Declare output schemas only where MCP defines them

Tool outputSchema is available from MCP 2025-06-18. A Tool whose effective scope includes an earlier protocol version cannot carry it unchanged. Omit it for the earlier version or provide disjoint variants.

Requirement

A Tool declares outputSchema only for MCP protocol versions whose Tool type defines it.

Details

Rule ID: mcp.tool.output-schema.protocol-version
Owner: mcp
Surface: Tool · Path: outputSchema
Kind: structural
Requirement level: Not specified in the rule record
Status: internal · Enforcement: internal

Protocol version applicability

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

Required by

• MCP Description 0.9.0, Section 9.1 (Tool Object field table)
  • 4. Versioning: req-field-protocol-version-availability

Based on

• Tier 1 — MCP schema.ts and normative specification text: MCP 2025-03-26 schema.ts, interface Tool (closed; no outputSchema)
  • validation/mcp-schemas/2025-03-26/schema.ts#L781-L810
• Tier 1 — MCP schema.ts and normative specification text: MCP 2025-06-18 schema.ts, interface Tool, outputSchema
  • validation/mcp-schemas/2025-06-18/schema.ts#L947-L955

Examples

Invalid example

Source: expected-invalid/output-schema-before-2025-06-18.json · Selected: /tools/0

{
  "name": "early_output",
  "inputSchema": {
    "type": "object"
  },
  "outputSchema": {
    "type": "object",
    "properties": {
      "total": {
        "type": "integer"
      }
    }
  }
}

Rule Source Code

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