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[Bug] Error with "thinking or redacted_thinking" tokens not being processable with Sonnet 5.

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
38/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Quiet
Tech stack
java
Domain
devtools

Research direction

Start by inspecting the logs referenced in the server error and compare the failing Sonnet 5 session with the successful Sonnet 4.6 continuation. Determine where the thinking or redacted_thinking content becomes unprocessable; done means the session can continue with Sonnet 5 without the 400 error.

Written by the indexing model from the issue text.

Description

bug

Environment

  • OS: Windows 11 Pro
  • Eclipse Version: Version: 2026-06 (4.40.0) / Build id: 20260604-0653
  • Plugin Version: GitHub Copilot GitHub Copilot 0.20.0.202607100822 com.microsoft.copilot.eclipse.feature

Describe the bug
While working on Sonnet 5, around the time i reach half of the max context (the context window icon shows half-circle full), i regularly get this message:

Sorry, an error occurred while generating a response. Details: unhandled status from server: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.1.content.1: thinking or redacted_thinking blocks in the latest assistant messa... Read more from logs.

To Reproduce
This happens consistently on Sonnet 5 that i use often, around the time where the context seems above 50% of max size. I definitely did not see that with any other models, but i also do not use other models often enough. To "fix", i switch to Sonnet 4.6 to continue my task and that works, but i can never get back to Sonnet 5 short of a whole new session. This happens no matter the "thinking" mode (low/high) i have used. This happens frequently, but not always under the "50%" condition i noted. I cannot reproduce this with a fixed set of steps.

Expected behavior
A session using Sonnet 5 shouldn't die this way.

Screenshots
The first screenshot is of the errors, then the second shows after i switched to Sonnet 4.6 to continue.

Image Image

Additional context
N/A

Dominant language
Java
Stars
125
Forks
63
Avg merge
1d 3h
Merged PRs (30d)
5

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