPTool
 
 # Some tests in this module spawn subprocess servers via stdio, each paying a
@@ -100,13 +102,26 @@ class InMemoryStdioMCPServer(StdioMCPServer):
         return FastMCPTransport(mcp=self.mcp)
 
 
-class TestConfigTransportLegacyOnly:
-    """`MCPConfigTransport.legacy_only` gating (regression for the over-broad flag).
+class LegacyFastMCPTransport(FastMCPTransport):
+    """In-memory transport that requires the handshake protocol era."""
+
+    legacy_only = True
+
+
+class LegacyInMemoryStdioMCPServer(InMemoryStdioMCPServer):
+    """In-memory config entry that behaves like a legacy-only backend."""
+
+    def to_transport(self) -> FastMCPTransport:
+        return LegacyFastMCPTransport(mcp=self.mcp)
+
+
+class TestConfigTransportEraNegotiation:
+    """`MCPConfigTransport` negotiates one era across every connection leg.
 
     A single-server config delegates directly to the underlying transport with no
-    proxy, so it must mirror that transport's era capability rather than being
-    forced legacy. Only the multi-server composite (backed by legacy-era
-    ProxyClients) is legacy-only.
+    proxy. A multi-server config discovers its backends before the composite client
+    negotiates, allowing an all-modern configuration to stay modern and a mixed
+    configuration to fall back consistently to the handshake era.
     """
 
     def test_single_modern_capable_server_is_not_forced_legacy(self):
@@ -129,8 +144,7 @@ class TestConfigTransportLegacyOnly:
         assert isinstance(transport.transport, SSETransport)
         assert transport.legacy_only is True
 
-    def test_multi_server_config_is_legacy_only(self):
-        """A multi-server composite is legacy-only regardless of backend eras."""
+    def test_multi_server_config_is_not_assumed_legacy_before_connect(self):
         config = {
             "mcpServers": {
                 "a": {"url": "https://a.example.com/mcp"},
@@ -138,7 +152,7 @@ class TestConfigTransportLegacyOnly:
             },
         }
         transport = MCPConfigTransport(config)
-        assert transport.legacy_only is True
+        assert transport.legacy_only is False
 
     def test_transforming_single_server_wrapper_is_legacy_only(self):
         """A single-server config that uses tool transforms or tag filters wraps
@@ -158,6 +172,151 @@ class TestConfigTransportLegacyOnly:
         assert transport.legacy_only is True
 
 
+def _make_protocol_era_server(name: str) -> FastMCP:
+    server = FastMCP(name)
+
+    @server.tool
+    async def protocol_era(ctx: Context) -> str:
+        assert ctx.request_context is not None
+        return ctx.request_context.protocol_version
+
+    @server.tool
+    def add(a: int, b: int) -> int:
+        return a + b
+
+    return server
+
+
+async def test_multi_server_auto_negotiates_modern_end_to_end():
+    """Modern backends keep the default multi-server client modern end to end."""
+    config = MCPConfig(
+        mcpServers={
+            "alpha": InMemoryStdioMCPServer(mcp=_make_protocol_era_server("alpha")),
+            "beta": InMemoryStdioMCPServer(mcp=_make_protocol_era_server("beta")),
+        }
+    )
+
+    async with Client(config) as client:
+        assert client.protocol_version == "2026-07-28"
+
+        tools = await client.list_tools()
+        assert {tool.name for tool in tools} == {
+            "alpha_add",
+            "alpha_protocol_era",
+            "beta_add",
+            "beta_protocol_era",
+        }
+
+        alpha_era = await client.call_tool("alpha_protocol_era", {})
+        beta_era = await client.call_tool("beta_protocol_era", {})
+        result = await client.call_tool("alpha_add", {"a": 2, "b": 3})
+
+    assert alpha_era.data == "2026-07-28"
+    assert beta_era.data == "2026-07-28"
+    assert result.data == 5
+
+
+async def test_multi_server_auto_falls_back_all_legs_when_one_backend_is_legacy():
+    """A mixed config never leaves the composite and its backends on different eras."""
+    config = MCPConfig(
+        mcpServers={
+            "modern": InMemoryStdioMCPServer(mcp=_make_protocol_era_server("modern")),
+            "legacy": LegacyInMemoryStdioMCPServer(
+                mcp=_make_protocol_era_server("legacy")
+            ),
+        }
+    )
+
+    async with Client(config) as client:
+        assert client.protocol_version not in MODERN_PROTOCOL_VERSIONS
+        modern_era = await client.call_tool("modern_protocol_era", {})
+        legacy_era = await client.call_tool("legacy_protocol_era", {})
+
+    assert modern_era.data not in MODERN_PROTOCOL_VERSIONS
+    assert legacy_era.data not in MODERN_PROTOCOL_VERSIONS
+
+
+@pytest.mark.parametrize(
+    ("mode", "is_modern"),
+    [("legacy", False), ("2026-07-28", True)],
+)
+async def test_multi_server_explicit_mode_reaches_every_backend(mode, is_modern):
+    config = MCPConfig(
+        mcpServers={
+            "alpha": InMemoryStdioMCPServer(mcp=_make_protocol_era_server("alpha")),
+            "beta": InMemoryStdioMCPServer(mcp=_make_protocol_era_server("beta")),
+        }
+    )
+
+    async with Client(config, mode=mode) as client:
+        alpha_era = await client.call_tool("alpha_protocol_era", {})
+        beta_era = await client.call_tool("beta_protocol_era", {})
+
+        assert (client.protocol_version in MODERN_PROTOCOL_VERSIONS) is is_modern
+
+    assert (alpha_era.data in MODERN_PROTOCOL_VERSIONS) is is_modern
+    assert (beta_era.data in MODERN_PROTOCOL_VERSIONS) is is_modern
+
+
+async def test_multi_server_proxy_client_auto_negotiates_modern_end_to_end():
+    """ProxyClient keeps its proxy options without losing the aggregate era."""
+    config = MCPConfig(
+        mcpServers={
+            "alpha": InMemoryStdioMCPServer(mcp=_make_protocol_era_server("alpha")),
+            "beta": InMemoryStdioMCPServer(mcp=_make_protocol_era_server("beta")),
+        }
+    )
+
+    async with ProxyClient(config, mode="auto") as client:
+        assert client.protocol_version == "2026-07-28"
+        alpha_era = await client.call_tool("alpha_protocol_era", {})
+        beta_era = await client.call_tool("beta_protocol_era", {})
+
+    assert alpha_era.data == "2026-07-28"
+    assert beta_era.data == "2026-07-28"
+
+
+async def test_multi_server_mode_is_resolved_when_proxy_client_connects():
+    config = MCPConfig(
+        mcpServers={
+            "alpha": InMemoryStdioMCPServer(mcp=_make_protocol_era_server("alpha")),
+            "beta": InMemoryStdioMCPServer(mcp=_make_protocol_era_server("beta")),
+        }
+    )
+    client = ProxyClient(config, mode="legacy")
+    client.mode = "auto"
+
+    async with client:
+        alpha_era = await client.call_tool("alpha_protocol_era", {})
+        beta_era = await client.call_tool("beta_protocol_era", {})
+
+    assert alpha_era.data == "2026-07-28"
+    assert beta_era.data == "2026-07-28"
+
+
+async def test_multi_server_shape_is_resolved_when_client_connects():
+    config = MCPConfig(
+        mcpServers={
+            "alpha": InMemoryStdioMCPServer(mcp=_make_protocol_era_server("alpha"))
+        }
+    )
+    client = Client(config)
+    config.add_server(
+        "beta", InMemoryStdioMCPServer(mcp=_make_protocol_era_server("beta"))
+    )
+
+    async with client:
+        assert client.protocol_version == "2026-07-28"
+        tools = await client.list_tools()
+
+    assert {tool.name for tool in tools} == {
+        "alpha_add",
+        "alpha_protocol_era",
+        "beta_add",
+        "beta_protocol_era",
+    }
+
+
 def test_parse_single_stdio_config():
     config = {
         "mcpServers": {
@@ -927,7 +1086,9 @@ async def test_multi_client_with_elicitation():
     config = MCPConfig(
         mcpServers={
             "test_server": InMemoryStdioMCPServer(mcp=_make_elicit_server()),
-            "test_server_2": InMemoryStdioMCPServer(mcp=_make_elicit_server()),
+            # One legacy-only backend makes the aggregate reconnect every leg
+            # under the handshake era, where server-initiated elicitation works.
+            "test_server_2": LegacyInMemoryStdioMCPServer(mcp=_make_elicit_server()),
         }
     )
 
@@ -1046,7 +1207,9 @@ async def test_multi_server_session_persistence():
     config = MCPConfig(
         mcpServers={
             "server1": InMemoryStdioMCPServer(mcp=_make_session_server()),
-            "server2": InMemoryStdioMCPServer(mcp=_make_session_server()),
+            # Session identity is a handshake-era feature. A legacy-only sibling
+            # verifies aggregate auto-negotiation preserves it on every backend.
+            "server2": LegacyInMemoryStdioMCPServer(mcp=_make_session_server()),
         }
     )
 

tests/utilities/test_inspect_icons.py

@@ -425,6 +425,7 @@ class TestFormatFunctions:
 
         # Check server version in MCP format
         assert data["serverInfo"]["version"] == "2.0.0"
+        assert data["instructions"] == "Test instructions"
 
         # MCP format SHOULD have environment fields
         assert "environment" in data