feat(func_metadata): expose skip_names argument

And move the `if param.name in skip_names:` check to the top

It allows users to ignore specific parameters, such as optional ones
and especially those starting with an underscore.

Author: NextFire

src/mcp/server/fastmcp/server.py

@@ -246,6 +246,7 @@ def add_tool(
         fn: AnyFunction,
         name: str | None = None,
         description: str | None = None,
+        skip_names: Sequence[str] = (),
     ) -> None:
         """Add a tool to the server.
 
@@ -256,11 +257,21 @@ def add_tool(
             fn: The function to register as a tool
             name: Optional name for the tool (defaults to function name)
             description: Optional description of what the tool does
+            skip_names: A list of parameter names to skip. These will not be included in
+                the model.
         """
-        self._tool_manager.add_tool(fn, name=name, description=description)
+        self._tool_manager.add_tool(
+            fn,
+            name=name,
+            description=description,
+            skip_names=skip_names,
+        )
 
     def tool(
-        self, name: str | None = None, description: str | None = None
+        self,
+        name: str | None = None,
+        description: str | None = None,
+        skip_names: Sequence[str] = (),
     ) -> Callable[[AnyFunction], AnyFunction]:
         """Decorator to register a tool.
 
@@ -271,6 +282,8 @@ def tool(
         Args:
             name: Optional name for the tool (defaults to function name)
             description: Optional description of what the tool does
+            skip_names: A list of parameter names to skip. These will not be included in
+                the model.
 
         Example:
             @server.tool()
@@ -295,7 +308,7 @@ async def async_tool(x: int, context: Context) -> str:
             )
 
         def decorator(fn: AnyFunction) -> AnyFunction:
-            self.add_tool(fn, name=name, description=description)
+            self.add_tool(fn, name=name, description=description, skip_names=skip_names)
             return fn
 
         return decorator

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

@@ -1,7 +1,7 @@
 from __future__ import annotations as _annotations
 
 import inspect
-from collections.abc import Callable
+from collections.abc import Callable, Sequence
 from typing import TYPE_CHECKING, Any
 
 from pydantic import BaseModel, Field
@@ -38,6 +38,7 @@ def from_function(
         name: str | None = None,
         description: str | None = None,
         context_kwarg: str | None = None,
+        skip_names: Sequence[str] = (),
     ) -> Tool:
         """Create a Tool from a function."""
         from mcp.server.fastmcp import Context
@@ -57,10 +58,10 @@ def from_function(
                     context_kwarg = param_name
                     break
 
-        func_arg_metadata = func_metadata(
-            fn,
-            skip_names=[context_kwarg] if context_kwarg is not None else [],
-        )
+        if context_kwarg is not None:
+            skip_names = (context_kwarg, *skip_names)
+
+        func_arg_metadata = func_metadata(fn, skip_names=skip_names)
         parameters = func_arg_metadata.arg_model.model_json_schema()
 
         return cls(

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

@@ -1,6 +1,6 @@
 from __future__ import annotations as _annotations
 
-from collections.abc import Callable
+from collections.abc import Callable, Sequence
 from typing import TYPE_CHECKING, Any
 
 from mcp.server.fastmcp.exceptions import ToolError
@@ -35,9 +35,15 @@ def add_tool(
         fn: Callable[..., Any],
         name: str | None = None,
         description: str | None = None,
+        skip_names: Sequence[str] = (),
     ) -> Tool:
         """Add a tool to the server."""
-        tool = Tool.from_function(fn, name=name, description=description)
+        tool = Tool.from_function(
+            fn,
+            name=name,
+            description=description,
+            skip_names=skip_names,
+        )
         existing = self._tools.get(tool.name)
         if existing:
             if self.warn_on_duplicate_tools:

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

@@ -130,12 +130,12 @@ def func_metadata(
     dynamic_pydantic_model_params: dict[str, Any] = {}
     globalns = getattr(func, "__globals__", {})
     for param in params.values():
+        if param.name in skip_names:
+            continue
         if param.name.startswith("_"):
             raise InvalidSignature(
                 f"Parameter {param.name} of {func.__name__} cannot start with '_'"
             )
-        if param.name in skip_names:
-            continue
         annotation = param.annotation
 
         # `x: None` / `x: None = None`

tests/server/fastmcp/test_func_metadata.py

@@ -180,18 +180,26 @@ def test_skip_names():
     """Test that skipped parameters are not included in the model"""
 
     def func_with_many_params(
-        keep_this: int, skip_this: str, also_keep: float, also_skip: bool
+        keep_this: int,
+        skip_this: str,
+        also_keep: float,
+        also_skip: bool,
+        _skip_this_too: int = 0,
     ):
-        return keep_this, skip_this, also_keep, also_skip
+        return keep_this, skip_this, also_keep, also_skip, _skip_this_too
 
     # Skip some parameters
-    meta = func_metadata(func_with_many_params, skip_names=["skip_this", "also_skip"])
+    meta = func_metadata(
+        func_with_many_params,
+        skip_names=["skip_this", "also_skip", "_skip_this_too"],
+    )
 
     # Check model fields
     assert "keep_this" in meta.arg_model.model_fields
     assert "also_keep" in meta.arg_model.model_fields
     assert "skip_this" not in meta.arg_model.model_fields
     assert "also_skip" not in meta.arg_model.model_fields
+    assert "_skip_this_too" not in meta.arg_model.model_fields
 
     # Validate that we can call with only non-skipped parameters
     model: BaseModel = meta.arg_model.model_validate({"keep_this": 1, "also_keep": 2.5})  # type: ignore