Wrong prompt is used for Custom Agents

Summary

When switching between Duo Agent Chat (default chat) and a custom/foundation agent the wrong prompt is used on the backend

Steps to reproduce

1. Navigate to a project, open a new chat window with Duo Agentic Chat, and ask a simple question - e.g. "Summarize this project"
2. Select a new agent to chat with my clicking the new chat and select an agent such as the Security Analyst Agent
3. Ask the custom agent the prompt "who are you?". It will correctly respond with details of the custom agent.
4. Start a fresh session, which typically would be done by a user wanting to clear the context window and history, again selecting the custom agent
5. Again, ask the agent "who are you". Note that this new session will respond as the normal Duo Agentic Chat agent.
6. to double check the prompt used you can find your session(s) in LangSmith and verify the prompts sent to the model. For the last "Who are you", you will see the default agent prompt

Example Project

What is the current bug behavior?

The wrong Chat Prompt Template appears to be used on the backend

What is the expected correct behavior?

The prompt template associated with the custom agent is used

Relevant logs and/or screenshots

See images embedded above

Output of checks

Results of GitLab environment info

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Results of GitLab application Check

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sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes

Patch release information for backports

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High-severity bug remediation

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