OpenHolidays MCP has well-structured tool definitions with complete input schemas and clear descriptions. All 4 tools have proper JSON Schema with typed properties and required fields. Descriptions are substantive (100-200 chars) and explain the Europe-focused scope and use cases. However, output schemas are not documented, LLMs cannot predict response structure. Parameter descriptions are good but lack format constraints (e.g., date format validation, language code validation). No error handling guidance is provided. Tool naming follows verb_noun convention (list_*, public_holidays, school_holidays) and is clear. Security is solid (keyless, read-only). Composition is good, tools chain well (list_countries → list_subdivisions → school_holidays). Missing: output schema documentation, input validation rules in descriptions, error recovery guidance.

• Document output schemas for all tools. Add 'outputSchema' field to each tool definition showing {country: string, count: number, holidays: [{name: string, start_date: string, end_date: string, nationwide: boolean, subdivisions: string[]}]}.
• Add format and constraint descriptions to parameters. E.g., 'valid_from: Start of date range (YYYY-MM-DD, must be ≤ valid_to, ISO 8601 format).' and 'language: ISO 639-1 code (e.g., EN, DE, FR; defaults to EN if unsupported).'
• Document error cases and recovery paths in tool descriptions. E.g., 'Returns error if country code is invalid (must be 2-letter ISO 3166-1 alpha-2). Call list_countries() to see supported countries.'
• Add pagination guidance. If results can be large, document expected size ranges and recommend filtering by date range. E.g., 'Typical result: 10-20 holidays per year. For multi-year ranges, consider splitting into annual queries.'
• Clarify transport and server initialization. Add a README or code comment showing how the server is launched (e.g., 'Runs as STDIO server via `node src/index.ts`' or 'Exposes HTTP on port 3000').