#"GPT-5.1 Thinking" forces fallback to "gpt-4o-mini" on all prompts (Account Routing Error)

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Reported by @spark blade

Bug Report: "GPT-5.1 Thinking" forces fallback to "gpt-4o-mini" on all prompts (Account Routing Error)
`Steps to Reproduce`

Steps to Reproduce *

  1. Select "ChatGPT 5.1 Thinking (Extended)" model in the UI.
  2. Send a complex prompt (e.g., Riemann Hypothesis proof) OR an identity check: "Please only answer with the exact name of the model you are currently using."
  3. Observe whether the "Thinking" chain initiates and check the output content.
`Expected Result`

The UI should show the "Thinking" process. The model should engage in deep reasoning (Chain of Thought). For the identity check, it should output gpt-5.1-thinking.

`Actual Result`

The model returns a response instantly with zero thinking steps. It self-identifies as gpt-4o-mini and provides shallow, low-quality answers. This happens 100% of the time regardless of prompt complexity, indicating a backend routing failure rather than adaptive behavior.

`Environment`

All Platforms (Web/Chrome/Safari/Edge, iOS Native App). Issue persists across different networks and after full cache/cookie reset.

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Additional Information

Please provide relevant details to help resolve the issue, such as:

  • ChatGPT Shared Link (if applicable).
  • Screenshots or videos demonstrating the problem.

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