PrefectHQ/fastmcp · error · RuntimeError

Failed to initialize server session

Error message

Failed to initialize server session

What it means

During the MCP initialize handshake the client waits for the server's response within a timeout. If negotiation times out, FastMCP raises this RuntimeError with the TimeoutError chained as cause. It means the server did not complete initialization in time.

Source

Thrown at fastmcp_slim/fastmcp/client/client.py:889

            with anyio.fail_after(timeout):
                if effective_mode == "legacy":
                    self._session_state.initialize_result = (
                        await self.session.initialize()
                    )
                elif effective_mode == "auto":
                    async with _conformant_discover_only(self.session):
                        await negotiate_auto(self.session)
                    # auto may have fallen back to the legacy handshake; surface its
                    # InitializeResult through the existing public property when so.
                    self._session_state.initialize_result = (
                        self.session.initialize_result
                    )
                else:
                    self.session.adopt(
                        self._prior_discover or _synthesize_discover(self.mode)
                    )
        except TimeoutError as e:
            raise RuntimeError("Failed to initialize server session") from e

    async def initialize(
        self,
        timeout: datetime.timedelta | float | int | None = None,
    ) -> mcp_types.InitializeResult:
        """Send an initialize request to the server.

        This method performs the MCP initialization handshake with the server,
        exchanging capabilities and server information. It is idempotent - calling
        it multiple times returns the cached result from the first call.

        The initialization happens automatically when entering the client context
        manager unless `auto_initialize=False` was set during client construction.
        Manual calls to this method are only needed when auto-initialization is disabled.

        With `mode="auto"` or a pinned modern version, connect-time negotiation may adopt
        the modern `server/discover` era, which has no `InitializeResult`; in that case
        this method raises. Read `protocol_version`, `server_info`,

View on GitHub (pinned to 1f02114297)

Solutions

1. Increase the timeout: `await client.initialize(timeout=30)`
2. Verify the server actually starts and responds (run it manually, check output)
3. Check network path and auth for remote transports
4. Retry once with a longer timeout before giving up

Example fix

// before
async with client:
    result = await client.initialize()  # default timeout

// after
async with client:
    result = await client.initialize(timeout=30.0)

# Health-check the server before the handshake (HTTP example)
# probe the endpoint with a short request first; for stdio, launch and
# confirm the process is still alive before connecting

try:
    result = await client.initialize(timeout=10)
except RuntimeError as e:
    if "Failed to initialize" in str(e):
        result = await client.initialize(timeout=60)
    else:
        raise

When it happens

Trigger: `client._negotiate()` (via auto_initialize on connect or `await client.initialize()`) exceeds the timeout because the server is slow to start, hung, or unreachable.

Common situations: Cold-starting a stdio server (heavy imports, slow container) exceeding the default timeout; remote server behind a slow proxy; wrong URL/port causing a silent hang; server blocking during auth.

Related errors

• Tool {func_name!r}: timeout cannot be enforced when run_in_t
• Invalid timeout type: {type(value)}
• module {__name__!r} has no attribute {name!r}
• Cannot resolve tool reference: {fn!r}
• Cannot specify both a name as first argument and as keyword

AI-assisted analysis of PrefectHQ/fastmcp@1f02114297 (2026-08-29). Data as JSON: /api/errors/681385a018518bf6. Report an issue: GitHub.