A side-by-side comparison of OpenAI's Chat Completions, Responses API, and Anthropic's Messages API, covering key differences, use cases, and how to avoid vendor lock-in with Portkey.

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A structured comparison of three major LLM API formats: OpenAI's Chat Completions (stateless, widest ecosystem), OpenAI's Responses API (agent-oriented with built-in tools and state management), and Anthropic's Messages API (Claude-native with extended thinking and prompt caching). The piece covers design goals, capabilities, and ideal use cases for each, then introduces Portkey as a gateway that supports all three formats with any provider, enabling provider switching without code changes while adding observability, fallbacks, and cost tracking.

Open AI Responses API vs. Chat Completions vs. Messages API

Open AI Responses API vs. Chat Completions vs. Messages API: At a glance

Making calls with each API through Portkey