Allow CallToolResult to be returned directly to support _meta field for OpenAI Apps

examples/fastmcp/direct_call_tool_result_return.py

@@ -0,0 +1,24 @@
+"""
+FastMCP Echo Server with direct CallToolResult return
+"""
+
+from typing import Annotated
+
+from pydantic import BaseModel
+
+from mcp.server.fastmcp import FastMCP
+from mcp.types import CallToolResult, TextContent
+
+mcp = FastMCP("Echo Server")
+
+
+class EchoResponse(BaseModel):
+    text: str
+
+
+@mcp.tool()
+def echo(text: str) -> Annotated[CallToolResult, EchoResponse]:
+    """Echo the input text with structure and metadata"""
+    return CallToolResult(
+        content=[TextContent(type="text", text=text)], structuredContent={"text": text}, _meta={"some": "metadata"}
+    )

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

@@ -22,7 +22,7 @@
 from mcp.server.fastmcp.exceptions import InvalidSignature
 from mcp.server.fastmcp.utilities.logging import get_logger
 from mcp.server.fastmcp.utilities.types import Audio, Image
-from mcp.types import ContentBlock, TextContent
+from mcp.types import CallToolResult, ContentBlock, TextContent
 
 logger = get_logger(__name__)
 
@@ -104,6 +104,12 @@ def convert_result(self, result: Any) -> Any:
         from function return values, whereas the lowlevel server simply serializes
         the structured output.
         """
+        if isinstance(result, CallToolResult):
+            if self.output_schema is not None:
+                assert self.output_model is not None, "Output model must be set if output schema is defined"
+                self.output_model.model_validate(result.structuredContent)
+            return result
+
         unstructured_content = _convert_to_content(result)
 
         if self.output_schema is None:
@@ -268,6 +274,14 @@ def func_metadata(
     output_info = FieldInfo.from_annotation(_get_typed_annotation(sig.return_annotation, globalns))
     annotation = output_info.annotation
 
+    # if the typehint is CallToolResult, the user either intends to return without validation
+    # or they provided validation as Annotated metadata
+    if isinstance(annotation, type) and issubclass(annotation, CallToolResult):
+        if output_info.metadata:
+            annotation = output_info.metadata[0]
+        else:
+            return FuncMetadata(arg_model=arguments_model)
+
     output_model, output_schema, wrap_output = _try_create_model_and_schema(annotation, func.__name__, output_info)
 
     if output_model is None and structured_output is True:

src/mcp/server/lowlevel/server.py

@@ -480,7 +480,7 @@ def call_tool(self, *, validate_input: bool = True):
         def decorator(
             func: Callable[
                 ...,
-                Awaitable[UnstructuredContent | StructuredContent | CombinationContent],
+                Awaitable[UnstructuredContent | StructuredContent | CombinationContent | types.CallToolResult],
             ],
         ):
             logger.debug("Registering handler for CallToolRequest")
@@ -504,7 +504,9 @@ async def handler(req: types.CallToolRequest):
                     # output normalization
                     unstructured_content: UnstructuredContent
                     maybe_structured_content: StructuredContent | None
-                    if isinstance(results, tuple) and len(results) == 2:
+                    if isinstance(results, types.CallToolResult):
+                        return types.ServerResult(results)
+                    elif isinstance(results, tuple) and len(results) == 2:
                         # tool returned both structured and unstructured content
                         unstructured_content, maybe_structured_content = cast(CombinationContent, results)
                     elif isinstance(results, dict):

tests/server/fastmcp/test_func_metadata.py

@@ -13,6 +13,7 @@
 from pydantic import BaseModel, Field
 
 from mcp.server.fastmcp.utilities.func_metadata import func_metadata
+from mcp.types import CallToolResult
 
 
 class SomeInputModelA(BaseModel):
@@ -834,6 +835,49 @@ def func_returning_unannotated() -> UnannotatedClass:
     assert meta.output_schema is None
 
 
+def test_tool_call_result_is_unstructured_and_not_converted():
+    def func_returning_call_tool_result() -> CallToolResult:
+        return CallToolResult(content=[])
+
+    meta = func_metadata(func_returning_call_tool_result)
+
+    assert meta.output_schema is None
+    assert isinstance(meta.convert_result(func_returning_call_tool_result()), CallToolResult)
+
+
+def test_tool_call_result_annotated_is_structured_and_converted():
+    class PersonClass(BaseModel):
+        name: str
+
+    def func_returning_annotated_tool_call_result() -> Annotated[CallToolResult, PersonClass]:
+        return CallToolResult(content=[], structuredContent={"name": "Brandon"})
+
+    meta = func_metadata(func_returning_annotated_tool_call_result)
+
+    assert meta.output_schema == {
+        "type": "object",
+        "properties": {
+            "name": {"title": "Name", "type": "string"},
+        },
+        "required": ["name"],
+        "title": "PersonClass",
+    }
+    assert isinstance(meta.convert_result(func_returning_annotated_tool_call_result()), CallToolResult)
+
+
+def test_tool_call_result_annotated_is_structured_and_invalid():
+    class PersonClass(BaseModel):
+        name: str
+
+    def func_returning_annotated_tool_call_result() -> Annotated[CallToolResult, PersonClass]:
+        return CallToolResult(content=[], structuredContent={"person": "Brandon"})
+
+    meta = func_metadata(func_returning_annotated_tool_call_result)
+
+    with pytest.raises(ValueError):
+        meta.convert_result(func_returning_annotated_tool_call_result())
+
+
 def test_structured_output_with_field_descriptions():
     """Test that Field descriptions are preserved in structured output"""
 

tests/server/test_lowlevel_output_validation.py

@@ -391,6 +391,47 @@ async def test_callback(client_session: ClientSession) -> CallToolResult:
     assert result.structuredContent == {"sentiment": "positive", "confidence": 0.95}
 
 
+@pytest.mark.anyio
+async def test_tool_call_result():
+    """Test returning ToolCallResult when no outputSchema is defined."""
+    tools = [
+        Tool(
+            name="get_info",
+            description="Get structured information",
+            inputSchema={
+                "type": "object",
+                "properties": {},
+            },
+            # No outputSchema for direct return of tool call result
+        )
+    ]
+
+    async def call_tool_handler(name: str, arguments: dict[str, Any]) -> CallToolResult:
+        if name == "get_info":
+            return CallToolResult(
+                content=[TextContent(type="text", text="Results calculated")],
+                structuredContent={"status": "ok", "data": {"value": 42}},
+                _meta={"some": "metadata"},
+            )
+        else:
+            raise ValueError(f"Unknown tool: {name}")
+
+    async def test_callback(client_session: ClientSession) -> CallToolResult:
+        return await client_session.call_tool("get_info", {})
+
+    result = await run_tool_test(tools, call_tool_handler, test_callback)
+
+    # Verify results
+    assert result is not None
+    assert not result.isError
+    assert len(result.content) == 1
+    assert result.content[0].type == "text"
+    assert result.content[0].text == "Results calculated"
+    assert isinstance(result.content[0], TextContent)
+    assert result.structuredContent == {"status": "ok", "data": {"value": 42}}
+    assert result.meta == {"some": "metadata"}
+
+
 @pytest.mark.anyio
 async def test_output_schema_type_validation():
     """Test outputSchema validates types correctly."""

tests/test_examples.py

@@ -44,6 +44,23 @@ async def test_complex_inputs():
         assert result.content[2].text == "charlie"
 
 
+@pytest.mark.anyio
+async def test_direct_call_tool_result_return():
+    """Test the CallToolResult echo server"""
+    from examples.fastmcp.direct_call_tool_result_return import mcp
+
+    async with client_session(mcp._mcp_server) as client:
+        result = await client.call_tool("echo", {"text": "hello"})
+        assert len(result.content) == 1
+        content = result.content[0]
+        assert isinstance(content, TextContent)
+        assert content.text == "hello"
+        assert result.structuredContent
+        assert result.structuredContent["text"] == "hello"
+        assert isinstance(result.meta, dict)
+        assert result.meta["some"] == "metadata"
+
+
 @pytest.mark.anyio
 async def test_desktop(monkeypatch: pytest.MonkeyPatch):
     """Test the desktop server"""