#Fine-tuned GPT-4o model output quality severely degraded since December 16, 2025

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wet skyBOT
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Reported by @full depot

Bug Report: Fine-tuned GPT-4o model output quality severely degraded since December 16, 2025
`Steps to Reproduce`

Call fine-tuned GPT-4o model (ft:gpt-4o-xxxx) via Chat Completions API
Send the same prompts that worked correctly until December 15, 2025
Observe model responses

`Expected Result`

Model should return properly structured JSON output with all required fields (sentence_id, translation, slashes, grammar_list) populated with meaningful content, as it did consistently before December 16, 2025.

`Actual Result`

Model outputs contain corrupted tokens:

  • Random "22" inserted throughout JSON keys and values (e.g., "22words", "22mot22ives", "2222word_idx")
  • Type errors: word_idx returns "false" (boolean) instead of integer
  • Structure collapse: "words" field returns ";" (string) instead of array
  • Nonsensical values: word field contains ";" instead of actual words

This pattern suggests token generation corruption, not just content quality degradation. The model appears to be hallucinating random numeric

`Environment`

api call

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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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