@app.tool() raises NameError for parameter/return types defined later in the module on Python 3.14+ (PEP 649/749 deferred annotations) #3668

Initial Checks

• I confirm that I'm using the newest release of my line (the latest 2.x, or the latest 1.x if I'm still on v1)
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Release line

2.x (current stable)

Description

Since Python 3.14 (PEP 649 / PEP 749), annotations are evaluated lazily, so this is valid without quotes or from __future__ import annotations:

@app.tool()
def search(q: Query) -> list[Hit]: ...

class Query(BaseModel): ...

MCPServer.tool() builds the Tool eagerly inside the decorator. The first thing it does is inspect.signature(fn) in find_resolved_parameters, which defaults to annotation_format=Format.VALUE. That forces fn.__annotate__ to run while Query doesn't exist yet, so registration crashes with a bare NameError:

File ".../mcp/server/mcpserver/tools/base.py", line 91, in from_function
    resolved_params = find_resolved_parameters(fn)
File ".../mcp/server/mcpserver/resolve.py", line 229, in find_resolved_parameters
    for name in inspect.signature(fn).parameters:
...
NameError: name 'Query' is not defined

Expected: either the tool registers (resolving annotations once the module has finished defining the types), or a clear InvalidSignature error that names the tool and the unresolved name.

Code path (v2.3.0; these files are byte-identical on main @ 91941ed):

(The same eager pattern is at resolve.py:376 and in the resource decorator at server.py:873.)

What I verified locally:

• I patched step 3 to use inspect.signature(fn, annotation_format=annotationlib.Format.FORWARDREF). Registration then fails at step 4 with InvalidSignature: Unable to evaluate type annotations for callable 'search' (cause: NameError: name 'Query' is not defined). So step 3 alone is not enough.
• With the decorator replaced by "collect fn, call app.add_tool(fn) after the classes are defined", the same function registers. list_tools() returns {'$ref': '#/$defs/Query2'} for the parameter, and call_tool validates and returns correctly. So deferring the build is enough.
• Quoting does not help: q: "Query" gives InvalidSignature on 3.14. So does from __future__ import annotations on 3.13.5. Only defining the types before the decorated function works today.
• The error type is inconsistent: unquoted annotations on 3.14+ raise a bare NameError (step 3), while quoted or __future__ annotations raise InvalidSignature (step 4).

Related but not duplicates:

• Tool context kwarg detection not compatible with string-valued/__future__ annotations #1129 (context kwarg detection with string annotations)
• wrapped tools using annotations fail #1391 (wrapped tools + __future__ annotations, wrong globals)
• find_context_parameter treats a Context return annotation as a parameter #3298 (find_context_parameter treats the return hint as a param)

Suggested fix (open to maintainers' preference):

1. Defer schema building for tools, and the same for resources/prompts. @app.tool() would record the function and options. The Tool (func_metadata, context/resolve detection) would be built on first list_tools/call_tool, or at server start, and cached. Any NameError then surfaces at that point as InvalidSignature naming the tool and the missing name. Registration-time validation that doesn't need types (name, lambda check, duplicates) can stay eager.
2. Independently, at resolve.py:229/:376 and server.py:873, only parameter names are used. On 3.14+ use inspect.signature(fn, annotation_format=annotationlib.Format.FORWARDREF); on older versions, inspect.signature(fn) plus fn.__code__-based names, or a try/except. Then a bare NameError can't escape from those sites.
3. Stop silently swallowing unresolvable hints in find_context_parameter and _type_hints. Once building is deferred, they'll resolve. If they still can't, raising InvalidSignature is safer than silently not injecting Context or dropping Resolve(...).
4. Add a test module run on 3.14 (already in the CI matrix) that defines tool param/return models below the decorated function.

Workaround: define all models used in tool signatures above the @app.tool() function, or call app.add_tool(fn) after the models are defined. Quoting the annotation does not work.

Example Code

# repro: tool parameter/return types defined *after* the tool (valid on Python 3.14+, PEP 649/749)
from pydantic import BaseModel
from mcp.server.mcpserver import MCPServer

app = MCPServer("repro")

@app.tool()
def search(q: Query) -> list[Hit]:
    """Search."""
    return [Hit(id=1)]

class Query(BaseModel):
    text: str

class Hit(BaseModel):
    id: int

if __name__ == "__main__":
    import asyncio
    print([t.name for t in asyncio.run(app.list_tools())])

Full traceback (fresh run, Python 3.14.8):

Traceback (most recent call last):
  File "repro_fwdref.py", line 7, in <module>
    @app.tool()
     ~~~~~~~~^^
  File ".../site-packages/mcp/server/mcpserver/server.py", line 725, in decorator
    self.add_tool(
  File ".../site-packages/mcp/server/mcpserver/server.py", line 647, in add_tool
    self._tool_manager.add_tool(
  File ".../site-packages/mcp/server/mcpserver/tools/tool_manager.py", line 51, in add_tool
    tool = Tool.from_function(
  File ".../site-packages/mcp/server/mcpserver/tools/base.py", line 91, in from_function
    resolved_params = find_resolved_parameters(fn)
  File ".../site-packages/mcp/server/mcpserver/resolve.py", line 229, in find_resolved_parameters
    for name in inspect.signature(fn).parameters:
                ~~~~~~~~~~~~~~~~~^^^^
  File ".../lib/python3.14/inspect.py", line 3334, in signature
    return Signature.from_callable(obj, follow_wrapped=follow_wrapped,
  File ".../lib/python3.14/inspect.py", line 3049, in from_callable
    return _signature_from_callable(obj, sigcls=cls,
  File ".../lib/python3.14/inspect.py", line 2519, in _signature_from_callable
    return _signature_from_function(sigcls, obj,
  File ".../lib/python3.14/inspect.py", line 2342, in _signature_from_function
    annotations = get_annotations(func, globals=globals, locals=locals, eval_str=eval_str,
  File ".../lib/python3.14/annotationlib.py", line 987, in get_annotations
    ann = _get_dunder_annotations(obj)
  File ".../lib/python3.14/annotationlib.py", line 1180, in _get_dunder_annotations
    ann = getattr(obj, "__annotations__", None)
  File "repro_fwdref.py", line 8, in __annotate__
    def search(q: Query) -> list[Hit]:
                  ^^^^^
NameError: name 'Query' is not defined

The traceback is identical on 3.14.8 free-threaded and 3.15.0rc3 (only the stdlib line numbers differ).

Python & MCP Python SDK

mcp 2.3.0 (also reproduced against main @ 91941ed: the relevant files are unchanged)
pydantic 2.14.0
Python 3.14.8 (main, Oct  3 2026) [Clang 22.1.3]               -> NameError
Python 3.14.8 free-threading build                             -> NameError
Python 3.15.0rc3 (main, Oct  3 2026) [Clang 22.1.3]            -> NameError
Python 3.13.5 + `from __future__ import annotations`           -> InvalidSignature
OS: Linux-6.12.94+-x86_64-with-glibc2.41 (uv-managed CPython builds)