Classic Duo Chat does not complete the output

Summary

When using for example /test prompt in Duo Chat (classic) to generate tests for a highlighted C code selection in the VSCode editor or passing the code as context in Duo Chat, the returned output starts but stops before it finishes. This results in unfinished code being presented to the user, regardless of where he/she is using the Duo Chat (classic).

Happens regardless gitlab SaaS or Self Managed, and regardless which version of GitLab used. Also regardless if used in Web IDE or in Desktop IDEs of chat (VSCode, VStudio etc.)

What is the current bug behavior?

The output stops before the test code is complete.

What is the expected correct behavior?

Tests should be generated completely

Steps to reproduce

1. open the attached example Csharp file in VSCode
   1. Gugus.cs
2. open the file n the IDE and select the content (CTRL/CMD+A)
   1. in Duo Chat execute the /tests command
3. alternatively, right-click on the selected code > GitLab Duo Chat > Generate tests
4. watch the output where the test code is generated
   1. the output is cut off in the code block but also it was observed that general chat output was cut off

Details

• The problem is that long responses from the LLM seem to get cut off at some point. We can often reproduce it when asking it to generate lots of tests for a code file, but it can happen in other cases as well.
• Customers report seeing this "cut off" issue for text responses as well as code blocks
• This happens in multiple IDEs (VS Code, IntelliJ, Visual Studio, Browser)
• This is happening in many versions of GitLab from at least 17.8 to the latest on GitLab.com
• It is not specific to Windows or Mac
• It happens with many different file types
• It does not seem related to /tests because we can reproduce it with multiple prompts
• We could not reproduce this with Duo Agentic Chat, even when trying to force it to output lots of information to the chat instead of having it write new files.
• This cannot be reproduced in Chat in the GitLab UI. The code answer is contained in a single block though, unlike the IDE answer which seems to break it out by classes.

References / Initial report

• https://gitlab.zendesk.com/agent/tickets/654161
  • Initial report by customer
• https://gitlab.com/gitlab-com/request-for-help/-/issues/3441#note_2807290315
  • Contains video of reproducing it

Relevant Slack Threads:

• https://gitlab.slack.com/archives/C06D5C70MD2/p1759516693249739