Copilot Chat (GPT-5) stopped working, shows "message too long" error and disappeared from model list

[GeorgeM89]

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Bug

Body

I was using ChatGPT-5 inside GitHub Copilot Chat and noticed strange behavior.
At first, everything was working, but after a long session of trying different prompts, Copilot suddenly started returning an error saying:

"Your message is too long"

However, my message contained only a couple of words.

Shortly after that, GPT-5 disappeared completely from the list of available models in Copilot Chat, and I could no longer select it.

Steps to Reproduce (not guaranteed):

1. Use GPT-5 in Copilot Chat for an extended period.
2. Send multiple prompts (sometimes long, sometimes short).
3. Eventually, even very short messages trigger a "too long" error.
4. After some time, GPT-5 vanishes from the model list.

Expected behavior:
Short prompts should not trigger a "too long" error, and GPT-5 should remain available as a selectable model.

Actual behavior:

• Error appears incorrectly on very short messages.
• GPT-5 disappears from the model dropdown entirely.

Environment:

• GitHub Copilot Chat via web

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[makspopovsk111]

Yesterday gpt 5 disappeared from copilot for me too

[majcek210]

You can try to relogin , clear cache or reload a chat, if it doesnt work its probably a github issue.
Hope this helps!