fix: Fix logic that determines standard resource vs. resource template to account for context param

Author: matthew-gries

src/mcp/server/fastmcp/resources/base.py

@@ -1,7 +1,9 @@
 """Base classes and interfaces for FastMCP resources."""
 
+from __future__ import annotations
+
 import abc
-from typing import Annotated
+from typing import TYPE_CHECKING, Annotated
 
 from pydantic import (
     AnyUrl,
@@ -15,6 +17,11 @@
 
 from mcp.types import Annotations, Icon
 
+if TYPE_CHECKING:
+    from mcp.server.fastmcp.server import Context
+    from mcp.server.session import ServerSessionT
+    from mcp.shared.context import LifespanContextT, RequestT
+
 
 class Resource(BaseModel, abc.ABC):
     """Base class for all resources."""
@@ -44,6 +51,9 @@ def set_default_name(cls, name: str | None, info: ValidationInfo) -> str:
         raise ValueError("Either name or uri must be provided")
 
     @abc.abstractmethod
-    async def read(self) -> str | bytes:
+    async def read(
+        self,
+        context: Context[ServerSessionT, LifespanContextT, RequestT] | None = None,
+    ) -> str | bytes:
         """Read the resource content."""
         pass  # pragma: no cover

src/mcp/server/fastmcp/resources/templates.py

@@ -2,16 +2,15 @@
 
 from __future__ import annotations
 
-import inspect
 import re
 from collections.abc import Callable
 from typing import TYPE_CHECKING, Any
 
-from pydantic import BaseModel, Field, validate_call
+from pydantic import BaseModel, Field
 
 from mcp.server.fastmcp.resources.types import FunctionResource, Resource
-from mcp.server.fastmcp.utilities.context_injection import find_context_parameter, inject_context
-from mcp.server.fastmcp.utilities.func_metadata import func_metadata
+from mcp.server.fastmcp.utilities.context_injection import find_context_parameter
+from mcp.server.fastmcp.utilities.func_metadata import FuncMetadata, func_metadata, is_async_callable
 from mcp.types import Annotations, Icon
 
 if TYPE_CHECKING:
@@ -33,6 +32,10 @@ class ResourceTemplate(BaseModel):
     fn: Callable[..., Any] = Field(exclude=True)
     parameters: dict[str, Any] = Field(description="JSON schema for function parameters")
     context_kwarg: str | None = Field(None, description="Name of the kwarg that should receive context")
+    fn_metadata: FuncMetadata = Field(
+        description="Metadata about the function including a pydantic model for arguments"
+    )
+    is_async: bool = Field(description="Whether the function is async")
 
     @classmethod
     def from_function(
@@ -56,16 +59,15 @@ def from_function(
         if context_kwarg is None:  # pragma: no branch
             context_kwarg = find_context_parameter(fn)
 
+        is_async = is_async_callable(fn)
+
         # Get schema from func_metadata, excluding context parameter
         func_arg_metadata = func_metadata(
             fn,
             skip_names=[context_kwarg] if context_kwarg is not None else [],
         )
         parameters = func_arg_metadata.arg_model.model_json_schema()
 
-        # ensure the arguments are properly cast
-        fn = validate_call(fn)
-
         return cls(
             uri_template=uri_template,
             name=func_name,
@@ -77,6 +79,8 @@ def from_function(
             fn=fn,
             parameters=parameters,
             context_kwarg=context_kwarg,
+            fn_metadata=func_arg_metadata,
+            is_async=is_async,
         )
 
     def matches(self, uri: str) -> dict[str, Any] | None:
@@ -96,13 +100,12 @@ async def create_resource(
     ) -> Resource:
         """Create a resource from the template with the given parameters."""
         try:
-            # Add context to params if needed
-            params = inject_context(self.fn, params, context, self.context_kwarg)
-
-            # Call function and check if result is a coroutine
-            result = self.fn(**params)
-            if inspect.iscoroutine(result):
-                result = await result
+            result = await self.fn_metadata.call_fn_with_arg_validation(
+                self.fn,
+                self.is_async,
+                params,
+                {self.context_kwarg: context} if self.context_kwarg is not None else None,
+            )
 
             return FunctionResource(
                 uri=uri,  # type: ignore

src/mcp/server/fastmcp/resources/types.py

@@ -1,28 +1,39 @@
 """Concrete resource implementations."""
 
+from __future__ import annotations
+
 import inspect
 import json
 from collections.abc import Callable
 from pathlib import Path
-from typing import Any
+from typing import TYPE_CHECKING, Any
 
 import anyio
 import anyio.to_thread
 import httpx
 import pydantic
 import pydantic_core
-from pydantic import AnyUrl, Field, ValidationInfo, validate_call
+from pydantic import AnyUrl, Field, ValidationInfo
 
 from mcp.server.fastmcp.resources.base import Resource
+from mcp.server.fastmcp.utilities.context_injection import find_context_parameter
 from mcp.types import Annotations, Icon
 
+if TYPE_CHECKING:
+    from mcp.server.fastmcp.server import Context
+    from mcp.server.session import ServerSessionT
+    from mcp.shared.context import LifespanContextT, RequestT
+
 
 class TextResource(Resource):
     """A resource that reads from a string."""
 
     text: str = Field(description="Text content of the resource")
 
-    async def read(self) -> str:
+    async def read(
+        self,
+        context: Context[ServerSessionT, LifespanContextT, RequestT] | None = None,
+    ) -> str:
         """Read the text content."""
         return self.text  # pragma: no cover
 
@@ -32,7 +43,10 @@ class BinaryResource(Resource):
 
     data: bytes = Field(description="Binary content of the resource")
 
-    async def read(self) -> bytes:
+    async def read(
+        self,
+        context: Context[ServerSessionT, LifespanContextT, RequestT] | None = None,
+    ) -> bytes:
         """Read the binary content."""
         return self.data  # pragma: no cover
 
@@ -50,13 +64,22 @@ class FunctionResource(Resource):
     - other types will be converted to JSON
     """
 
-    fn: Callable[[], Any] = Field(exclude=True)
+    fn: Callable[..., Any] = Field(exclude=True)
+    context_kwarg: str | None = Field(default=None, description="Name of the kwarg that should receive context")
 
-    async def read(self) -> str | bytes:
+    async def read(
+        self,
+        context: Context[ServerSessionT, LifespanContextT, RequestT] | None = None,
+    ) -> str | bytes:
         """Read the resource by calling the wrapped function."""
         try:
-            # Call the function first to see if it returns a coroutine
-            result = self.fn()
+            # Inject context if needed
+            kwargs: dict[str, Any] = {}
+            if self.context_kwarg is not None and context is not None:
+                kwargs[self.context_kwarg] = context
+
+            # Call the function
+            result = self.fn(**kwargs)
             # If it's a coroutine, await it
             if inspect.iscoroutine(result):
                 result = await result
@@ -83,14 +106,14 @@ def from_function(
         mime_type: str | None = None,
         icons: list[Icon] | None = None,
         annotations: Annotations | None = None,
-    ) -> "FunctionResource":
+    ) -> FunctionResource:
         """Create a FunctionResource from a function."""
         func_name = name or fn.__name__
         if func_name == "<lambda>":  # pragma: no cover
             raise ValueError("You must provide a name for lambda functions")
 
-        # ensure the arguments are properly cast
-        fn = validate_call(fn)
+        # Find context parameter if it exists
+        context_kwarg = find_context_parameter(fn)
 
         return cls(
             uri=AnyUrl(uri),
@@ -99,6 +122,7 @@ def from_function(
             description=description or fn.__doc__ or "",
             mime_type=mime_type or "text/plain",
             fn=fn,
+            context_kwarg=context_kwarg,
             icons=icons,
             annotations=annotations,
         )
@@ -137,7 +161,10 @@ def set_binary_from_mime_type(cls, is_binary: bool, info: ValidationInfo) -> boo
         mime_type = info.data.get("mime_type", "text/plain")
         return not mime_type.startswith("text/")
 
-    async def read(self) -> str | bytes:
+    async def read(
+        self,
+        context: Context[ServerSessionT, LifespanContextT, RequestT] | None = None,
+    ) -> str | bytes:
         """Read the file content."""
         try:
             if self.is_binary:
@@ -153,7 +180,10 @@ class HttpResource(Resource):
     url: str = Field(description="URL to fetch content from")
     mime_type: str = Field(default="application/json", description="MIME type of the resource content")
 
-    async def read(self) -> str | bytes:
+    async def read(
+        self,
+        context: Context[ServerSessionT, LifespanContextT, RequestT] | None = None,
+    ) -> str | bytes:
         """Read the HTTP content."""
         async with httpx.AsyncClient() as client:  # pragma: no cover
             response = await client.get(self.url)
@@ -191,7 +221,10 @@ def list_files(self) -> list[Path]:  # pragma: no cover
         except Exception as e:
             raise ValueError(f"Error listing directory {self.path}: {e}")
 
-    async def read(self) -> str:  # Always returns JSON string  # pragma: no cover
+    async def read(
+        self,
+        context: Context[ServerSessionT, LifespanContextT, RequestT] | None = None,
+    ) -> str:  # Always returns JSON string  # pragma: no cover
         """Read the directory listing."""
         try:
             files = await anyio.to_thread.run_sync(self.list_files)

src/mcp/server/fastmcp/server.py

@@ -372,7 +372,7 @@ async def read_resource(self, uri: AnyUrl | str) -> Iterable[ReadResourceContent
             raise ResourceError(f"Unknown resource: {uri}")
 
         try:
-            content = await resource.read()
+            content = await resource.read(context=context)
             return [ReadResourceContents(content=content, mime_type=resource.mime_type)]
         except Exception as e:  # pragma: no cover
             logger.exception(f"Error reading resource {uri}")
@@ -571,21 +571,18 @@ async def get_weather(city: str) -> str:
             )
 
         def decorator(fn: AnyFunction) -> AnyFunction:
-            # Check if this should be a template
+            # Extract signature and parameters
             sig = inspect.signature(fn)
-            has_uri_params = "{" in uri and "}" in uri
-            has_func_params = bool(sig.parameters)
+            uri_params = set(re.findall(r"{(\w+)}", uri))
+            context_param = find_context_parameter(fn)
+            func_params = {p for p in sig.parameters.keys() if p != context_param}
 
-            if has_uri_params or has_func_params:
-                # Check for Context parameter to exclude from validation
-                context_param = find_context_parameter(fn)
-
-                # Validate that URI params match function params (excluding context)
-                uri_params = set(re.findall(r"{(\w+)}", uri))
-                # We need to remove the context_param from the resource function if
-                # there is any.
-                func_params = {p for p in sig.parameters.keys() if p != context_param}
+            # Determine if this should be a template
+            has_uri_params = len(uri_params) != 0
+            has_func_params = len(func_params) != 0
 
+            if has_uri_params or has_func_params:
+                # Validate that URI params match function params
                 if uri_params != func_params:
                     raise ValueError(
                         f"Mismatch between URI parameters {uri_params} and function parameters {func_params}"

src/mcp/server/fastmcp/tools/base.py

@@ -1,7 +1,5 @@
 from __future__ import annotations as _annotations
 
-import functools
-import inspect
 from collections.abc import Callable
 from functools import cached_property
 from typing import TYPE_CHECKING, Any
@@ -10,7 +8,7 @@
 
 from mcp.server.fastmcp.exceptions import ToolError
 from mcp.server.fastmcp.utilities.context_injection import find_context_parameter
-from mcp.server.fastmcp.utilities.func_metadata import FuncMetadata, func_metadata
+from mcp.server.fastmcp.utilities.func_metadata import FuncMetadata, func_metadata, is_async_callable
 from mcp.types import Icon, ToolAnnotations
 
 if TYPE_CHECKING:
@@ -60,7 +58,7 @@ def from_function(
             raise ValueError("You must provide a name for lambda functions")
 
         func_doc = description or fn.__doc__ or ""
-        is_async = _is_async_callable(fn)
+        is_async = is_async_callable(fn)
 
         if context_kwarg is None:  # pragma: no branch
             context_kwarg = find_context_parameter(fn)
@@ -107,12 +105,3 @@ async def run(
             return result
         except Exception as e:
             raise ToolError(f"Error executing tool {self.name}: {e}") from e
-
-
-def _is_async_callable(obj: Any) -> bool:
-    while isinstance(obj, functools.partial):  # pragma: no cover
-        obj = obj.func
-
-    return inspect.iscoroutinefunction(obj) or (
-        callable(obj) and inspect.iscoroutinefunction(getattr(obj, "__call__", None))
-    )

src/mcp/server/fastmcp/utilities/func_metadata.py

@@ -1,3 +1,4 @@
+import functools
 import inspect
 import json
 from collections.abc import Awaitable, Callable, Sequence
@@ -531,3 +532,13 @@ def _convert_to_content(
         result = pydantic_core.to_json(result, fallback=str, indent=2).decode()
 
     return [TextContent(type="text", text=result)]
+
+
+def is_async_callable(obj: Any) -> bool:
+    """Check if an object is an async callable."""
+    while isinstance(obj, functools.partial):  # pragma: no cover
+        obj = obj.func
+
+    return inspect.iscoroutinefunction(obj) or (
+        callable(obj) and inspect.iscoroutinefunction(getattr(obj, "__call__", None))
+    )