Preserve kw-only defaults when rebuilding functions for resolved annotations

[jlowin]

Motivation

• When get_cached_typeadapter resolved forward/Annotated hints it rebuilt the callable with types.FunctionType but only copied positional defaults, causing __kwdefaults__ to be lost and keyword-only parameters with defaults to become required in generated schemas and validation.
• This changed runtime behavior for functions using future annotations and broke schema/validation consumers by making optional kw-only args mandatory.

Description

• Capture __kwdefaults__ from the original callable (for both functions and bound methods) during the cloning path in get_cached_typeadapter and assign it to the rebuilt function via new_func.__kwdefaults__ in src/fastmcp/utilities/types.py.
• Preserve existing behavior for processed annotations while ensuring kw-only defaults remain intact so Pydantic TypeAdapter produces the same schema/validation semantics as the original function.
• Add a focused regression test test_kwonly_defaults_preserved_when_annotations_are_processed to tests/utilities/test_types.py that verifies schema default and validation for a kw-only Annotated parameter.

Testing

• Ran uv sync which completed successfully in this environment.
• Ran the full test suite with uv run pytest -n auto, which exercises repository tests but reported unrelated timeouts/failures in this environment (13 failed, 1 error) that are not caused by this focused fix.
• Ran the focused test uv run pytest tests/utilities/test_types.py -k kwonly_defaults_preserved_when_annotations_are_processed which passed and confirmed the regression is fixed.
• Ran uv run pytest tests/utilities/test_typeadapter.py and uv run ruff check src/fastmcp/utilities/types.py tests/utilities/test_types.py which both passed; uv run prek run --all-files failed due to an external pre-commit hook network fetch error unrelated to the code change.

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Codex Task

[marvin-context-protocol]

Test Failure Analysis

(Edited to reflect latest workflow run analysis — previous comment addressed the formatting failure.)

Summary: 2 tests in tests/server/auth/test_oauth_consent_page.py timed out on Python 3.10 only. These tests are unrelated to this PR's changes and represent a pre-existing flakiness issue.

Root Cause: TestConsentBindingCookie::test_parallel_flows_do_not_interfere and TestConsentBindingCookie::test_consent_disabled_skips_binding_check both make a real outbound HTTP request to https://github.com/login/oauth/access_token via AsyncOAuth2Client.fetch_token() (with a 30-second timeout). In the CI environment on Python 3.10, this network request stalls long enough to exceed pytest's 5-second test timeout, causing the tests to deadlock. On Python 3.13 the same tests pass — likely due to a faster connection failure or different networking behavior on that runner.

This PR only changes src/fastmcp/utilities/types.py and tests/utilities/test_types.py; neither of the failing tests touches those files.

Suggested Solution: The failing tests should mock the token exchange call so they do not depend on network connectivity. In each test, patch AsyncOAuth2Client.fetch_token to raise immediately:

from unittest.mock import AsyncMock, patch

with patch(
    "fastmcp.server.auth.oauth_proxy.proxy.AsyncOAuth2Client.fetch_token",
    new_callable=AsyncMock,
    side_effect=Exception("mocked: token exchange unavailable"),
):
    r = c.get(f"/auth/callback?code=fake&state={txn_id}", follow_redirects=False)
assert r.status_code != 403  # still passes — failure is 500, not 403

This should be fixed in a separate PR that targets tests/server/auth/test_oauth_consent_page.py. The current PR can be merged — its changes are correct and unrelated to the failure.

Detailed Analysis

Failing tests (Python 3.10 only):

FAILED tests/server/auth/test_oauth_consent_page.py::TestConsentBindingCookie::test_parallel_flows_do_not_interfere - Failed: Timeout (>5.0s) from pytest-timeout.
FAILED tests/server/auth/test_oauth_consent_page.py::TestConsentBindingCookie::test_consent_disabled_skips_binding_check - Failed: Timeout (>5.0s) from pytest-timeout.

Deadlock stack trace (test client waiting indefinitely for app response):

anyio/from_thread.py:334 → concurrent/futures/_base.py:453 → threading.py:320: waiter.acquire()
E  Failed: Timeout (>5.0s) from pytest-timeout.

Root path through the code:

Both tests reach _handle_idp_callback in proxy.py, which creates an AsyncOAuth2Client and calls:

# proxy.py:1712 — with HTTP_TIMEOUT_SECONDS = 30
oauth_client = AsyncOAuth2Client(timeout=HTTP_TIMEOUT_SECONDS, ...)
idp_tokens = await oauth_client.fetch_token(url="https://github.com/login/oauth/access_token", ...)

The callback comment in the test says "It will fail at token exchange (500) but not at consent verification" — expecting a fast network error. In the Python 3.10 runner the connection attempt to GitHub hangs instead, exceeding the 5-second pytest limit.

Why Python 3.13 passes: The CI annotation confirms GitHub is unreachable (Failed to connect to github.com port 443 after 134093 ms). Python 3.13 appears to surface the connection error fast enough; Python 3.10 does not.

Related Files

• tests/server/auth/test_oauth_consent_page.py — contains TestConsentBindingCookie class with the two failing tests (lines 513 and 638)
• src/fastmcp/server/auth/oauth_proxy/proxy.py:1712 — AsyncOAuth2Client.fetch_token() call with 30-second timeout
• src/fastmcp/server/auth/oauth_proxy/models.py:32 — HTTP_TIMEOUT_SECONDS = 30

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🤖 Generated with Claude Code

[jlowin]

Auto-reviewed: all CI checks green, review comments addressed. Merging.