### 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: <token>" \ "https://gitlab.com/api/v4/projects/:id" | \ jq '.remove_source_branch_after_merge') curl --request POST \ --header "PRIVATE-TOKEN: <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 <!-- This bug happens on GitLab.com --> #### Results of GitLab environment info <details> <summary>Expand for output related to GitLab environment info</summary> <pre> Reproduced on GitLab.com (SaaS) GitLab version: Latest (as of 2026-01-23) API version: v4 </pre> </details> #### Results of GitLab application Check <details> <summary>Expand for output related to the GitLab application check</summary> <pre> N/A - This issue is on GitLab.com </pre> </details> ### 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](https://gitlab.com/gitlab-org/gitlab/-/issues/18283) - Original feature for configurable defaults - [MR !29787](https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/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.