Validate JSON data against a JSON Schema (draft-07/2019-09/2020-12) via @mukundakatta/validate-mcp (npx)

Recipe: Validate JSON against a JSON Schema via @mukundakatta/validate-mcp

Setup

{ "mcpServers": { "validate": { "command": "npx", "args": ["-y", "@mukundakatta/validate-mcp@latest"] } } }

Surface

• Package: @mukundakatta/[email protected] (MIT, 9.4 kB, 3 deps: ajv + ajv-formats + MCP SDK)
• Transport: stdio
• Tools: 1 — validate
• Binary: mcp-validate
• Schema support: draft-07 / 2019-09 / 2020-12 (Ajv under the hood)
• Formats: email, uri, date-time, hostname, ipv4, ipv6, etc. (via ajv-formats)

Tool: validate

Input: { schema: <JSON Schema object>, data: <any JSON value> } Output: { valid: boolean, errors: Array<{ path, keyword, message, schema_path }> }

Errors use JSON-pointer paths (/name, /age, /) so agents can map violations back to specific fields programmatically.

Trace 1 — valid data (all constraints pass)

→ tools/call  validate
  data:   { "name": "Alice", "age": 30, "email": "[email protected]" }
  schema: { type: "object", required: ["name","age","email"],
            properties: { name: {type:"string",minLength:1}, age: {type:"integer",minimum:0,maximum:150}, email: {type:"string",format:"email"} },
            additionalProperties: false }
← { "valid": true, "errors": [] }

Trace 2 — invalid data (4 violations detected)

→ tools/call  validate
  data:   { "name": "", "age": -5, "email": "not-an-email", "extra": "field" }
  schema: (same as above)
← {
    "valid": false,
    "errors": [
      { "path": "/",      "keyword": "additionalProperties", "message": "must NOT have additional properties" },
      { "path": "/name",  "keyword": "minLength",             "message": "must NOT have fewer than 1 characters" },
      { "path": "/age",   "keyword": "minimum",               "message": "must be >= 0" },
      { "path": "/email", "keyword": "format",                "message": "must match format \"email\"" }
    ]
  }

Failure modes observed

• isError: true with "unknown tool: <name>" when tool name is wrong (only validate exists)
• Schema compiles fresh each call — no caching across calls (caller's responsibility)

Notes

• Uses allErrors: true so all violations are returned, not just the first
• strict: false in Ajv config means lenient schema parsing (won't reject unknown keywords)
• The macOS /tmp → /private/tmp symlink breaks the import.meta.url === file://${process.argv[1]} startup guard — use /private/tmp or a relative cwd when spawning via node directly