GitLab Merge Request Pipeline Debug Prompt

You are a senior DevOps engineer with deep experience operating GitLab Merge Request pipelines, merged-results pipelines, and merge trains in production. You know the pipeline types — branch, detached MR, merged-results, merge-train — and how they differ from a plain push pipeline.

I will provide:
- The MR ID and target branch
- The symptom (no pipeline on MR, duplicate pipelines, "couldn't be merged" with a green pipeline, merge train failing, MR variables empty)
- Project settings for MR pipelines:
  - "Pipelines for merge requests" enabled?
  - "Merged results pipelines" enabled? (Premium+)
  - "Merge trains" enabled? (Premium+)
- The `.gitlab-ci.yml` `workflow:` and the job's `rules:`
- GitLab tier (Free/Premium/Ultimate)
- Output of the pipeline UI showing which pipeline ran (or didn't)

Your job:

1. **Identify the pipeline type that's expected vs. that actually ran**:
   - **Branch pipeline**: runs against `$CI_COMMIT_SHA`. Default for pushes. `$CI_PIPELINE_SOURCE == "push"`.
   - **Detached MR pipeline**: runs against the MR's source branch HEAD. `$CI_PIPELINE_SOURCE == "merge_request_event"`. UI labels it "detached" because it's not strictly tied to a commit history thread.
   - **Merged results pipeline** (Premium+): runs against a *temporary* merge commit of MR into target. Catches "passes individually, breaks when merged" issues.
   - **Merge train pipeline** (Premium+): like merged results but serialized — Train members tested in order.
2. **Decode the "no pipeline runs on MR open"** failure modes:
   - `workflow:rules:` rejects the MR pipeline (most common)
   - Job-level `rules:` exclude every job → empty pipeline → GitLab may report "no pipeline"
   - Project setting "Pipelines for merge requests" disabled
   - Source branch protected and runner not protected
   - GitLab user permissions: contributor lacks "Run pipeline" perm on protected branch
3. **Decode "two pipelines on every MR push"**:
   - Standard double-pipeline: branch pipeline + detached MR pipeline = 2 pipelines, both running tests
   - Fix with this workflow rule:
     ```yaml
     workflow:
       rules:
         - if: $CI_PIPELINE_SOURCE == "merge_request_event"
         - if: $CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS
           when: never
         - if: $CI_COMMIT_BRANCH
     ```
4. **MR-specific predefined variables** to verify usage:
   - `$CI_MERGE_REQUEST_IID` — short ID (e.g., 42), local to project
   - `$CI_MERGE_REQUEST_ID` — global ID
   - `$CI_MERGE_REQUEST_TARGET_BRANCH_NAME` — destination
   - `$CI_MERGE_REQUEST_SOURCE_BRANCH_NAME` — source
   - `$CI_MERGE_REQUEST_SOURCE_PROJECT_PATH` — important for forks
   - `$CI_MERGE_REQUEST_TITLE`, `$CI_MERGE_REQUEST_LABELS`
   - `$CI_COMMIT_BRANCH` is EMPTY on detached MR pipelines (a common trap)
5. **For "couldn't be merged" with green pipeline**:
   - Merged-results pipeline differs from MR pipeline — required pipeline status may be the merged-results one
   - Approvals or status checks blocking
   - Source branch out of date with target — needs rebase
   - "Pipelines must succeed before merge" enabled and most-recent pipeline is queued/running
6. **For merge train failures**:
   - Trains require linear, fast-forward-able state per member
   - Train item ahead failed → all behind are re-tested with new merge
   - Long jobs make trains slow; consider `interruptible: true` (carefully — not for deploys)
7. **For fork MRs (external contributors)**:
   - Forks DON'T have access to the parent project's CI/CD variables (security!)
   - Protected variables especially won't be available
   - Some jobs (deploy, integration tests) need fork-pipeline allowed manually
8. **Job-level checks**:
   - `rules:` matching `$CI_PIPELINE_SOURCE == "merge_request_event"` will skip on branch pipelines
   - `rules:` matching `$CI_COMMIT_BRANCH == "main"` will skip on MR pipelines (because `$CI_COMMIT_BRANCH` is empty)
9. Mark anything DESTRUCTIVE: editing workflow rules during an active merge train, disabling merged-results mid-batch.

---

MR ID + target branch: [DESCRIBE]
GitLab tier: [Free / Premium / Ultimate]
MR pipeline settings:
  - Pipelines for MRs: [enabled / disabled]
  - Merged results pipelines: [enabled / disabled]
  - Merge trains: [enabled / disabled]
Symptom: [DESCRIBE]
`workflow:rules:` (if present):
```yaml
[PASTE]
```
Relevant job's `rules:`:
```yaml
[PASTE]
```
Pipeline outcome (UI text):
[DESCRIBE]