fix: skip output schema validation when tool returns is_error=True

When a tool with an inferred output_schema returns
CallToolResult(is_error=True), the SDK's convert_result() calls
model_validate(None) unconditionally, raising a pydantic error that
replaces the intended error message.

Skip output schema validation when the result signals an error,
matching the TypeScript SDK fix (modelcontextprotocol/typescript-sdk#655).

Fixes #2429

Author: kimsehwan96

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

@@ -103,7 +103,7 @@ 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.is_error:
                 assert self.output_model is not None, "Output model must be set if output schema is defined"
                 self.output_model.model_validate(result.structured_content)
             return result

tests/server/mcpserver/test_func_metadata.py

@@ -14,7 +14,7 @@
 
 from mcp.server.mcpserver.exceptions import InvalidSignature
 from mcp.server.mcpserver.utilities.func_metadata import func_metadata
-from mcp.types import CallToolResult
+from mcp.types import CallToolResult, TextContent
 
 
 class SomeInputModelA(BaseModel):
@@ -875,6 +875,31 @@ 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_is_error_skips_validation():
+    """Test that is_error=True skips output schema validation.
+
+    Regression test for https://github.com/modelcontextprotocol/python-sdk/issues/2429
+    """
+
+    class DivideOutput(BaseModel):
+        result: float
+
+    def func_returning_error() -> Annotated[CallToolResult, DivideOutput]:
+        return CallToolResult(
+            content=[TextContent(type="text", text="Division by zero")],
+            is_error=True,
+        )
+
+    meta = func_metadata(func_returning_error)
+    assert meta.output_schema is not None
+
+    result = meta.convert_result(func_returning_error())
+    assert isinstance(result, CallToolResult)
+    assert result.is_error is True
+    assert isinstance(result.content[0], TextContent)
+    assert result.content[0].text == "Division by zero"
+
+
 def test_tool_call_result_in_optional_is_rejected():
     """Test that Optional[CallToolResult] raises InvalidSignature"""