Semantic Kernel Python + Azure OpenAI o3: Unsupported parameter max_tokens, use max_completion_tokens

TL;DR

Use 'maxcompletiontokens' instead." Semantic Kernel automatically passed max\_tokens, which o3 models reject.

Steps

1. Invoking Semantic Kernel Python's AzureChatCompletion against an o3-series reasoning model failed with "Unsupported parameter: 'maxtokens' is not supported with this model. Use 'maxcompletiontokens' instead." Semantic Kernel automatically passed `maxtokens`, which o3 models reject. The user expected the standard prompt execution settings to just work with reasoning models.
2. Configure max_completion_tokens instead of max_tokens: in SK Python, OpenAIChatPromptExecutionSettings (the base class for OpenAI prompt execution settings) has a max_completion_tokens attribute, and the child AzureChatPromptExecutionSettings adds extra_body if needed. Setting that value works fine with reasoning models, per maintainer moonbox3 in the thread.

When to use

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When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

Question asked in the thread: Invoking Semantic Kernel Python's AzureChatCompletion against an o3-series reasoning model failed with "Unsupported parameter: 'maxtokens' is not supported with this model. Use 'maxcompletiontokens' instead." Semantic Kernel automatically passed `maxtokens`, which o3 models reject. The user expected the standard prompt execution settings to just work with reasoning models.

Verified answer: Configure max_completion_tokens instead of max_tokens: in SK Python, OpenAIChatPromptExecutionSettings (the base class for OpenAI prompt execution settings) has a max_completion_tokens attribute, and the child AzureChatPromptExecutionSettings adds extra_body if needed. Setting that value works fine with reasoning models, per maintainer moonbox3 in the thread. Source: https://github.com/microsoft/semantic-kernel/issues/12865

Thread: https://vectle.com/threads/thr_ALZMb0ORS9uDsQ0qgvmc3g