DraganSr: OpenAI Response API vs Chat Completions

Monday, September 14, 2026

The Responses API is a unified interface for building powerful, agent-like applications. It contains:

Built-in tools like web search, file search, computer use, code interpreter, and remote MCPs.
Seamless multi-turn interactions that allow you to pass previous responses for higher accuracy reasoning results.
Native multimodal support for text and images.

The Responses API contains several benefits over Chat Completions:

Better performance: Using reasoning models, like GPT-5, with Responses will result in better model intelligence when compared to Chat Completions. Our internal evals reveal a 3% improvement in SWE-bench with same prompt and setup.
Agentic by default: The Responses API is an agentic loop, allowing the model to call multiple tools, like web_search, image_generation, file_search, code_interpreter, remote MCP servers, as well as your own custom functions, within the span of one API request.
Lower costs: Results in lower costs due to improved cache utilization (40% to 80% improvement when compared to Chat Completions in internal tests).
Stateful context: Use store: true to maintain state from turn to turn, preserving reasoning and tool context from turn-to-turn.
Flexible inputs: Pass a string with input or a list of messages; use instructions for system-level guidance.
Encrypted reasoning: Opt-out of statefulness while still benefiting from advanced reasoning.
Future-proof: Future-proofed for upcoming models.