[v1.x] Complete the FastMCP Settings model at import time (#3352)

src/mcp/server/fastmcp/server.py

@@ -1087,6 +1087,11 @@ class FastMCP(Generic[LifespanResultT]):
             raise ValueError(str(e))
 
 
+# `Settings.lifespan` refers to FastMCP, which is only defined above; complete the model now so
+# settings sources never see an unresolved annotation when a FastMCP instance is created.
+Settings.model_rebuild()
+
+
 class StreamableHTTPASGIApp:
     """
     ASGI application for Streamable HTTP server transport.

tests/server/fastmcp/test_server.py

@@ -10,6 +10,7 @@ from starlette.routing import Mount, Route
 from mcp.server.fastmcp import Context, FastMCP
 from mcp.server.fastmcp.prompts.base import Message, UserMessage
 from mcp.server.fastmcp.resources import FileResource, FunctionResource
+from mcp.server.fastmcp.server import Settings
 from mcp.server.fastmcp.utilities.types import Audio, Image
 from mcp.server.session import ServerSession
 from mcp.server.transport_security import TransportSecuritySettings
@@ -31,6 +32,11 @@ if TYPE_CHECKING:
     from mcp.server.fastmcp import Context
 
 
+def test_settings_model_is_complete_at_import():
+    """The Settings model resolves its FastMCP annotation at import, so building one needs no deferred rebuild."""
+    assert Settings.__pydantic_complete__
+
+
 class TestServer:
     @pytest.mark.anyio
     async def test_create_server(self):