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[v1.x] Rebuild FastMCP Settings once FastMCP is defined #3295

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claude wants to merge 1 commit into from
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[v1.x] Rebuild FastMCP Settings once FastMCP is defined #3295

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7 changes: 7 additions & 0 deletions src/mcp/server/fastmcp/server.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1087,6 +1087,13 @@ async def get_prompt(self, name: str, arguments: dict[str, Any] | None = None) -
raise ValueError(str(e))


# `Settings.lifespan` forward-references `FastMCP`, which is defined after `Settings`,
# so the annotation is unresolved when the class body executes. Rebuild the model now
# that `FastMCP` exists, otherwise settings sources may fail to resolve the field and
# pydantic-settings >= 2.15 warns about the incomplete definition on instantiation.
Settings.model_rebuild()


class StreamableHTTPASGIApp:
"""
ASGI application for Streamable HTTP server transport.
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15 changes: 15 additions & 0 deletions tests/server/fastmcp/test_server.py
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Expand Up @@ -183,6 +183,21 @@ async def test_add_resource_decorator_incorrect_usage(self):
def get_data(x: str) -> str: # pragma: no cover
return f"Data: {x}"

def test_settings_model_is_fully_defined_after_import(self):
"""Regression test for #3294: `Settings.lifespan` forward-references `FastMCP`,
which is defined later in the module, so `Settings` must be rebuilt once
`FastMCP` exists. An unresolved forward reference leaves the model incomplete:
settings sources may fail to resolve the field, and pydantic-settings >= 2.15
warns (`IncompleteFieldDefinitionWarning`) on every `FastMCP()` construction.
"""
from typing import ForwardRef

from mcp.server.fastmcp.server import Settings

assert Settings.__pydantic_complete__ is True
lifespan = Settings.model_fields["lifespan"]
assert not isinstance(lifespan.annotation, ForwardRef)


class TestDnsRebindingProtection:
"""Tests for automatic DNS rebinding protection on localhost."""
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