Use the applicable elicitation form vocabulary

The allowed form-schema vocabulary depends on the MCP protocol version. Check the fields of requestedSchema against that vocabulary rather than assuming a general Tool JSON Schema can be copied into an elicitation form unchanged.

Requirement

In MCP 2025-06-18, a form requestedSchema has no $schema, no array or titled enum properties, and default only on boolean properties.

Details

Rule ID: mcp.elicitation.form-schema.protocol-shape
Owner: mcp
Surface: ElicitRequest · Path: requestedSchema
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	applies	not-applicable	not-applicable

Required by

MCP Description 0.9.0, Section 12.3
- 12. Elicitation Declarations: req-elicitation-form-vocabulary

Based on

Tier 1 — MCP schema.ts and normative specification text: MCP 2025-06-18 schema.ts, ElicitRequest requestedSchema and PrimitiveSchemaDefinition (default only on BooleanSchema; EnumSchema with enum and enumNames)
Tier 1 — MCP schema.ts and normative specification text: MCP 2025-11-25 schema.ts, requestedSchema $schema, default on all primitive schemas, titled and multi-select enum schemas

Examples

Invalid example

Source: expected-invalid/elicitation-form-schema-2025-06-18.json · Selected: /prompts/0/elicitations/0/requestedSchema

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "properties": {
    "colors": {
      "type": "array",
      "items": {
        "type": "string",
        "enum": [
          "red",
          "blue"
        ]
      }
    },
    "label": {
      "type": "string",
      "default": "main"
    }
  }
}

Rule Source Code

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