OpenAI max_tokens vs max_completion_tokens Error

This error usually appears when a request body mixes parameters from different OpenAI model families. Validate JSON first, then align token fields with the target model.

Symptoms

  • The API rejects a request that worked with a different model.
  • The response mentions an unsupported or unknown token parameter.
  • A coding agent changed the model name but kept the old request body.

Likely causes

  • Reasoning and text models can expect different token parameter names.
  • SDK examples from older projects may use fields that no longer match the chosen model.
  • The request body is copied from a different endpoint or model family.

Fix steps

  1. Check the request with the OpenAI API Request Checker and choose the intended model family.
  2. Remove suspicious sampling or token fields before adding them back one at a time.
  3. Keep one minimal known-good request in your docs or tests.

Verify the fix

  • Validate the JSON body.
  • Run a minimal request with cURL or your SDK.
  • Confirm the final parameter names against official API docs.

FAQ

Can both fields be sent together?

Usually no. Keep the request aligned with the target endpoint and model family.

Should I change the model or the parameter?

Start by deciding the model family, then adjust parameters to match it.

Related tools and guides

OpenAI OpenAI API Request Checker

Statically inspect an OpenAI API request body for common JSON and parameter mistakes before sending it to the API.

 JSON JSON Validator and Formatter

Validate JSON, format it for reading, or minify it for compact API payloads without sending data to an external API.

 cURL cURL Command Builder

Build readable cURL commands for API requests with method, headers, JSON body and safe auth placeholders.

 OpenAI Debug OpenAI API Request JSON Before Sending

Most API request bugs are easier to fix before the request is sent. Start with valid JSON, then check model-family parameters and only then debug application code.

 cURL cURL API Debugging Workflow for Developers

cURL is the simplest way to separate API behavior from SDK, framework or frontend code. Build a minimal request first, then compare it with the failing implementation.

Last updated: May 18, 2026