GPT-5.6 Sol Pro in Chat mode reverting to Instant-level routing or reasoning budget

What version of the Codex App are you using (From “About Codex” dialog)?

Version 26.715.31925

What subscription do you have?

Pro 20x

What platform is your computer?

Darwin 25.5.0 arm64 arm

What issue are you seeing?

When I manually select GPT-5.6 Sol Pro in Chat mode within the unified ChatGPT/Codex app, prompts that would ordinarily trigger a substantial Pro reasoning run now complete in approximately 5 seconds.

The resulting answers are materially shallower, less careful, and lower-quality than previous GPT-5.6 Sol Pro responses. They appear behaviorally similar to responses from an Instant model or from a model running with a substantially reduced reasoning budget.

There is no visible error message. The interface continues to display GPT-5.6 Sol Pro as the selected model, but the latency and output quality do not appear consistent with the selected mode.

This behavior is reproducible in both:

the unified desktop app; and

the unified ChatGPT web interface.

It has also been independently reproduced on another ChatGPT Pro 5x account.

I cannot determine from the client whether the wrong model is being selected at inference time or whether GPT-5.6 Sol Pro is receiving an unexpectedly low effective reasoning budget. The observable symptoms are consistent with either:

silent routing or fallback to an Instant model;

incorrect model configuration or metadata;

an unexpectedly reduced reasoning-effort setting;

an account-level or rollout feature-flag problem; or

a regression associated with the recent unification of the ChatGPT and Codex interfaces.

What steps can reproduce the bug?

Selecting "Pro" reasoning effort where the model is GPT-5.6 Sol

What is the expected behavior?

_No response_

Additional information

_No response_