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Upgrade guide: staying on 3.x is an option for sampling servers

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Jeremiah Lowin 2026-07-27 09:37:15 -04:00
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docs/getting-started/upgrading/from-fastmcp-3.mdx
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@ -243,7 +243,7 @@ FastMCP servers built on the SDK v2 serve multiple protocol eras from the same s
All three *pushed*: the server sent a request down a live back-channel and blocked for the answer, and the sessionless protocol has no such channel. Since `fastmcp.Client` now negotiates the modern protocol by default, a method like that would fail against a default client. What the protocol removed is the pushing, not the asking — sampling, elicitation, and roots all still reach the client through the [guard pattern](/servers/elicitation#elicitation-on-the-modern-protocol), where a tool *returns* an `InputRequiredResult` describing what it needs, the client answers, and it calls again with the answer attached.
Migrating differs by capability. For **roots**, the guard pattern is the direct replacement: a server asks once and has what it needs, so the extra round buys the whole answer, and taking the paths as tool arguments is simpler still when the caller can just supply them. For **sampling**, the guard route works the same way, but generation usually belongs in your server, because every round is a full request-response cycle and a generation loop pays that cost repeatedly. [Call an LLM from your server](/servers/sampling) with your own API key and your tool behaves the same for every client, including the many that never implemented sampling; reach for the guard route when the point is specifically to use the caller's model.
Migrating differs by capability. For **roots**, the guard pattern is the direct replacement: a server asks once and has what it needs, so the extra round buys the whole answer, and taking the paths as tool arguments is simpler still when the caller can just supply them. For **sampling**, the guard route works the same way, but generation usually belongs in your server, because every round is a full request-response cycle and a generation loop pays that cost repeatedly. [Call an LLM from your server](/servers/sampling) with your own API key and your tool behaves the same for every client, including the many that never implemented sampling; reach for the guard route when the point is specifically to use the caller's model. If borrowing the caller's model *is* your server — you hold no key of your own, and the token bill was never yours to pay — staying on FastMCP 3.x is the honest answer until that changes.
| Context feature | Earlier eras (session-based) | `2026-07-28` (sessionless) |
| --- | --- | --- |
@ -268,7 +268,7 @@ Most servers upgrade untouched. Work down this list to find the ones that don't:
1. **Bump your environment.** Raise any pin below `pydantic>=2.12`; upgrade FastAPI if your resolver complains about Starlette `<1.0.1`.
2. **Fix imports that moved out.** Replace `from mcp.types import X` with `from mcp_types import X`, and update any import from the [removed modules](#moved-imports) (`fastmcp.server.proxy`, `fastmcp.server.openapi`, `fastmcp.server.apps`, the `fastmcp.tools.tool` / `resources.resource` / `prompts.prompt` component shims).
3. **Update removed server APIs.** Swap `as_proxy` → `create_proxy`, `import_server` → `mount`, `mount(prefix=)` → `mount(namespace=)`, and the [other removed methods and keywords](#removed-server-methods-and-mount-keywords).
4. **Replace `ctx.sample` and `ctx.list_roots`.** Both are gone from `Context`, as are `FastMCP(sampling_handler=...)` and `sampling_handler_behavior=`. Call an LLM directly from your server for generation; ask for roots through the guard pattern, or take file paths as tool arguments.
4. **Replace `ctx.sample` and `ctx.list_roots`.** Both are gone from `Context`, as are `FastMCP(sampling_handler=...)` and `sampling_handler_behavior=`. Call an LLM directly from your server for generation; ask for roots through the guard pattern, or take file paths as tool arguments. A server whose purpose is to use the caller's model should stay on FastMCP 3.x rather than migrate.
5. **Update removed tool parameters.** Replace tool `serializer=` (return a `ToolResult`), `exclude_args=` (use `Depends()`), and `StreamableHttpTransport(sse_read_timeout=)`.
6. **Fix `McpError` construction.** Positional `McpError(ErrorData(...))` becomes keyword `McpError(code=..., message=...)`. Catching is unchanged.
7. **Move httpx to httpx2.** Grep for `except httpx.` and for custom `httpx_client_factory` / `httpx.Auth` objects handed to FastMCP, and swap the import to `httpx2`.