API shape comparison
- Responses: single endpoint, input is a messages array, tool calls are items
- Messages: single endpoint, content is a parts array, tool calls are tool_use parts
- Both stream, both multimodal, both with hosted tools
Picking guidance
- Want Claude → anthropic_messages
- Mostly GPT with a fallback chain → responses
- AiApiy / aggregator services → responses (their proxies expose this wire)
- China-hosted models → most are responses-compatible