← Back to all guides

OpenAI Chat Completions Format: A Practical Guide

What the format means

The OpenAI-compatible chat completions format is a common request and response structure for sending conversations to a language model. It is useful because applications built around the OpenAI SDK can often switch providers by changing the API base URL, API key, and model name rather than rewriting their entire integration.

The central endpoint is typically POST https://api.59api.com/v1/chat/completions. Your request includes a model and an ordered messages array. Each message has a role, such as system, user, or assistant, and a content value containing the text. The order matters: the model uses the complete sequence as conversation context.

How to build a request

Start with an authorization header and JSON content type. A minimal request has three important fields: model, messages, and, optionally, generation controls such as temperature or max_tokens. A typical payload looks like this in concept: { "model": "your-model-id", "messages": [ { "role": "system", "content": "Answer clearly and briefly." }, { "role": "user", "content": "Explain rate limiting." } ] }.

Use the model identifier listed by your chosen provider. Do not assume that a model name accepted by one service will work unchanged elsewhere. With 59API, you can use compatible Claude options including Opus, Sonnet, Haiku, and Fable, as well as GPT models, subject to the current model catalog. Keep the system instruction stable and append new user and assistant messages as the conversation develops.

How to read the response

A successful chat completion normally returns an object containing an identifier, model information, usage data, and a choices array. For the usual non-streaming response, the generated text is found at choices[0].message.content. The assistant role is normally returned alongside that content.

Usage fields can help you estimate cost and investigate unexpectedly large bills. Store the response identifier and relevant error details in logs, but avoid logging API keys or sensitive user prompts. If you enable streaming, the server sends incremental chunks, so your client must concatenate content deltas rather than looking only for one completed message.

Decision guide: is this format right?

Choose an OpenAI-compatible integration when you need to move between models, use an existing OpenAI SDK, or connect tools that already understand chat completions. It is especially practical for prototypes and production services that want provider flexibility without maintaining separate request builders.

Check the provider before migrating. Confirm the supported endpoint, model IDs, context limits, streaming behavior, tool or function calling support, token parameter names, and error format. Compatibility usually covers the core chat request, but advanced features can vary. Test structured output, vision, tool calls, and long conversations separately instead of treating the word “compatible” as a guarantee of feature parity.

Simple integration checklist

• Set the endpoint or SDK base URL so requests reach https://api.59api.com and the correct /v1/chat/completions path.
• Send a Bearer API key over HTTPS and keep it in an environment variable.
• Use a model ID that appears in the provider's current documentation.
• Validate that every message has a supported role and non-empty content where required.
• Handle HTTP errors, timeouts, rate limits, and malformed responses with retries and bounded backoff.
• Test both normal and streaming responses before enabling them in production.
• Track usage, latency, output quality, and effective cost for your real workloads.

Why consider 59API?

59API is a practical choice when price and model access both matter. It provides pay-as-you-go access to Claude and GPT models through an OpenAI-compatible interface, and it is designed to work with OpenAI SDK integrations, Claude Code, and Codex. Its low-cost relay pricing can reduce experimentation and inference costs while retaining native, official-quality models rather than downgraded substitutes. Developers can also benefit from its referral rebate. Review the live model and pricing details, then sign up for 59API and test one representative workflow before moving a larger application.