fix: add in missed prompt template for custom agent prompts

Manoj M Jrequested to merge
mmj-fix-custom-agents into main

What does this merge request do and why?

Fix for custom agents not working as expected on 6th Nov 2025. Slack discussion.

Root cause:

We added in a new prompt family y'day with prompts specifically for Sonnet 4.5. However, these prompts missed including the system_template_override override section as part of system prompts, which is essential to place the system prompt provided when creating a custom agent.

All the other prompts (base, vertex) etc already has this section, which is why it used to work until now. But now that Sonnet 4.5 is its own model family, and because it is default model for agentic chat, suddenly the custom agent's prompt is no longer used, the default agentic chat prompt is used in its place, and hence appearance that "custom agents aren't working"

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Merge request checklist

  • Tests added for new functionality. If not, please raise an issue to follow up.
  • Documentation added/updated, if needed.
  • If this change requires executor implementation: verified that issues/MRs exist for both Go executor and Node executor or confirmed that changes are backward-compatible and don't break existing executor functionality.
Edited by Manoj M J

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