Support from __future__ import annotations (#1199)

src/fastmcp/tools/tool.py

@@ -3,7 +3,15 @@ from __future__ import annotations
 import inspect
 from collections.abc import Callable
 from dataclasses import dataclass
-from typing import TYPE_CHECKING, Annotated, Any, Generic, Literal, TypeVar
+from typing import (
+    TYPE_CHECKING,
+    Annotated,
+    Any,
+    Generic,
+    Literal,
+    TypeVar,
+    get_type_hints,
+)
 
 import mcp.types
 import pydantic_core
@@ -371,7 +379,20 @@ class ParsedFunction:
         input_schema = compress_schema(input_schema, prune_params=prune_params)
 
         output_schema = None
-        output_type = inspect.signature(fn).return_annotation
+        # Get the return annotation from the signature
+        sig = inspect.signature(fn)
+        output_type = sig.return_annotation
+
+        # If the annotation is a string (from __future__ annotations), resolve it
+        if isinstance(output_type, str):
+            try:
+                # Use get_type_hints to resolve the return type
+                # include_extras=True preserves Annotated metadata
+                type_hints = get_type_hints(fn, include_extras=True)
+                output_type = type_hints.get("return", output_type)
+            except Exception:
+                # If resolution fails, keep the string annotation
+                pass
 
         if output_type not in (inspect._empty, None, Any, ...):
             # there are a variety of types that we don't want to attempt to

src/fastmcp/utilities/types.py

@@ -8,7 +8,15 @@ from collections.abc import Callable
 from functools import lru_cache
 from pathlib import Path
 from types import EllipsisType, UnionType
-from typing import Annotated, TypeAlias, TypeVar, Union, get_args, get_origin
+from typing import (
+    Annotated,
+    TypeAlias,
+    TypeVar,
+    Union,
+    get_args,
+    get_origin,
+    get_type_hints,
+)
 
 import mcp.types
 from mcp.types import Annotations
@@ -35,6 +43,54 @@ def get_cached_typeadapter(cls: T) -> TypeAdapter[T]:
     However, this isn't feasible for user-generated functions. Instead, we use a
     cache to minimize the cost of creating them as much as possible.
     """
+    # For functions, we need to ensure TypeAdapter can resolve forward
+    # references
+    # Normally this could be done by setting e.g. parent_depth=3 to reflect the
+    # globals in the parent stack, but this utility function can't make that assumption.
+    if inspect.isfunction(cls) or inspect.ismethod(cls):
+        # Only try to resolve annotations if the function has them
+        if hasattr(cls, "__annotations__") and cls.__annotations__:
+            try:
+                # Use include_extras=True to preserve Annotated metadata
+                resolved_hints = get_type_hints(cls, include_extras=True)
+                # Check if we need to create a new function with resolved annotations
+                if resolved_hints != cls.__annotations__:
+                    # Create a new function object with resolved annotations
+                    import types
+
+                    # Handle both functions and methods
+                    if inspect.ismethod(cls):
+                        actual_func = cls.__func__
+                        code = actual_func.__code__
+                        globals_dict = actual_func.__globals__
+                        name = actual_func.__name__
+                        defaults = actual_func.__defaults__
+                        closure = actual_func.__closure__
+                    else:
+                        code = cls.__code__
+                        globals_dict = cls.__globals__
+                        name = cls.__name__
+                        defaults = cls.__defaults__
+                        closure = cls.__closure__
+
+                    new_func = types.FunctionType(
+                        code,
+                        globals_dict,
+                        name,
+                        defaults,
+                        closure,
+                    )
+                    new_func.__dict__.update(cls.__dict__)
+                    new_func.__module__ = cls.__module__
+                    new_func.__qualname__ = getattr(cls, "__qualname__", cls.__name__)
+                    new_func.__annotations__ = resolved_hints
+                    return TypeAdapter(new_func)
+            except Exception:
+                # If resolution fails, this might be due to closure-scoped types
+                # that aren't available in the function's globals. In this case,
+                # we'll let TypeAdapter handle the string annotations directly.
+                pass
+
     return TypeAdapter(cls)
 
 
@@ -77,12 +133,21 @@ def find_kwarg_by_type(fn: Callable, kwarg_type: type) -> str | None:
     Includes union types that contain the kwarg_type, as well as Annotated types.
     """
     if inspect.ismethod(fn) and hasattr(fn, "__func__"):
-        sig = inspect.signature(fn.__func__)
-    else:
-        sig = inspect.signature(fn)
+        fn = fn.__func__
 
+    # Try to get resolved type hints
+    try:
+        # Use include_extras=True to preserve Annotated metadata
+        type_hints = get_type_hints(fn, include_extras=True)
+    except Exception:
+        # If resolution fails, use raw annotations if they exist
+        type_hints = getattr(fn, "__annotations__", {})
+
+    sig = inspect.signature(fn)
     for name, param in sig.parameters.items():
-        if is_class_member_of_type(param.annotation, kwarg_type):
+        # Use resolved hint if available, otherwise raw annotation
+        annotation = type_hints.get(name, param.annotation)
+        if is_class_member_of_type(annotation, kwarg_type):
             return name
     return None
 

tests/server/test_server_interactions.py

@@ -31,6 +31,20 @@ from fastmcp.utilities.json_schema import compress_schema
 from fastmcp.utilities.types import Audio, File, Image
 
 
+def _normalize_anyof_order(schema):
+    """Normalize the order of items in anyOf arrays for consistent comparison."""
+    if isinstance(schema, dict):
+        if "anyOf" in schema:
+            # Sort anyOf items by their string representation for consistent ordering
+            schema = schema.copy()
+            schema["anyOf"] = sorted(schema["anyOf"], key=str)
+        # Recursively normalize nested objects
+        return {k: _normalize_anyof_order(v) for k, v in schema.items()}
+    elif isinstance(schema, list):
+        return [_normalize_anyof_order(item) for item in schema]
+    return schema
+
+
 class PersonTypedDict(TypedDict):
     name: str
     age: int
@@ -917,7 +931,12 @@ class TestToolOutputSchema:
 
             type_schema = compress_schema(TypeAdapter(annotation).json_schema())
             assert len(tools) == 1
-            assert tools[0].outputSchema == type_schema
+
+            # Normalize anyOf ordering for comparison since union type order
+            # can vary between environments when using annotation resolution
+            actual_schema = _normalize_anyof_order(tools[0].outputSchema)
+            expected_schema = _normalize_anyof_order(type_schema)
+            assert actual_schema == expected_schema
 
     async def test_disabled_output_schema_no_structured_content(self):
         mcp = FastMCP()

tests/tools/test_tool_future_annotations.py

@@ -0,0 +1,170 @@
+from __future__ import annotations
+
+from typing import Any, cast
+
+import mcp.types
+import pytest
+
+from fastmcp import Context, FastMCP
+from fastmcp.client import Client
+from fastmcp.tools.tool import ToolResult
+from fastmcp.utilities.types import Image
+
+fastmcp_server = FastMCP()
+
+
+@fastmcp_server.tool
+def simple_with_context(ctx: Context) -> str:
+    """Simple tool with context parameter."""
+    return f"Request ID: {ctx.request_id}"
+
+
+@fastmcp_server.tool
+def complex_types(
+    data: dict[str, Any], items: list[int], ctx: Context
+) -> dict[str, str | int]:
+    """Tool with complex type annotations."""
+    return {"count": len(items), "request_id": ctx.request_id}
+
+
+@fastmcp_server.tool
+def optional_context(name: str, ctx: Context | None = None) -> str:
+    """Tool with optional context."""
+    if ctx:
+        return f"Hello {name} from request {ctx.request_id}"
+    return f"Hello {name}"
+
+
+@fastmcp_server.tool
+def union_with_context(value: int | str, ctx: Context) -> ToolResult:
+    """Tool returning ToolResult with context."""
+    return ToolResult(content=f"Value: {value}, Request: {ctx.request_id}")
+
+
+@fastmcp_server.tool
+def returns_image(ctx: Context) -> Image:
+    """Tool that returns an Image."""
+    # Create a simple 1x1 white pixel PNG
+    png_data = b"\x89PNG\r\n\x1a\n\x00\x00\x00\rIHDR\x00\x00\x00\x01\x00\x00\x00\x01\x08\x02\x00\x00\x00\x90wS\xde\x00\x00\x00\x0cIDATx\x9cc\xf8\x0f\x00\x00\x01\x01\x00\x05\x18\xd4c\x00\x00\x00\x00IEND\xaeB`\x82"
+    return Image(data=png_data, format="png")
+
+
+@fastmcp_server.tool
+async def async_with_context(ctx: Context) -> str:
+    """Async tool with context."""
+    return f"Async request: {ctx.request_id}"
+
+
+class TestFutureAnnotations:
+    async def test_simple_with_context(self):
+        async with Client(fastmcp_server) as client:
+            result = await client.call_tool("simple_with_context", {})
+            assert "Request ID:" in cast(mcp.types.TextContent, result.content[0]).text
+
+    async def test_complex_types(self):
+        async with Client(fastmcp_server) as client:
+            result = await client.call_tool(
+                "complex_types", {"data": {"key": "value"}, "items": [1, 2, 3]}
+            )
+            # Check the result is valid JSON with expected values
+            import json
+
+            data = json.loads(cast(mcp.types.TextContent, result.content[0]).text)
+            assert data["count"] == 3
+            assert "request_id" in data
+
+    async def test_optional_context(self):
+        async with Client(fastmcp_server) as client:
+            result = await client.call_tool("optional_context", {"name": "World"})
+            assert (
+                "Hello World from request"
+                in cast(mcp.types.TextContent, result.content[0]).text
+            )
+
+    async def test_union_with_context(self):
+        async with Client(fastmcp_server) as client:
+            result = await client.call_tool("union_with_context", {"value": 42})
+            assert (
+                "Value: 42, Request:"
+                in cast(mcp.types.TextContent, result.content[0]).text
+            )
+
+    async def test_returns_image(self):
+        async with Client(fastmcp_server) as client:
+            result = await client.call_tool("returns_image", {})
+            assert result.content[0].type == "image"
+            assert result.content[0].mimeType == "image/png"
+
+    async def test_async_with_context(self):
+        async with Client(fastmcp_server) as client:
+            result = await client.call_tool("async_with_context", {})
+            assert (
+                "Async request:" in cast(mcp.types.TextContent, result.content[0]).text
+            )
+
+    async def test_modern_union_syntax_works(self):
+        """Test that modern | union syntax works with future annotations."""
+        # This demonstrates that our solution works with | syntax when types
+        # are available in module globals
+
+        # Define a tool with modern union syntax
+        @fastmcp_server.tool
+        def modern_union_tool(value: str | int | None) -> str | None:
+            """Tool using modern | union syntax throughout."""
+            if value is None:
+                return None
+            return f"processed: {value}"
+
+        async with Client(fastmcp_server) as client:
+            # Test with string
+            result = await client.call_tool("modern_union_tool", {"value": "hello"})
+            assert (
+                "processed: hello"
+                in cast(mcp.types.TextContent, result.content[0]).text
+            )
+
+            # Test with int
+            result = await client.call_tool("modern_union_tool", {"value": 42})
+            assert (
+                "processed: 42" in cast(mcp.types.TextContent, result.content[0]).text
+            )
+
+            # Test with None
+            result = await client.call_tool("modern_union_tool", {"value": None})
+            # When function returns None, FastMCP returns empty content
+            assert (
+                len(result.content) == 0
+                or cast(mcp.types.TextContent, result.content[0]).text == "null"
+            )
+
+
+@pytest.mark.xfail(
+    reason="Closure-scoped types cannot be resolved with 'from __future__ import annotations'. "
+    "When using future annotations, all type annotations become strings that need to be evaluated "
+    "using eval() in the function's global namespace. Types defined only in closure scope "
+    "(like local imports or type aliases) are not available in the function's __globals__ "
+    "and therefore cannot be resolved by get_type_hints()."
+)
+def test_closure_scoped_types_limitation():
+    """
+    This test demonstrates that closure-scoped types don't work with future annotations.
+
+    The fundamental issue is that 'from __future__ import annotations' converts all
+    annotations to strings, and those strings can only be resolved using the function's
+    global namespace, not local variables from closures.
+    """
+
+    def create_failing_closure():
+        # This import is only available in the closure scope
+
+        mcp = FastMCP()
+
+        @mcp.tool
+        def closure_tool(value: str | None) -> str:
+            """This will fail because Optional can't be resolved from closure import."""
+            return str(value)
+
+        return mcp
+
+    # This should raise an error during tool registration
+    create_failing_closure()