When to Use Responses API Instead of Chat Completions

You should usually use the Responses API instead of Chat Completions when you are starting a new OpenAI integration, especially if the workflow may grow beyond single-turn text generation. OpenAI still supports Chat Completions, but its documentation recommends Responses for new projects and calls out better fit for reasoning models, tools, and stateful flows.

The practical rule is simple: use Chat Completions only when your application is intentionally narrow and you want the most minimal chat-style interface. Use Responses when you expect tool use, richer control, multi-step behavior, or future expansion.

Why this decision matters

Choosing the API early affects how much migration work you create later. A lot of teams start with a small chat feature and then discover they need retrieval, structured outputs, tool calls, persistent state, or agent routing. Responses is better aligned with that expansion path, which is why it makes sense to choose it before your application grows.

Use Responses API when

• you are building a new OpenAI-powered product feature
• you expect to add tool calls or external functions
• you want built-in tools such as web search or file search
• you are using reasoning models and want the better-supported path
• you need stateful interaction, or may need it later
• you want the API path OpenAI is actively prioritizing

Chat Completions may still be enough when

• the feature is truly single-turn and text-only
• your app already works well on Chat Completions and does not need new capabilities
• you want the narrowest possible message-based interface for a contained use case

Quick decision table

Key differences in practice

The biggest practical difference is that Responses is built to handle more than a single assistant message. It can represent tool calls, outputs, and richer interaction items more naturally. OpenAI also highlights statefulness, built-in tools, and future model support as part of the reason to favor it.

For GPT-5.x specifically, OpenAI documents that Responses can pass prior chain-of-thought context between turns, which can improve intelligence, reduce generated reasoning tokens, increase cache hit rates, and lower latency compared with Chat Completions on the same broader workflow pattern.

What this means for reasoning models

If you are using reasoning models, the case for Responses gets stronger. OpenAI’s reasoning guide explicitly says reasoning models work better with the Responses API. So even if your current use case looks simple, choosing Chat Completions may become limiting once the model workload becomes more reasoning-heavy.

What this means for future maintenance

The maintenance argument is often stronger than the feature argument. If you know the feature may grow, starting on Responses saves future migration work. If you start on Chat Completions for speed, document that choice as a deliberate temporary shortcut rather than the long-term architecture.

Limitations and edge cases

Responses is not automatically better just because it is newer. A very small app with fixed prompts, no state, and no tools may not gain much from the broader interface on day one. The important thing is to be honest about whether the use case will stay that small.

Where templates help

Templates help most when the workflow already fits a known pattern such as retrieval, extraction, support triage, or agent-assisted routing. In those cases, it is easier to start from a Responses-friendly workflow template than to build a temporary Chat Completions version and migrate later.

FAQ

Is Chat Completions still supported?

Yes. OpenAI still supports it.

Does OpenAI recommend Responses for new apps?

Yes. OpenAI documentation explicitly recommends Responses for new projects.

Should I migrate every old Chat Completions endpoint immediately?

Not necessarily. But for new development, it is usually better to start with Responses unless the use case is intentionally narrow and stable.

Conclusion

If you are deciding today, Responses API should usually be your default. Chat Completions still has a place for small, fixed, text-only features, but it is no longer the best general starting point for most new OpenAI workflows.