Merge Request with non-existent target branch shows no diffs and provides poor error messaging

Description

When a merge request is created targeting a branch that doesn't exist, the MR shows no diffs and provides confusing error states without clearly indicating the root cause.

Steps to Reproduce

  1. Have a project with default branch master that has commits
  2. Create a merge request via API with source_branch: feature/my-feature and target_branch: main (where main doesn't exist)
  3. The MR is created successfully but shows:
    • "diff_refs": null
    • "has_conflicts": true
    • "changes_count": null
    • No diffs visible in UI
    • No clear indication that the target branch doesn't exist

Expected Behavior

  • The API should validate that the target branch exists and return a clear error (400 Bad Request) if it doesn't
  • If the MR is somehow created, the UI should clearly indicate "Target branch 'main' does not exist"
  • GitLab Duo Chat/Agents should be aware of the project's default branch and prevent targeting non-existent branches

Actual Behavior

  • MR is created successfully despite targeting a non-existent branch
  • No clear error message about missing target branch
  • User must manually discover the issue and change target branch
  • The MR appears broken with no explanation

Workaround

Update the MR to target an existing branch (e.g., master)

Impact

  • Confusing for GitLab Duo Chat users who may not know the project's branch structure
  • Wastes time debugging what appears to be a GitLab bug
  • Poor API validation allows invalid state

Environment

  • GitLab Version: 18.10.0 (GitLab.com SaaS)
  • API Version: v4

Additional Context

Issue text was generated by GitLab Duo Chat (Claude Sonnet 4.5 - Anthropic model)

Edited by 🤖 GitLab Bot 🤖