feat(ci): wake up PRs targeting merge-train branches after merge

.claude/skills/merge-train-infra/SKILL.md

@@ -0,0 +1,126 @@
+---
+name: merge-train-infra
+description: Reference for merge-train automation internals -- workflows, scripts, CI integration, and configuration. Use when modifying or debugging merge-train infrastructure.
+---
+
+# Merge-Train Infrastructure
+
+This skill covers the automation internals of the merge-train system. For contributor-facing guidance (creating PRs, labels, handling failures), see the `merge-trains` skill.
+
+## Automation Lifecycle
+
+The merge-train system is fully automated via GitHub Actions in `.github/workflows/merge-train-*.yml`:
+
+1. **PR Creation** (`merge-train-create-pr.yml`): Triggered on push to `merge-train/*` branches. Creates a PR targeting `next` with the `ci-no-squash` label (and `ci-full-no-test-cache` for spartan). Skips merge commits and commits already in `next`.
+
+2. **Body Updates** (`merge-train-update-pr-body.yml`): Triggered on push to `merge-train/**`. Updates the PR body with meaningful commits (those containing PR references like `(#1234)`). The body uses `BEGIN_COMMIT_OVERRIDE` / `END_COMMIT_OVERRIDE` markers for release-please.
+
+3. **Next Integration** (`merge-train-next-to-branches.yml`): Triggered on push to `next`. Merges `next` into each active merge-train branch via `scripts/merge-train/merge-next.sh`. Uses `continue-on-error: true` so a conflict in one branch does not block others. Skips branches whose PR already has auto-merge enabled.
+
+4. **Auto-Merge** (`merge-train-auto-merge.yml`): Runs hourly via cron (`0 * * * *`). Calls `scripts/merge-train/auto-merge.sh` for both merge-train (4-hour inactivity) and backport-train (8-hour inactivity) branches. Uses separate GitHub tokens: `AZTEC_BOT_GITHUB_TOKEN` for API calls and `MERGE_TRAIN_GITHUB_TOKEN` for approvals. Will not auto-merge if the last merge-queue CI run failed or was cancelled.
+
+5. **Recreation & Wakeup** (`merge-train-recreate.yml`): Triggered when a PR is closed (merged). If the merged PR's head branch starts with `merge-train/`, recreates the branch from the base branch (usually `next`). Then runs `scripts/merge-train/wakeup-prs.sh` to add the `ci-wakeup-pr-after-merge` label to all open PRs targeting the branch that have passed CI and have automerge enabled. This triggers a CI re-run (typically a no-op via tree-hash cache) so those PRs can proceed through the merge queue. The label is immediately removed by a step in `ci3.yml` so it can be re-applied on subsequent merges.
+
+6. **Failure Notification** (`merge-queue-dequeue-notify.yml`): Triggered when a PR is dequeued from the merge queue. If the PR's head branch starts with `merge-train/` and the PR was NOT merged, sends a Slack notification via `ci3/merge_train_failure_slack_notify`.
+
+## CI Integration Details
+
+### CI Mode Selection (`.github/ci3_labels_to_env.sh`)
+
+Merge-train branches influence CI mode:
+- `merge_group` events or `ci-merge-queue` label → `merge-queue` mode
+- If the merge-group event is for `merge-train/spartan` → upgraded to `merge-queue-heavy` mode (10 parallel grind runs instead of 4)
+- Target branch `merge-train/docs` → `ci-docs` mode
+- Target branch `merge-train/barretenberg` → `ci-barretenberg` mode
+
+### CI Concurrency (`.github/workflows/ci3.yml`)
+
+```yaml
+group: ci3-${{ (startsWith(github.event.pull_request.head.ref, 'merge-train/') && github.run_id) || ... }}
+```
+
+Merge-train PRs get **full concurrency** (each run has its own unique group via `github.run_id`), while non-merge-train PRs share a group by branch name with cancel-in-progress.
+
+### Instance Postfix (`.github/ci3.sh`)
+
+```bash
+if [[ "${PR_HEAD_REF:-}" == merge-train/* ]]; then
+    export INSTANCE_POSTFIX=${PR_COMMITS:-}
+fi
+```
+
+Merge-train PRs get a unique instance postfix (commit count) to allow parallel EC2 instances.
+
+### CI Modes in bootstrap.sh
+
+- `ci-docs`: Only builds and tests documentation
+- `ci-barretenberg`: Only builds and tests barretenberg (AVM disabled)
+- `ci-barretenberg-full`: Full barretenberg CI including acir_tests
+- `merge-queue`: 4x AMD64 full + 1x ARM64 fast in parallel
+- `merge-queue-heavy`: 10x AMD64 full + 1x ARM64 fast in parallel (used for `merge-train/spartan`)
+
+### Test History Tracking (`ci3/run_test_cmd`)
+
+```bash
+if [[ "$is_merge_queue" -eq 1 || ("${TARGET_BRANCH:-}" =~ ^v[0-9]) || ("${TARGET_BRANCH:-}" == merge-train/*) ]]; then
+    track_test_history=1
+fi
+```
+
+### Failure Notification (`ci3/bootstrap_ec2`)
+
+When a CI run fails on an EC2 instance, it calls `merge_train_failure_slack_notify` to send failure notifications to the appropriate Slack channel based on the branch name.
+
+## Creating a New Merge Train
+
+1. Create a branch from `next` with naming pattern `merge-train/{team}`
+2. Add the branch to the matrix in `.github/workflows/merge-train-next-to-branches.yml`
+3. Add the branch-to-Slack-channel mapping in `ci3/merge_train_failure_slack_notify`
+4. Optionally add CI mode overrides in `.github/ci3_labels_to_env.sh` and `bootstrap.sh`
+5. Push code to the branch -- automation handles PR creation from there
+
+## Key Files Reference
+
+### Workflows
+
+| File | Purpose |
+|---|---|
+| `.github/workflows/merge-train-readme.md` | User-facing documentation |
+| `.github/workflows/merge-train-create-pr.yml` | Auto-creates PRs for train branches |
+| `.github/workflows/merge-train-auto-merge.yml` | Hourly cron to auto-merge inactive trains |
+| `.github/workflows/merge-train-next-to-branches.yml` | Syncs `next` into all train branches; defines active branches |
+| `.github/workflows/merge-train-recreate.yml` | Recreates branch after merge |
+| `.github/workflows/merge-train-update-pr-body.yml` | Updates PR body with commit list |
+| `.github/workflows/merge-queue-dequeue-notify.yml` | Slack notification on merge-queue dequeue |
+| `.github/workflows/squashed-pr-check.yml` | Squash enforcement (skipped for `ci-no-squash`) |
+
+### Scripts
+
+| File | Purpose |
+|---|---|
+| `scripts/merge-train/auto-merge.sh` | Auto-merge logic -- checks inactivity, last CI status, approves and merges |
+| `scripts/merge-train/merge-next.sh` | Merges `next` into a train branch, handles conflicts, cancels stale CI runs |
+| `scripts/merge-train/update-pr-body.sh` | Updates PR body with meaningful commits |
+| `scripts/merge-train/squash-pr.sh` | Squashes PR commits (used by `ci-squash-and-merge` label) |
+| `scripts/merge-train/wakeup-prs.sh` | Adds `ci-wakeup-pr-after-merge` label to qualifying PRs after branch recreation |
+
+### CI Configuration
+
+| File | Purpose |
+|---|---|
+| `.github/ci3_labels_to_env.sh` | CI mode selection based on labels and target branches |
+| `.github/ci3.sh` | Instance postfix for merge-train parallelism |
+| `ci3/merge_train_failure_slack_notify` | Slack failure notification with branch-to-channel mapping |
+| `ci3/run_test_cmd` | Test history tracking for merge-train branches |
+| `ci3/bootstrap_ec2` | EC2 failure notification trigger |
+| `bootstrap.sh` | CI mode definitions (`ci-docs`, `ci-barretenberg`, etc.) |
+
+### Other Scripts
+
+| File | Purpose |
+|---|---|
+| `scripts/auto_close_issues.py` | Auto-closes issues referenced in merged merge-train PRs (GitHub's native auto-close doesn't work for intermediate branches) |
+| `scripts/find_orphaned_issues_in_prs.py` | Finds PRs in merge-train commits that reference still-open issues |
+| `scripts/dedupe_release_notes.py` | Deduplicates release notes from merge-train merges |
+| `scripts/commits` | Pretty git log that groups merge-train children by subsystem |
+| `scripts/filter_history` | Filters git history, identifying merge-train merge commits as "containers" |

.claude/skills/merge-trains/SKILL.md

@@ -0,0 +1,82 @@
+---
+name: merge-trains
+description: Guide for working with merge-train branches -- creating PRs, choosing the right base branch, understanding labels, handling failures, and bypassing checks.
+---
+
+# Working with Merge Trains
+
+## What Is a Merge Train?
+
+A merge train is an automated batching system (inspired by [Rust rollups](https://forge.rust-lang.org/release/rollups.html)) that groups multiple PRs together for coordinated integration into the `next` branch. Instead of each PR going through the merge queue individually, teams push their PRs into a shared `merge-train/*` branch. Periodically, that branch is merged as a single unit into `next`.
+
+## Active Merge-Train Branches
+
+| Branch | Team / Domain | Slack Channel |
+|---|---|---|
+| `merge-train/avm` | AVM, barretenberg vm2 folder | `#team-bonobos` |
+| `merge-train/barretenberg` | Barretenberg folder, but not vm2 folder | `#honk-team` |
+| `merge-train/ci` | CI infrastructure / ci3 | `#help-ci` |
+| `merge-train/docs` | Documentation | `#dev-rels` |
+| `merge-train/fairies` | aztec-nr | `#team-fairies` |
+| `merge-train/spartan` | Spartan / infra / yarn-project sequencer and prover orchestration | `#team-alpha` |
+
+## How to Use a Merge Train
+
+### Targeting a Merge Train with Your PR
+
+1. Create your feature branch **off the appropriate merge-train branch** (not our default branch `next`).
+2. Open your PR targeting that merge-train branch (e.g., base: `merge-train/barretenberg`).
+3. When your PR is approved and merged, it gets squashed into the merge-train branch.
+4. The merge-train PR (which targets `next`) automatically accumulates your commit.
+
+### Key Rules for Contributors
+
+- **Base branch matters**: Always branch from the branch specified in the CI_BASE_BRANCH environment variable. If it is not set, then ask the user their intent and offer to set CI_BASE_BRANCH in their shell's RC file. 
+- **Your PR is squashed into the train**: Individual PRs targeting a merge-train branch are squash-merged as usual. You should not use the merge commit merge method, but the squash method.
+- **The train itself is NOT squashed**: The merge-train PR (e.g., `merge-train/barretenberg` -> `next`) is merged with a **merge commit**, preserving the individual squashed commits. This is why the `ci-no-squash` label is automatically applied.
+- **You generally don't need to worry about the train PR itself** -- it is fully automated (creation, body updates, approval, merge, and recreation). You only need to pay attention to it if an alert is sent to your team channel.
+
+## CI Behavior for Merge Trains
+
+- **Specialized CI modes**: PRs targeting `merge-train/docs` run docs-only CI. PRs targeting `merge-train/barretenberg` run barretenberg-only CI. This avoids running the full test suite for domain-specific changes.
+- **Merge-queue mode**: When the merge-train PR enters GitHub's merge queue, it runs the full `merge-queue` CI mode (4 parallel grind runs on AMD64 + 1 ARM64). `merge-train/spartan` uses the heavier `merge-queue-heavy` mode (10 grind runs).
+- **Full concurrency**: Merge-train PRs get unique CI concurrency groups (using `github.run_id`), so multiple CI runs can proceed in parallel without cancelling each other.
+- **Test history tracking**: Test results are tracked for merge-train PRs, same as merge-queue runs.
+
+## Handling Merge-Train Failures
+
+### When CI Fails on the Merge-Train PR
+
+Two options from the [merge-train-readme.md](https://github.com/AztecProtocol/aztec-packages/blob/next/.github/workflows/merge-train-readme.md):
+
+**Option 1: Direct Fix** -- Push a fix directly to the merge-train branch. Use bypass merge to expedite (all users have this permission). You can use the ci-skip label to no-op CI if really necessary.
+
+**Option 2: Fix in Next** -- Merge a revert or workaround into `next`. The fix will auto-propagate to the merge-train via the `merge-train-next-to-branches` workflow. Best when the root cause is in `next` or multiple trains are affected.
+
+### When Auto-Merge Is Blocked
+
+The auto-merge script will **not** enable auto-merge if the last merge-queue CI run for the PR concluded with `failure` or `cancelled`. Someone needs to either fix the issue and push, or force-merge.
+
+### Merge Conflicts from Next
+
+When merging `next` into a train branch causes conflicts, the `merge-next.sh` script:
+- Aborts the merge
+- Posts a comment on the latest `next` commit listing the conflicted files
+- The team must manually resolve conflicts on their train branch
+
+## Bypassing Checks / Force-Merging
+
+If the user needs to bypass CI checks for their merge-train PR (e.g., a known flaky failure, an urgent merge, or CI infrastructure issues):
+
+1. **Confirm intent**: Always confirm with the user that they want to skip CI, since this merges untested code into `next`.
+2. **Add the `ci-skip` label**: Apply the `ci-skip` label to the merge-train PR. This causes CI to skip entirely. Use: `gh pr edit <PR_NUMBER> --add-label ci-skip`
+3. **Force merge in the UI**: The user can then use GitHub's "Merge without waiting for requirements to be met" button (bypass merge) in the PR UI. All users have this permission. 
+
+**Important**: Only do this when the user explicitly asks to bypass checks. Always confirm first since it skips all CI validation.
+
+## Backport Trains
+
+A related system exists for backport branches (`backport-to-*`). These use the same auto-merge mechanism but with different settings:
+- Branch pattern: `backport-to-`
+- Inactivity threshold: 8 hours (instead of 4)
+- Merge strategy: merge commit

.github/workflows/ci3.yml

@@ -44,6 +44,12 @@ jobs:
     if: github.event.pull_request.head.repo.fork != true && (github.event.pull_request.draft == false || contains(github.event.pull_request.labels.*.name, 'ci-draft'))
     environment: ${{ startsWith(github.ref, 'refs/tags/v') && 'master' || '' }}
     steps:
+      - name: Remove wakeup label
+        if: contains(github.event.pull_request.labels.*.name, 'ci-wakeup-pr-after-merge')
+        env:
+          GH_TOKEN: ${{ secrets.AZTEC_BOT_GITHUB_TOKEN }}
+        run: gh pr edit ${{ github.event.pull_request.number }} --remove-label ci-wakeup-pr-after-merge --repo ${{ github.repository }} || true
+
       - name: Checkout
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
         with:

.github/workflows/merge-train-next-to-branches.yml

@@ -16,6 +16,7 @@ jobs:
           - merge-train/barretenberg
           - merge-train/ci
           - merge-train/docs
+          - merge-train/fairies
           - merge-train/spartan
     steps:
       - name: Checkout

.github/workflows/merge-train-readme.md

@@ -1,49 +1,20 @@
 # Merge Train Documentation
 
-## Overview
+## Skills
 
-The merge train is an automated system for managing pull requests in a coordinated queue. It helps prevent merge conflicts and ensures smooth integration of multiple PRs targeting the same base branch.
-Inspired by [rust rollups](https://forge.rust-lang.org/release/rollups.html), but that's a non-ideal name for our domain.
+The merge-train system is documented via Claude Code skills in `.claude/skills/`. These are structured markdown files designed to be consumed by LLMs (such as Claude) to answer interactive questions about the system.
 
-## How It Works
+| Skill | Description |
+|---|---|
+| [`merge-trains`](../../.claude/skills/merge-trains/SKILL.md) | Contributor-facing guide -- creating PRs, choosing the right base branch, understanding labels, CI behavior, handling failures, and bypassing checks. |
+| [`merge-train-infra`](../../.claude/skills/merge-train-infra/SKILL.md) | Infrastructure reference -- workflows, scripts, CI integration, configuration, and how to create a new merge train. |
 
-1. **Merge Train Branches**: Special branches prefixed with `merge-train/` (e.g., `merge-train/barretenberg`, `merge-train/docs`)
-2. **Automatic Recreation**: When a merge train PR is merged, the branch is automatically recreated with an empty commit
-3. **PR Body Updates**: The merge train PR body is automatically updated with a list of commits whenever changes are pushed
-4. **Auto-merge**: PRs that have been inactive for 4+ hours are automatically approved and merged (if they contain meaningful commits)
-5. **Next Branch Integration**: Changes from the `next` branch are automatically merged into active merge train branches
+### Using the Skills
 
-## Using the Merge Train
+Ask Claude Code questions like:
+- "How do I add my PR to a merge train?"
+- "What happens when a merge-train CI run fails?"
+- "How do I create a new merge-train branch?"
+- "How does the auto-merge system work?"
 
-### Creating a New Merge-Train
-
-1. **Create Branch**: Fork from `next` with naming pattern `merge-train/{team}`
-2. **Enable Auto-Sync**: Add your branch to `.github/workflows/merge-train-next-to-branches.yml`
-3. **Open PR**: Create a PR from your merge-train branch to `next` - automation handles the rest
-
-### Adding Your PR to the Merge Train
-
-1. Create your feature branch and make your changes
-2. Instead of targeting `master` or `next` directly, target the appropriate merge train branch (e.g., `merge-train/barretenberg`)
-3. When your PR merges the changes will be included in the next merge train cycle
-
-### Merge Train Lifecycle
-
-1. **Creation**: A merge train PR is created automatically when changes are pushed to the branch and labeled with `ci-no-squash`.
-2. **Accumulation**: Feature PRs are merged into the merge train branch, squashed
-3. **Auto-merge**: After 4 hours of inactivity (with meaningful commits), the train is automatically merged with a merge-commit (not a squash)
-4. **Recreation**: The cycle starts again with a new merge train
-
-## Handling Merge Failures
-
-When a merge-train fails due to issues from `next`:
-
-### Option 1: Direct Fix
-- Push a fix directly to the merge-train branch
-- Use bypass merge to expedite (all users have this permission)
-- Best for small, quick fixes
-
-### Option 2: Fix in Next
-- Merge a revert or workaround into `next`
-- The fix will auto-propagate to merge-train via automation
-- Best when key assumptions are broken or multiple trains affected
+Claude will load the relevant skill and provide contextual answers based on the actual automation code.

.github/workflows/merge-train-recreate.yml

@@ -36,3 +36,10 @@ jobs:
           # This will generally only succeed if the branch does not exist, as a fallback if the branch is unprotected and gets deleted.
           git push origin "$MERGE_TRAIN_BRANCH"
 
+      - name: Wake up PRs targeting this branch
+        env:
+          GH_TOKEN: ${{ secrets.AZTEC_BOT_GITHUB_TOKEN }}
+        run: |
+          MERGE_TRAIN_BRANCH="${{ github.event.pull_request.head.ref }}"
+          ./scripts/merge-train/wakeup-prs.sh "$MERGE_TRAIN_BRANCH"
+

ci3/merge_train_failure_slack_notify

@@ -27,6 +27,8 @@ elif [[ "$REF_NAME" == "merge-train/ci" ]]; then
   channel="#help-ci"
 elif [[ "$REF_NAME" == "merge-train/docs" ]]; then
   channel="#dev-rels"
+elif [[ "$REF_NAME" == "merge-train/fairies" ]]; then
+  channel="#team-fairies"
 elif [[ "$REF_NAME" == "merge-train/spartan" ]]; then
   channel="#team-alpha"
 else