GitLab Duo summarize code changes endlessly loads when source branch is identical to target branch

Summary

When attempting to generate a merge request's description by summarising code changes with GitLab Duo, if the source branch is identical to target branch (no changes in MR), then the summarize code changes button endlessly loads with no feedback to the user.

Steps to reproduce

1. Create a new project in a group/instance with Duo Enterprise
2. Create a new branch from main/master
3. Push the new branch with no changes
4. Create a new MR with the new branch as the source and main/master as the target
5. Select Summarize code changes
6. Notice how the button endlessly loads and no description is generated.

Example Project

I have a private project where I can invite GitLab team members to demonstrate this in action

What is the current bug behavior?

Summarize code changes with GitLab Duo button endlessly loads when source and target branch content is the same. There is no feedback for the user that they match, which can cause the impression that Duo is in a "hung" state and is not functioning as expected.

What is the expected correct behavior?

The user should receive some feedback from Duo to indicate that the branches match and that it is unable to summarise the content.

Relevant logs and/or screenshots

Bad Example - Source and Target branch match

duo-summarise-bad-example

Good Example - Source and Target branch differ

duo-summarise-good-example

Output of checks

This bug happens on GitLab.com and Self-Managed 18.4.2

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
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

If the bug fix needs to be backported in a patch release to a version under the maintenance policy, please follow the steps on the patch release runbook for GitLab engineers.

Refer to the internal "Release Information" dashboard for information about the next patch release, including the targeted versions, expected release date, and current status.

High-severity bug remediation

To remediate high-severity issues requiring an internal release for single-tenant SaaS instances, refer to the internal release process for engineers.