fastmcp/docs/getting-started/upgrading/from-fastmcp-3.mdx

---
title: Upgrading from FastMCP 3
sidebarTitle: "From FastMCP 3.x"
description: What changes when you upgrade to FastMCP 4, which builds on the MCP Python SDK v2
icon: up
---

FastMCP 4 builds on the MCP Python SDK v2, and that is the source of every change in this guide. The SDK v2 makes two sweeping changes to the protocol layer: it splits the protocol types out of `mcp.types` into a standalone `mcp_types` package, and it renames every protocol field from camelCase to snake_case (`inputSchema` → `input_schema`, `mimeType` → `mime_type`, `isError` → `is_error`, and so on).

FastMCP 4 absorbs almost all of this for you. Field access is bridged so your existing reads keep working, and the imports you were taught have a stable home in FastMCP itself. The sections below describe what FastMCP handles for you, the small number of changes you must make in your own code, and the deprecation timeline for the compatibility shims.

## Environment requirements

The SDK v2 raises FastMCP's dependency floors, which matters before any of your code runs.

**pydantic >= 2.12 is now the floor.** If your project pins an older pydantic (for example `pydantic==2.11.*`), installing this FastMCP release fails with an unsatisfiable-resolution error from your installer — bump your pin to `>=2.12` first. If you don't pin pydantic at all, installers upgrade it silently as part of the FastMCP upgrade.

**The server extra floors Starlette >= 1.0.** Modern FastAPI (0.11x and later) already runs on Starlette 1.x, so mounting a FastMCP server inside a FastAPI app coexists cleanly — verified with FastAPI 0.138.2. Only very old FastAPI versions pinned below Starlette 1.0 conflict; upgrade FastAPI if your resolver complains about Starlette.

## What FastMCP absorbs

### Legacy camelCase field access keeps working

Objects that FastMCP hands back to you — the results of `client.list_tools()`, `client.call_tool_mcp()`, `client.read_resource()`, and the parameter objects passed to your sampling and elicitation handlers — are SDK v2 objects with snake_case fields. FastMCP installs a compatibility bridge at import time that routes the old camelCase names to their new snake_case fields, so code written against FastMCP 2.x still reads correctly:

```python
from fastmcp import Client

async with Client("my_mcp_server.py") as client:
    tools = await client.list_tools()
    schema = tools[0].inputSchema  # still works, warns once
```

Each bridged read emits a `FastMCPDeprecationWarning` pointing you at the snake_case name (`tools[0].input_schema` here). The bridge covers the fields users actually read: `inputSchema`/`outputSchema` on tools, `mimeType` on resources and content, `isError`/`structuredContent` on tool results, `nextCursor` on paginated results, `serverInfo`/`protocolVersion` on the initialize result, the sampling parameter fields (`systemPrompt`, `maxTokens`, `stopSequences`, `modelPreferences`, `toolChoice`), and `requestedSchema` on elicitation parameters.

The bridge is controlled by the `mcp_camelcase_compat` setting, which defaults to on. Set it to `False` (or the environment variable `FASTMCP_MCP_CAMELCASE_COMPAT=false`) to turn the shims off, in which case only the snake_case names resolve:

```python
import fastmcp

fastmcp.settings.mcp_camelcase_compat = False
```

See [Settings](/more/settings) for the full reference.

### Protocol types moved to `mcp_types`

The `mcp.types` module no longer exists. Every protocol type — `TextContent`, `ImageContent`, `Tool`, `ErrorData`, `Icon`, `PromptMessage`, `SamplingMessage`, `ToolAnnotations`, notification and request wrapper types like `ToolListChangedNotification`, and everything else — now lives in the standalone `mcp_types` package. Update your imports to point there:

```python
from mcp_types import TextContent, Tool, ToolAnnotations
```

`fastmcp.types` still exists, but holds only types FastMCP defines itself (currently just `Textarea`, used to render a multiline textarea in form-based UIs) — it does not re-export protocol types.

### `McpError` has an alias

`fastmcp.exceptions.McpError` is an alias of the SDK's `MCPError`. Catching errors is unchanged — `except McpError` still catches SDK-raised errors, and reading `err.error.code` still works:

```python
from fastmcp.exceptions import McpError

try:
    ...
except McpError as err:
    print(err.error.code)
```

### Behavior preserved across the SDK boundary

A few client behaviors that touch the SDK are preserved so you don't have to change anything:

- `Client(timeout=...)` accepts both a `timedelta` and a plain float number of seconds, as before.
- `client.ping()` returns a `bool`.
- `client.transport.get_session_id()` returns `None` on protocol eras that have no session, rather than raising. (The SDK v2 removed session-id access from its streamable HTTP transport; FastMCP reconstructs it on the transport object.)

## What you must change

Three things are on you.

**Your own `mcp.types` imports.** FastMCP can re-export types, but it can't rewrite imports in your code. Any `from mcp.types import X` or `import mcp.types` in your server or client fails at import time with:

```
ModuleNotFoundError: No module named 'mcp.types'
```

The raw message gives no hint toward the fix, so if you see it after upgrading, this is why. Switch to `from mcp_types import X`.

**`McpError` construction.** The v1 pattern of wrapping an `ErrorData` and passing it positionally fails under SDK v2 with:

```
TypeError: MCPError.__init__() missing 1 required positional argument: 'message'
```

Note the message prints the class as `MCPError` (uppercase) even though your code wrote `McpError` — the old name is an alias for the SDK's renamed class. Construct the error with keyword arguments instead:

```python
from fastmcp.exceptions import McpError

# Before (raises TypeError under SDK v2):
#   raise McpError(ErrorData(code=-32000, message="Client not supported"))

# After:
raise McpError(code=-32000, message="Client not supported")
```

Catching and `err.error.code` are unchanged — only construction moved.

**Raw session access sees v2 objects.** If you reach past FastMCP's client and server surfaces into `client.session`, `ctx.session`, or the internals of `ctx.request_context`, you're now holding raw SDK v2 objects with snake_case fields and the v2 method signatures. FastMCP does not wrap these; code that depends on their v1 shape needs updating.

**FastMCP now uses httpx2 exclusively.** FastMCP has replaced `httpx` with [httpx2](https://pypi.org/project/httpx2/), a next-generation httpx fork, across its entire HTTP stack — client transports and every server-side path (auth providers, the OpenAPI integration, the version check). `httpx` is no longer a FastMCP dependency. If you pass a custom client or factory into a FastMCP client transport — `StreamableHttpTransport(httpx_client_factory=...)`, `SSETransport(httpx_client_factory=...)`, `OAuth(httpx_client_factory=...)`, or a custom `httpx.Auth` as `Client(auth=...)` — those objects must now be httpx2. httpx2 is a drop-in fork with the same public API, so the change is an import swap:

```python
# Before
import httpx

transport = StreamableHttpTransport(
    "https://example.com/mcp",
    httpx_client_factory=lambda **kwargs: httpx.AsyncClient(verify=False, **kwargs),
)

# After
import httpx2

transport = StreamableHttpTransport(
    "https://example.com/mcp",
    httpx_client_factory=lambda **kwargs: httpx2.AsyncClient(verify=False, **kwargs),
)
```

The `client` you pass to `FastMCP.from_openapi(client=...)` (and `OpenAPIProvider(client=...)`) is now type-hinted `httpx2.AsyncClient`. FastMCP does not gate on the type, so an existing `httpx.AsyncClient` keeps working at runtime via duck-typing this release — but switching it to `httpx2.AsyncClient` clears the type hint and is the supported path going forward. HTTP made inside your own tools is entirely yours and is unaffected either way.

**The subtlest break is exception handlers, and no type checker will catch it.** `httpx` very likely remains installed in your environment (the Anthropic, OpenAI, and Google SDKs all depend on it), so code that catches old-httpx exceptions around FastMCP calls still imports and still type-checks — it just never matches, because FastMCP now raises `httpx2` exceptions. The handler silently becomes dead code:

```python
import httpx  # still installed transitively — this import works

try:
    result = await client.call_tool("fetch", {"url": url})
except httpx.ConnectError:  # dead code: FastMCP now raises httpx2.ConnectError
    return fallback()
```

Grep your codebase for `except httpx.` and move those handlers to `httpx2`. The exception hierarchies match name-for-name, so the fix is an import swap — the hard part is remembering to look. One place you are covered automatically: exceptions raised *inside your tools and resources* (for example, a tool whose own old-httpx call gets a 429) are still mapped to `ToolError`/`ResourceError` by FastMCP's error boundary, which recognizes both libraries' exceptions during the transition.

Two runtime behaviors shift with httpx2, and because the switch is now wholesale they apply to **all** FastMCP HTTP — including server-auth upstream calls, not just the client path. TLS verification uses the operating system's trust store (via `truststore`, honoring `SSL_CERT_FILE`/`SSL_CERT_DIR`) instead of the bundled certifi CA set, so corporate-CA or certifi-pinned setups may verify differently. And the FastMCP HTTP loggers are renamed from `httpx`/`httpcore.*` to `httpx2`/`httpcore2.*` — update any logging filters that select the HTTP stack by logger name.

## Deprecation timeline

The camelCase bridge is a migration aid, not a permanent fixture. It works today and warns on every bridged read so you can find and update the affected call sites. Plan to migrate your reads to snake_case: the shims will be removed in a future release, after which only the snake_case names resolve — the same state you get today by setting `mcp_camelcase_compat = False`. Turning the setting off is a good way to surface every remaining camelCase read in your code as a hard `AttributeError` before the shims go away.

## SDK deprecation warnings you may see

Ordinary use of `ctx.info` (client logging) and `ctx.sample` now emits an SDK-level `MCPDeprecationWarning`:

```
The logging/sampling capability is deprecated as of 2026-07-28 (SEP-2577)
```

These warnings come from the MCP SDK, not from FastMCP. For logging they are benign: `ctx.info` keeps working on session-based connections exactly as the protocol table below describes, and the SDK is only signaling the protocol's direction. For sampling, FastMCP additionally emits its own `FastMCPDeprecationWarning`: `ctx.sample` and `ctx.sample_step` are deprecated and slated for removal, so treat that warning as a prompt to migrate to server-side LLM calls rather than as informational.

## Protocol version support

FastMCP servers built on the SDK v2 serve multiple protocol eras from the same server. The SDK negotiates the era each client speaks: the sessionless `2026-07-28` era (which discovers capabilities through `server/discover`) and earlier session-based handshake versions are all handled simultaneously. This formally supersedes FastMCP's earlier "latest protocol only" stance — a single server now works with clients across the protocol transition.

Not every Context feature is available on every era yet. The imperative push APIs that call back into the client mid-execution — `ctx.elicit`, `ctx.sample`, and `ctx.list_roots` — depend on the session-based back-channel of the earlier eras, so on a `2026-07-28` connection they raise a clear, era-aware error rather than reaching the client. Elicitation itself still reaches the user on the modern era, through the guard pattern: a tool *returns* an `InputRequiredResult` describing what it needs, and the client answers with a fresh call (see [Elicitation on the modern protocol](/servers/elicitation#elicitation-on-the-modern-protocol)). Logging notifications and the request/response features flow on every era.

Sampling is the exception that does not come back, and the reason is the protocol rather than an unfinished FastMCP feature. SEP-2577 deprecated server-initiated sampling, so `ctx.sample` and `ctx.sample_step` are **deprecated** and will be removed in a future FastMCP release. Elicitation moved to the guard pattern because the modern protocol still carries elicitation requests; sampling has no equivalent path because the protocol deprecated the pattern itself. The migration is to call an LLM directly from your server rather than borrowing the client's model. See [Sampling](/servers/sampling) for details.

| Context feature | Earlier eras (session-based) | `2026-07-28` (sessionless) |
| --- | --- | --- |
| `ctx.info` / logging notifications | Supported | Supported |
| Tools, resources, prompts, completions | Supported | Supported |
| `ctx.elicit` | Supported | Use the guard pattern (return `InputRequiredResult`) |
| `ctx.sample` / `ctx.sample_step` | Supported (deprecated) | Removed — call an LLM server-side |
| `ctx.list_roots` | Supported | Via the guard pattern (`input_requests` carries roots requests) |
| `Middleware.on_initialize` | Runs on connect | Never runs — there is no `initialize` handshake |
| Session state (`ctx.set_state` across calls) | Persists for the session | Does not persist — every request is a fresh connection |
| Tasks (via the FastMCP client) | Supported | Not yet |

If your tools rely on `ctx.elicit` or `ctx.list_roots`, they continue to work against clients on the earlier eras; on the modern era, reach for the guard pattern instead (see [Elicitation on the modern protocol](/servers/elicitation#elicitation-on-the-modern-protocol)). Sampling is deprecated on every era and will not return on modern connections — migrate those tools to server-side LLM calls.

Two of these bite by default now, because **`fastmcp.Client` defaults to `mode="auto"`** in v4 — an ordinary `Client(server)` negotiates the newest protocol both sides share, which against a FastMCP server is the sessionless `2026-07-28` era. On that era there is no `initialize` handshake, so a `Middleware.on_initialize` hook never runs; and each request is a fresh connection, so state written with `ctx.set_state` in one call is not visible in the next. A server that gates access in `on_initialize` or relies on per-session state must keep its clients on the session-based era. The narrow escape is per-client: `Client(server, mode="legacy")`. The durable, server-side answer is to declare the versions the server actually serves so a modern client is refused at connect time rather than silently losing those features — see the server's protocol-version restriction (added alongside this change).