[v1.x] Complete the FastMCP Settings model at import time

Settings.lifespan is annotated with FastMCP, which is defined further down
the module, so pydantic left the Settings model incomplete until the first
FastMCP() call rebuilt it. pydantic-settings 2.15 now emits
IncompleteFieldDefinitionWarning whenever a settings source inspects such a
field, which means a warning on every FastMCP() construction (and, with
warnings treated as errors, a failing test suite on the "highest"
dependency resolution).

Call Settings.model_rebuild() once FastMCP exists so the model is complete
before anything instantiates it.

Author: maxisbey

src/mcp/server/fastmcp/server.py

@@ -1087,6 +1087,11 @@ async def get_prompt(self, name: str, arguments: dict[str, Any] | None = None) -
             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 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 @@
     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):