DouaneCode MCP has well-structured tool definitions with clear naming (verb_noun pattern: search_, get_), comprehensive parameter descriptions, and proper input schemas. All 4 tools follow consistent patterns. However, output schemas are not documented, LLMs cannot predict response structure for planning. Parameter descriptions are strong (60-80 chars avg), but lack explicit constraints (enums, ranges, patterns). Error handling is present but generic; recovery guidance is minimal. No tool annotations (readOnlyHint, idempotentHint) despite all tools being read-only. Missing pagination guidance for search_customs_code despite potentially large result sets.

• Document output schemas for all 4 tools. Example: search_customs_code returns {results: [{code: string, label: string, hierarchy: string[], ...}], total: int, limit: int}. Include field types and descriptions.
• Add JSON Schema constraints: limit with minimum/maximum; chapter with pattern '^[0-9]{2}$'; code with pattern '^[0-9]{4,10}$'; origin with enum or pattern for ISO-2 country codes.
• Add tool annotations to FastMCP tool decorators: @mcp.tool(readOnlyHint=True, idempotentHint=True) for all tools to signal safety properties to clients.
• Enhance error messages with recovery guidance. E.g., 'Rate limit reached (429). Upgrade your plan at https://douanecode.fr/tarifs or retry after 24 hours.' instead of generic message.
• Add pagination support to search_customs_code: accept offset/limit, return total count and next_offset. Document in description: 'Returns up to limit results; use offset to paginate through large result sets.'