Single tool with complete JSON Schema and basic descriptions. Tool name 'speak' is verb-led and clear. Schema includes proper constraints (speedScale: 0.5-2.0, volumeScale: 0-2.0, required params). However, descriptions are in Japanese and relatively brief (under 100 chars for most params). No output schema documented. Error handling exists but returns plain text instead of structured guidance. Tool does one thing well but lacks production-grade documentation polish and LLM-optimization for English-language agents.

• Provide English descriptions for all parameters. Replace 'VOICEVOXを使用してテキストを読み上げます' with 'Synthesize and play audio from text using VOICEVOX (text-to-speech engine). This is a command with side effects and should not be retried on partial failures.'
• Document the complete output schema. E.g., {"type": "object", "properties": {"status": {"type": "string", "description": "Success or error code"}, "message": {"type": "string"}, "audio_duration_ms": {"type": "number"}}}
• Add speaker discovery tool. Create a 'list_speakers' or 'get_speakers' tool that returns available speaker IDs, names, and metadata so agents can pick valid speakers without trial-and-error.
• Enhance error responses with recovery guidance. Instead of 'エラー: Speaker 99 not found', return {"error": "invalid_speaker", "message": "Speaker ID 99 not found. Use list_speakers() to see available options." } with isError: true.
• Clarify async behavior in the description. Explain that async=true (default) queues audio immediately but does not wait; async=false blocks the tool until playback completes. Agents need to understand timing implications.
• Add minimum/maximum validation for speedScale and volumeScale in the tool implementation and document that values outside [0.5, 2.0] and [0, 2.0] respectively will be rejected.
• Consider accepting speaker names or aliases in addition to numeric IDs. E.g., parameter 'speaker' could accept 'speaker_id' (number) OR 'speaker_name' (string like 'zundamon'), with resolution inside the tool.