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fix(server): allow tools with outputSchema to surface errors via unstructured content #2447

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fix(server): allow tools with outputSchema to surface errors via unstructured content #2447

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18 changes: 13 additions & 5 deletions src/mcp/server/lowlevel/server.py
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Original file line number Diff line number Diff line change
Expand Up @@ -537,6 +537,8 @@ async def handler(req: types.CallToolRequest):
# output normalization
unstructured_content: UnstructuredContent
maybe_structured_content: StructuredContent | None
is_error = False

if isinstance(results, types.CallToolResult):
return types.ServerResult(results)
elif isinstance(results, types.CreateTaskResult):
Expand All @@ -556,12 +558,18 @@ async def handler(req: types.CallToolRequest):
else: # pragma: no cover
return self._make_error_result(f"Unexpected return type from tool: {type(results).__name__}")

# output validation
# output validation: a tool with an outputSchema must still
# be able to surface a tool-execution failure via unstructured
# content. If structured output is missing for such a tool,
# treat the unstructured payload as a tool-error result
# (isError=true) instead of replacing it with a hard
# "outputSchema defined but no structured output returned"
# error that obscures the original failure message. This
# mirrors the TypeScript SDK fix in
# modelcontextprotocol/typescript-sdk#655.
if tool and tool.outputSchema is not None:
if maybe_structured_content is None:
return self._make_error_result(
"Output validation error: outputSchema defined but no structured output returned"
)
is_error = True
else:
try:
jsonschema.validate(instance=maybe_structured_content, schema=tool.outputSchema)
Expand All @@ -573,7 +581,7 @@ async def handler(req: types.CallToolRequest):
types.CallToolResult(
content=list(unstructured_content),
structuredContent=maybe_structured_content,
isError=False,
isError=is_error,
)
)
except UrlElicitationRequiredError:
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