feat(tool): validate and enforce default values

- Added validation to ensure all parameters listed in `exclude_args` exist in the function.
- Ensured parameters in `exclude_args` have default values to avoid runtime errors.
- Added test.

src/fastmcp/tools/tool.py

@@ -76,6 +76,18 @@ class Tool(BaseModel):
             if param.kind == inspect.Parameter.VAR_KEYWORD:
                 raise ValueError("Functions with **kwargs are not supported as tools")
 
+        if exclude_args:
+            for arg_name in exclude_args:
+                if arg_name not in sig.parameters:
+                    raise ValueError(
+                        f"Parameter '{arg_name}' in exclude_args does not exist in function."
+                    )
+                param = sig.parameters[arg_name]
+                if param.default == inspect.Parameter.empty:
+                    raise ValueError(
+                        f"Parameter '{arg_name}' in exclude_args must have a default value."
+                    )
+
         func_name = name or getattr(fn, "__name__", None) or fn.__class__.__name__
 
         if func_name == "<lambda>":

tests/server/test_tool_exclude_args.py

@@ -1,5 +1,6 @@
 from typing import Any
 
+import pytest
 from mcp.types import TextContent
 
 from fastmcp import Client, FastMCP
@@ -24,6 +25,21 @@ async def test_tool_exclude_args_in_tool_manager():
         assert args not in tools[0].parameters
 
 
+async def test_tool_exclude_args_without_default_value_raises_error():
+    """Test that excluding args without default values raises ValueError"""
+    mcp = FastMCP("Test Server")
+
+    with pytest.raises(ValueError):
+
+        @mcp.tool(exclude_args=["state"])
+        def echo(message: str, state: dict[str, Any] | None) -> str:
+            """Echo back the message provided."""
+            if state:
+                # State was read
+                pass
+            return message
+
+
 async def test_add_tool_method_exclude_args():
     """Test that tool exclude_args work with the add_tool method."""
     mcp = FastMCP("Test Server")