Single tool with adequate schema and description, but moderate gaps in parameter constraints, error handling guidance, and output documentation. The tool name is clear (send_whatsapp), description explains the action and prerequisite format, and input schema has types and descriptions. However, no output schema is documented, error handling returns raw exception messages without recovery guidance, and parameter format constraints are stated in prose rather than as formal constraints (regex, enum). The server follows fastmcp conventions correctly but lacks patterns for idempotency, validation clarity, and structured error recovery.

• Add a documented output schema: `{"success": boolean, "message_sid": string, "status": string, "timestamp": string}`. This enables LLMs to know what fields to expect and what to extract.
• Replace prose format constraint with a JSON Schema pattern: `"to_number": {"type": "string", "pattern": "^whatsapp:\\+[0-9]{1,15}$", "description": "Recipient's WhatsApp number..."}`.
• Enhance error responses with recovery guidance: instead of 'Failed to send WhatsApp message...', return structured error objects like `{"success": false, "error_code": "rate_limited", "message": "Rate limit reached. Wait 60s before retrying.", "next_action": "retry_in_seconds: 60"}`.
• Add tool annotations to declare side effects: in fastmcp, use `@mcp.tool(destructiveHint=True)` or add a field to the tool definition indicating this is a write operation.
• Document idempotency: state clearly whether calling send_whatsapp twice with identical args will send two messages or one (add deduplication by MessageSID if supported by Twilio).
• Add rate-limit and timeout handling: catch Twilio's rate-limit exceptions separately from general errors and advise the LLM to retry with exponential backoff.