Supply structured content when an output schema needs it

A successful Tool example for a Tool with outputSchema must carry the required structuredContent. The schema describes that structured value, not the unstructured content array. Check success and error examples separately before deciding which payload is missing.

Requirement

When a Tool declares outputSchema, a successful result contains structuredContent.

Details

Rule ID: mcp.tool.structured-content.output-schema
Owner: mcp
Surface: CallToolResult · Path: structuredContent
Kind: semantic
Requirement level: must
Status: internal · Enforcement: internal

Protocol version applicability

2024-11-05	2025-03-26	2025-06-18	2025-11-25	2026-07-28
not-applicable	not-applicable	applies	applies	applies

Required by

• MCP Description 0.9.0, Section 9.3
  • 9. Tools: req-tool-example-structured-content

Based on

• Tier 1 — MCP schema.ts and normative specification text: MCP 2025-06-18 to 2026-07-28 specification, Server Features, Tools, Output Schema (“Servers MUST provide structured results that conform to this schema”)
  • https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/docs/specification/2025-06-18/server/tools.mdx#L309-L382
  • https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/docs/specification/2025-11-25/server/tools.mdx#L335-L408
  • https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/docs/specification/2026-07-28/server/tools.mdx#L509-L631

Notes

Conformance of structuredContent to outputSchema is checked by mcpdesc.tool.example.schema-conformance. Error results are governed by mcpdesc.tool.example.error-result.

Examples

Invalid example

Source: expected-invalid/invalid-tool-example-result-semantics.json · Selected: /tools/0/examples/missing/result

{
  "resultType": "complete",
  "content": [
    {
      "type": "text",
      "text": "Missing structured value"
    }
  ]
}

Invalid example

Source: expected-invalid/invalid-tool-interaction-example-semantics.json · Selected: /tools/0/interactionExamples/broken/result

{
  "content": [
    {
      "type": "text",
      "text": "Release preparation completed."
    }
  ]
}

Rule Source Code

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