GPT-6 models are sent max_tokens and rejected (not treated as reasoning models) #1605

Issue

GPT-6 deployments, such as gpt-6-astra, fail with a 400 whenever SimpleChat sets a response length:

Unsupported parameter: 'max_tokens' is not supported with this model. Use 'max_completion_tokens' instead.

This affects:

• every workflow model call once the model's token limits are configured, because the workflow budget always sets a response length;
• chat when Response Length is set.

Found on a SimpleChat POC deployment running the React v2 branch.

Steps to Reproduce

1. Add a GPT-6 deployment, such as gpt-6-astra, as a model and configure its token limits (context window, output limit, total-generation accounting).
2. Run a workflow whose agent uses that model. Alternatively, chat with it with Response Length set.

Expected Behavior

SimpleChat sends max_completion_tokens and no temperature, as it does for GPT-5 and o-series models.

Actual Behavior

ModelEndpointBehavior.is_openai_reasoning_model in application/single_app/model_endpoint_clients.py recognizes only gpt-5, o1, o3 and o4 names (OPENAI_REASONING_MODEL_PREFIXES). GPT-6 deployments get max_tokens and a temperature, and the request is rejected. The same check exists on Development, main and the React v2 branch.

Impact

Workflows that use a GPT-6 model fail on their first model call. Prompt variables, content screening and orchestration share the same policy, so they send GPT-6 the same wrong parameters.

Notes

• Fix: recognize gpt-6 like gpt-5, as a name prefix and as a family name inside deployment or display names. Add a regression test for the GPT-6 names and the workflow budget's parameter choice.
• Already fixed on the React v2 branch in 0.261.219 (test_gpt6_reasoning_model_parameters.py, docs/explanation/fixes/GPT6_REASONING_MODEL_PARAMETERS_FIX.md). Needs porting to Development.