fix: preserve FastMCP tool error results

Author: he-yufeng

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

@@ -112,7 +112,7 @@ def convert_result(self, result: Any) -> Any:
         the structured output.
         """
         if isinstance(result, CallToolResult):
-            if self.output_schema is not None:
+            if self.output_schema is not None and not result.isError:
                 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

tests/server/fastmcp/test_func_metadata.py

@@ -13,7 +13,7 @@
 from pydantic import BaseModel, Field
 
 from mcp.server.fastmcp.utilities.func_metadata import func_metadata
-from mcp.types import CallToolResult
+from mcp.types import CallToolResult, TextContent
 
 
 class SomeInputModelA(BaseModel):
@@ -878,6 +878,22 @@ def func_returning_annotated_tool_call_result() -> Annotated[CallToolResult, Per
         meta.convert_result(func_returning_annotated_tool_call_result())
 
 
+def test_tool_call_result_annotated_error_skips_structured_validation():
+    class PersonClass(BaseModel):
+        name: str
+
+    def func_returning_tool_error() -> Annotated[CallToolResult, PersonClass]:  # pragma: no cover
+        return CallToolResult(content=[TextContent(type="text", text="Division by zero")], isError=True)
+
+    meta = func_metadata(func_returning_tool_error)
+    result = meta.convert_result(func_returning_tool_error())
+
+    assert isinstance(result, CallToolResult)
+    assert result.isError is True
+    assert result.structuredContent is None
+    assert result.content[0].text == "Division by zero"
+
+
 def test_tool_call_result_in_optional_is_rejected():
     """Test that Optional[CallToolResult] raises InvalidSignature"""