Keep newer client input responses closed

In MCP 2026-07-28, sampling, elicitation, and roots responses are closed input responses rather than open results. Remove undefined fields; _meta is defined only for the sampling response. Earlier versions’ open-result behavior does not carry forward to these types.

Requirement

In MCP 2026-07-28, sampling, elicitation, and roots responses are input responses, not results, and accept only the fields their types define.

Details

Rule ID: mcp.input-response.closed
Owner: mcp
Surface: InputResponse
Kind: structural
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	not-applicable	not-applicable	applies

Required by

• MCP Description 0.9.0, Sections 3.6 and 9.4
  • 3. Document Structure: req-carried-closed-reject
  • 9. Tools: req-interaction-protocol-version-valid

Based on

• Tier 1 — MCP schema.ts and normative specification text: MCP 2026-07-28 schema.ts, type InputResponse (CreateMessageResult, ListRootsResult, ElicitResult); none extends Result; only CreateMessageResult defines _meta, through SamplingMessage
  • validation/mcp-schemas/2026-07-28/schema.ts#L541-L542
  • validation/mcp-schemas/2026-07-28/schema.ts#L2190-L2228
  • validation/mcp-schemas/2026-07-28/schema.ts#L3120-L3149
  • validation/mcp-schemas/2026-07-28/schema.ts#L2725-L2744

Examples

Invalid example

Source: expected-invalid/closed-step-responses-2026.json · Selected: /tools/0/interactionExamples/approved/steps/0/response

{
  "roots": [
    {
      "uri": "file:///workspace/example"
    }
  ],
  "_meta": {
    "trace": "r1"
  }
}

Rule Source Code

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