Add modern-di-fastmcp integration #462

Ship modern-di-fastmcp as a separate-repo integration, following
docs/integrations/writing-integrations.md.

Feasibility

Prototyped against fastmcp==3.4.6 and fastmcp==4.0.3, running unchanged on both.

Injection is fastmcp.dependencies.Depends(...) as a parameter default.

Injected params never reach the LLM: FastMCP strips them from the tool signature before schema
generation, via without_injected_parameters in fastmcp/server/dependencies.py, applied to
tools, resources and resource templates. Verified with two injected params present, the generated
schema listed only the real one, and a client attempt to supply an injected param was rejected.

Also verified in the prototype: APP open/close, one shared REQUEST container per MCP call,
per-request finalizers firing, and container.override() intact.

Estimated ~90 LOC in main.py, mid-range for the family's 76-213.

Public surface

mcp = FastMCP("my-server")
container = setup_di(mcp, Container(groups=[Dependencies]))
container.validate()


@mcp.tool
def list_users(
    limit: int,
    service: UserService = FromDI(Dependencies.user_service),  # noqa: B008
) -> list[str]:
    return service.list(limit)

The client sees only limit.

Decisions needed before implementation

1. FromDI form: use the parameter-default spelling. Still wants an ADR in the integration's
   own repo; one was drafted and dropped unmerged, so the reasoning lives here for now. FromDI is used as
   service: UserService = FromDI(...) with a scoped # noqa: B008 exemption; the
   Annotated[T, FromDI(...)] form the family mandates is not offered, because FastMCP detects the
   marker only as a default. uncalled_for.introspection.get_dependency_parameters iterates
   signature.parameters and keeps those where isinstance(parameter.default, Dependency);
   annotation metadata is never consulted, and there is no annotation-reading counterpart.

   Verified on 3.4.6 and 4.0.3, one tool per form on the same server: the default spelling generates
   schema ['name'] and resolves, while the Annotated spelling generates ['name', 'svc'] and
   then fails the call with ToolError, missing required argument. It does not merely fail to
   inject, it advertises a parameter to the model and rejects the call the model makes.

2. Which root-lifecycle hook: settled, use add_provider. ServerExtension (4.x only) looked
   semantically right, but its identifier is a required reverse-DNS string, validated at
   registration and advertised under ServerCapabilities.extensions. Using one for a lifecycle
   callback would announce a protocol capability the integration does not implement, whereas
   Provider's thinness is invisible to clients. add_provider also works on both 3.x and 4.x,
   avoiding a fastmcp>=4 floor. lite-bootstrap reached the same conclusion independently and
   recorded it in ADR 0001-fastmcp-teardown-via-provider-lifespan.md; cross-reference it rather
   than re-deriving. Note that ADR's revisit trigger names "a documented public way to compose a
   lifespan post-construction", which FastMCP 4's add_extension arguably satisfies, so it reads as
   fired until someone amends it.

3. What the ContextProvider binds to. The prototype binds MiddlewareContext.
   fastmcp.Context would be more useful, but MiddlewareContext.fastmcp_context is
   Context | None, so it cannot be bound unconditionally.

Contract items needing a framework-specific answer

Two checklist items the generic spec cannot answer for
this host:

4. Root reopen on restart. The checklist requires the root container to reopen on startup so a
   restart does not depend on the implicit-reuse warning (ContainerClosedWarning) and gets
   finalizers wired to shutdown. Unaddressed: what happens when the same FastMCP server is started
   twice against one root container, given the lifecycle runs through Provider.lifespan() rather
   than a composed lifespan.

5. close_async vs close_sync. FastMCP's lifespan is async, so close_async is the
   expected match, but the spec asks for this to be stated rather than assumed.

Known constraints

• No SESSION scope. No session-close hook exists, so it cannot be offered with deterministic
  teardown. Scope table is APP + REQUEST.
• pyproject dependency spelling must account for fastmcp being a metapackage over
  fastmcp-slim in 4.x.
• Churn. Three majors in ~17 months, repo moved jlowin to PrefectHQ, DI engine swapped twice,
  sse_app()/streamable_http_app() removed in v4, and the engine underneath (uncalled-for) is at
  0.4.0. This would be the family's least stable host. Mitigated by depending only on
  fastmcp.dependencies.Depends, which FastMCP re-exports and documents.

Not verified

• Whether Middleware.on_message fires on every dispatch path in every transport. The prototype
  exercised the in-memory client only, though on_message is documented as covering all MCP traffic.
• Behaviour under FastMCP background tasks (fastmcp[tasks] / Docket), where a tool may run outside
  a request and a REQUEST-scoped container would not exist.

Context

• FastMCP ships its own DI (Depends, Shared, Current*), so this pays off mainly for someone who
  already has a modern-di graph, or who wants one container shared across a FastAPI app and an MCP
  server mounted beside it.
• Prior art: dishka-fastmcp, fastmcp-dishka, wireup, fastmcp-injector.
• The PyPI name modern-di-fastmcp is free.