Preserve fields on open results with a warning

An open MCP result can carry fields that its protocol version does not define. Those values are preserved without assigning them later-version meaning, and a warning identifies the affected version. Review the field or split protocol variants if the warning is not intended.

Requirement

A field that an open MCP result type allows but the MCP protocol version does not define is accepted and preserved without meaning, with a warning attributed to that protocol version.

Details

Rule ID: mcp.result.additional-properties.preserve
Owner: mcp
Surface: Result
Kind: preservation
Requirement level: may
Status: internal · Enforcement: internal

Protocol version applicability

`2024-11-05`	`2025-03-26`	`2025-06-18`	`2025-11-25`	`2026-07-28`
applies	applies	applies	applies	applies

Required by

MCP Description 0.9.0, Section 3.6 (Carried MCP Values)
- 3. Document Structure: req-carried-open-preserve

Based on

Tier 1 — MCP schema.ts and normative specification text: MCP 2024-11-05 to 2026-07-28 schema.ts, interface Result (index signature [key: string]: unknown, inherited by every result type)
Tier 1 — MCP schema.ts and normative specification text: MCP 2024-11-05 to 2025-11-25 schema.ts, CreateMessageResult, ElicitResult, and ListRootsResult extend Result

Notes

The warnings of this rule for a document equal the union of its warnings for each protocol-version-specific Effective Protocol View (scripts/test-views.mjs).

Examples

Accepted with a warning

Source: expected-warning/open-result-unknown-properties.json · Selected: /tools/0/examples/traced/result

{
  "content": [
    {
      "type": "text",
      "text": "found"
    }
  ],
  "vendorTrace": {
    "id": "a1"
  }
}

Accepted with a warning

Source: expected-warning/open-results-shared-across-protocol-versions.json · Selected: /prompts/0/examples/default/result

{
  "resultType": "complete",
  "messages": [
    {
      "role": "assistant",
      "content": {
        "type": "text",
        "text": "Hello."
      }
    }
  ]
}

Accepted with a warning

Source: expected-warning/open-step-responses-before-2026.json · Selected: /tools/0/interactionExamples/approved/steps/0/response

{
  "roots": [
    {
      "uri": "file:///workspace/example",
      "_meta": {
        "trace": "root1"
      }
    }
  ],
  "_meta": {
    "trace": "r1"
  },
  "acmeTrace": {
    "id": "a1"
  }
}

Accepted with a warning

Source: expected-warning/component-protocol-scope.json · Selected: /components/resourceExamples/current/result

{
  "resultType": "complete",
  "ttlMs": 60,
  "cacheScope": "private",
  "contents": [
    {
      "uri": "data://status",
      "text": "ready"
    }
  ]
}

Rule Source Code

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