Represent Tool execution errors as error results

Tool execution errors are completed MCP Tool results with isError true, not a separate description-only error catalogue. Check the error result’s content and avoid presenting successful structured output as the error payload.

Requirement

A Tool execution-error example result sets isError to true and does not contain structuredContent.

Details

Rule ID: mcpdesc.tool.example.error-result
Owner: mcpdesc
Surface: Tool example · Path: result
Kind: mcpdesc-semantic
Requirement level: must
Protocol versions: independent of the MCP protocol version
Status: internal · Enforcement: internal

Required by

• MCP Description 0.9.0, Section 9.3 (an execution-error result MUST NOT contain structuredContent; outputSchema does not validate error content)
  • 9. Tools: req-tool-example-error-result
  • 9. Tools: req-tool-example-success

Notes

MCP does not forbid structuredContent on error results; MCP Description requires its absence so that example structuredContent always conforms to outputSchema.

Examples

Invalid example

Source: expected-invalid/invalid-tool-example-result-semantics.json · Selected: /tools/1/examples/business-error/result

{
  "resultType": "complete",
  "content": [
    {
      "type": "text",
      "text": "Rejected"
    }
  ],
  "structuredContent": {
    "code": "rejected"
  },
  "isError": true
}

Rule Source Code

tool.yaml at immutable tag v0.9.0-rc.3.