Keep fields within their supported protocol scope

A declaration must be valid in every protocol version in its effective scope. A field introduced later cannot be shared with earlier versions merely because the document also declares the later version. Omit the field or describe disjoint scoped variants.

Requirement

A declaration field is used only in protocol versions whose MCP type defines it: title and _meta from 2025-06-18, Tool annotations from 2025-03-26, icons and Tool schema $schema from 2025-11-25, Tool execution only in 2025-11-25, and the completions capability from 2025-03-26.

Details

Rule ID: mcp.declaration.field.protocol-version
Owner: mcp
Surface: Tool, Resource, ResourceTemplate, Prompt, PromptArgument, ServerCapabilities
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`
applies	applies	applies	applies	applies

Required by

MCP Description 0.9.0, Sections 8.2, 9.1, 9.2, 10.1, 10.2, and 11.1
- 4. Versioning: req-field-protocol-version-availability
- 1. Tools: req-tool-schema-dialect-keyword

Based on

Tier 1 — MCP schema.ts and normative specification text: MCP 2024-11-05 to 2026-07-28 schema.ts, interfaces BaseMetadata, Icons, Tool, Resource, ResourceTemplate, Prompt, PromptArgument, and ServerCapabilities

Notes

outputSchema, the outputSchema root, and the tasks capability have their own rules; the rule property identifies which applies.

Examples

Invalid example

Source: expected-invalid/protocol-version-inapplicable-execution.json · Selected: /tools/0

{
  "name": "run_job",
  "inputSchema": {
    "type": "object",
    "additionalProperties": false
  },
  "execution": {
    "taskSupport": "optional"
  }
}

Invalid example

Source: expected-invalid/protocol-version-inapplicable-tool-schema-dialect.json · Selected: /tools/0/inputSchema

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "additionalProperties": false
}

Rule Source Code

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