allow output schema to be None distinct from empty dict which makes not much sense

Author: davemssavage

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

@@ -23,7 +23,7 @@ class Tool(BaseModel):
     name: str = Field(description="Name of the tool")
     description: str = Field(description="Description of what the tool does")
     parameters: dict[str, Any] = Field(description="JSON schema for tool parameters")
-    output: dict[str, Any] = Field(description="JSON schema for tool output")
+    output: dict[str, Any] | None = Field(description="JSON schema for tool output", default=None, )
     fn_metadata: FuncMetadata = Field(
         description="Metadata about the function including a pydantic model for tool"
         " arguments"
@@ -99,7 +99,7 @@ async def run(
                 if self.context_kwarg is not None
                 else None,
             )
-            if self.output.get("type") == "object":
+            if self.output and self.output.get("type") == "object":
                 return DataContent(
                     type="data",
                     data=result,

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

@@ -46,7 +46,7 @@ def model_dump_one_level(self) -> dict[str, Any]:
 
 class FuncMetadata(BaseModel):
     arg_model: Annotated[type[ArgModelBase], WithJsonSchema(None)]
-    output_schema: dict[str, Any]
+    output_schema: dict[str, Any] | None
     # We can add things in the future like
     #  - Maybe some args are excluded from attempting to parse from JSON
     #  - Maybe some args are special (like context) for dependency injection
@@ -137,7 +137,7 @@ def func_metadata(
     sig = _get_typed_signature(func)
     params = sig.parameters
     dynamic_pydantic_model_params: dict[str, Any] = {}
-    output_schema: dict[str, Any] = {}
+    output_schema: dict[str, Any] | None = None
     globalns = getattr(func, "__globals__", {})
     for param in params.values():
         if param.name.startswith("_"):