API: Merge request creation ignores project's remove_source_branch_after_merge setting

Summary

When creating a merge request via the REST API (POST /projects/:id/merge_requests) without explicitly setting the remove_source_branch parameter, the API does not respect the project's configured remove_source_branch_after_merge setting. Instead, it defaults to false (unchecked), creating inconsistent behavior between UI-created and API-created merge requests.

Steps to reproduce

  1. Configure a project with default "Delete source branch" enabled:

    • Navigate to Settings → General → Merge requests
    • Enable "Enable 'Delete source branch' option by default"
    • Save changes
    • Verify the setting via API: GET /api/v4/projects/:id shows "remove_source_branch_after_merge": true

  2. Create a merge request via the REST API without specifying remove_source_branch parameter: curl --request POST
    --header "PRIVATE-TOKEN: <your_token>"
    --header "Content-Type: application/json"
    --data '{ "source_branch": "test-branch", "target_branch": "main", "title": "Test MR via API" }'
    "https://gitlab.com/api/v4/projects/:id/merge_requests" 3. Check the response and observe: { "force_remove_source_branch": false, "should_remove_source_branch": false } 4. Compare: Create an MR via the GitLab UI in the same project - the "Delete source branch" checkbox is checked by default (respects project setting)

Example Project

This bug can be reproduced on any GitLab.com project where:

  • The project has remove_source_branch_after_merge enabled in settings
  • MRs are created via the REST API without the remove_source_branch parameter

What is the current bug behavior?

When the remove_source_branch parameter is omitted from the API request:

  • The merge request is created with force_remove_source_branch: false
  • The "Delete source branch" checkbox is unchecked in the UI
  • The project's remove_source_branch_after_merge setting is completely ignored
  • API-created MRs behave differently from UI-created MRs in the same project

What is the expected correct behavior?

When the remove_source_branch parameter is not provided in the API request:

  1. The API should check the project's remove_source_branch_after_merge setting
  2. Apply that project default to the newly created merge request
  3. Behave consistently with how the GitLab UI creates merge requests
  4. Only use an explicit value when the remove_source_branch parameter is actually provided in the request

This would make API-created MRs respect project-level configuration, just like UI-created MRs do.

Relevant logs and/or screenshots

API Response showing the issue: { "id": 123456, "iid": 83, "title": "Test MR via API", "source_branch": "test-branch", "target_branch": "main", "force_remove_source_branch": false, "should_remove_source_branch": false, ... }Project setting (confirmed enabled): { "id": 12345, "name": "example-project", "remove_source_branch_after_merge": true, ... }Current workaround (requires extra API call):

Must explicitly fetch project setting and pass it

PROJECT_SETTING=$(curl --header "PRIVATE-TOKEN: "
"https://gitlab.com/api/v4/projects/:id" |
jq '.remove_source_branch_after_merge')

curl --request POST
--header "PRIVATE-TOKEN: "
--data "{ "source_branch": "test-branch", "target_branch": "main", "title": "Test MR", "remove_source_branch": $PROJECT_SETTING }"
"https://gitlab.com/api/v4/projects/:id/merge_requests"### Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info 
Reproduced on GitLab.com (SaaS)
GitLab version: Latest (as of 2026-01-23)
API version: v4

Results of GitLab application Check

Expand for output related to the GitLab application check 
N/A - This issue is on GitLab.com

Possible fixes

The issue likely exists in the merge request creation service. When remove_source_branch is not provided in the API params, the service should:

  1. Fetch the project's remove_source_branch_after_merge attribute
  2. Use it as the default value for force_remove_source_branch
  3. Only override when explicitly provided in the request

Potential location (approximate):

  • lib/api/merge_requests.rb - API endpoint handler
  • app/services/merge_requests/create_service.rb - Service that creates MRs

The fix should ensure that omitting the parameter results in project default behavior, not hardcoded false.

Additional Context

Impact:

  • CI/CD automation tools don't respect team preferences
  • CLI tools (glab, custom scripts, MCP servers) create MRs with wrong defaults
  • Inconsistent UX between UI and API
  • Teams with "delete by default" enabled still accumulate stale branches from API-created MRs

Related:

  • Issue #18283 - Original feature for configurable defaults
  • MR !29787 - Implementation of remove_source_branch_after_merge
  • The project setting was added in GitLab 12.5, but the API may not have been updated to respect it

Patch release information for backports

Not applicable at this stage.

Edited by 🤖 GitLab Bot 🤖