[DNM] Support functools.partial as tools, prompts, and resources

[strawgate]

Supersedes #3269. Closes #3266.

Bug fix

functools.partial objects failed in two ways:

1. Registration: @mcp.tool(partial_fn) raised TypeError because inspect.isroutine() returns False for partials
2. Execution: mcp.add_tool(partial_fn) with update_wrapper caused Pydantic to follow __wrapped__ back to the original signature, ignoring bound arguments — tools accepted the wrong parameters at call time

Both issues affected prompts and resources too, not just tools.

Consolidation (no behavior changes)

The fix required touching callable-handling code scattered across 11 files. Rather than adding 13 more isinstance(fn, functools.partial) checks (as #3269 did), this PR centralizes the patterns into focused utilities:

Utility	Replaces	Callsites
is_callable_object()	inspect.isroutine() checks in decorator gates	7
get_callable_name()	getattr(fn, "__name__") or fn.__class__.__name__	4
prepare_callable()	Duplicated unwrap blocks (partial/callable class/staticmethod)	5
set_fastmcp_meta()	target = fn.__func__; target.__fastmcp__ = metadata	6
TaskConfig.normalize()	Identical 6-line if/elif/else normalization blocks	4

Each utility does one thing and is directly tested.

import functools
from fastmcp import FastMCP

mcp = FastMCP("demo")

def add(x: int, y: int) -> int:
    return x + y

add_ten = functools.partial(add, y=10)
functools.update_wrapper(add_ten, add)

mcp.tool(add_ten)        # was TypeError
mcp.add_tool(add_ten)    # was ValidationError at call time

🤖 Generated with Claude Code

[marvin-context-protocol]

(Edited to reflect the latest CI failure — supersedes prior analysis.)

tl;dr: ruff check flags one SIM105 violation in fastmcp_slim/fastmcp/utilities/callable_utils.py:80. Replace the inner try/except AttributeError: pass around setattr with contextlib.suppress(AttributeError) and push.

Root Cause: The new partial-unwrapping block copies wrapper attributes onto the rewritten functools.partial. The inner setattr is wrapped in try: ... except AttributeError: pass, which ruff's SIM105 rule disallows in favor of contextlib.suppress. Only one error — ruff format, ty, loq, and codespell all passed.

Fix: In fastmcp_slim/fastmcp/utilities/callable_utils.py:

import contextlib
# ...
for attr in functools.WRAPPER_ASSIGNMENTS:
    try:
        val = getattr(old, attr)
    except AttributeError:
        pass
    else:
        with contextlib.suppress(AttributeError):
            setattr(fn, attr, val)

(The outer try/except/else is fine because it has an else: branch — SIM105 only fires when the body is a bare try/except/pass.)

Log excerpt

SIM105 Use `contextlib.suppress(AttributeError)` instead of `try`-`except`-`pass`
  --> fastmcp_slim/fastmcp/utilities/callable_utils.py:80:17
   |
78 |                 pass
79 |             else:
80 | /               try:
81 | |                   setattr(fn, attr, val)
82 | |               except AttributeError:
83 | |                   pass
   | |__________________________^
84 |
85 |     # Callable classes (not routines, not partials) → unwrap to __call__

Found 1 error. No fixes available (1 hidden fix can be enabled with the --unsafe-fixes option).

Related files

• fastmcp_slim/fastmcp/utilities/callable_utils.py (lines 80–83) — the only file flagged.
• PR is still labeled DON'T MERGE / [DNM] in the title, so this fix only needs to land for CI hygiene on the branch.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve partial doc metadata when creating resources

Calling prepare_callable(fn) here reconstructs functools.partial objects that were wrapped with functools.update_wrapper, which drops copied attributes like __doc__; later in FunctionResource.from_function the default description is derived from inspect.getdoc(fn), so resources created from wrapped partials now get the generic functools.partial docstring instead of the original function doc. This is a user-visible regression for mcp.add_resource(partial_fn)/@resource(...) flows that rely on inferred descriptions.

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[chatgpt-codex-connector]

Prepare partials before validating resource-template params

Call-site validation for FunctionResourceTemplate.from_function runs on the original callable before this prepare_callable step, so functools.update_wrapper-wrapped partials are still validated against the unbound signature. In practice, a partial that binds a required parameter (for example partial(fn, fmt='json') when fmt has no default) is incorrectly rejected as missing URI params, even though the partial already supplies it. Running prepare_callable before the earlier signature/parameter checks avoids this false ValueError and makes wrapped partials behave consistently with the new partial support.

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[chatgpt-codex-connector]

Derive app tool names for partial callables

Allowing functools.partial through _dispatch_decorator now routes app.tool(partial_fn) into _register, but that path still requires getattr(fn, "__name__", None), which is missing for partials unless users called functools.update_wrapper or passed name=. In practice, app.tool(functools.partial(add, y=10)) now fails with ValueError("Cannot determine tool name..."), while other tool entry points (Tool.from_function/mcp.add_tool) can infer names for the same callable. This leaves partial support inconsistent and broken for the FastMCPApp.tool API.

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[chatgpt-codex-connector]

Preserve annotation metadata when cloning wrapped partials

Rebuilding a functools.partial here drops attributes copied by functools.update_wrapper (notably annotation/module context), so partials that rely on deferred annotations (e.g. from __future__ import annotations with custom types like 'User') can no longer be introspected by downstream schema generation and fail with NameError during tool/prompt/resource registration. This is a regression for wrapped partials that previously carried enough metadata for type resolution; the clone should retain the copied metadata while removing only __wrapped__.

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[chatgpt-codex-connector]

Prepare partials before classifying resources

For functools.update_wrapper-wrapped partials whose bound arguments leave no callable parameters, this path still hands the original partial to add_resource. add_resource decides static resource vs template with inspect.signature() before prepare_callable() strips __wrapped__, so a static URI like @mcp.resource("data://fixed") on partial(fn, fmt="json") is misclassified as a template and raises ValueError("URI template must contain at least one parameter") instead of registering the resource.

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