diff --git a/README.md b/README.md
index d130119..83409d4 100644
--- a/README.md
+++ b/README.md
@@ -22,6 +22,7 @@ behind a runner contract you control (or pass inline via `run`).
 | `actions/plan`   | Action | Preview an infrastructure change |
 | `actions/apply`  | Action | Apply an infrastructure change |
 | `actions/notify` | Action | Post the pipeline result as a rich Slack card |
+| `actions/enforce` | Action | Merge-time gate — revert a merge whose PR had an empty description and return `block` (+ alert content) so the caller stands its release down |
 
 ## Design
 
@@ -274,6 +275,96 @@ version:
     changelog-on-rc: true
 ```
 
+## enforce — inputs & outputs
+
+Merge-time gate that enforces non-empty PR descriptions. Call it as the **first
+step** of a job at the top of your pipeline, gated to the branch you protect.
+On a push whose merge commit traces to a PR with an empty (or whitespace-only)
+body, it reverts the merge on that branch, edits the PR to explain, and returns
+`block=true`; downstream jobs read `needs.<guard-job>.outputs.block != 'true'`
+and stand down. A PR-fetch API error fails **safe** (skips, never
+false-reverts); a rejected push or a revert conflict keeps `block=true` and
+reports "revert by hand".
+
+It reverts a merge commit via `git revert -m 1` and a squash/rebase merge via
+the `before..HEAD` range, so all three GitHub merge styles are fully undone.
+
+Because it's a composite action, the alert is posted in the **same job**: the
+Slack webhook (typically an environment-scoped secret) resolves inline, so no
+separate output-threaded alert job is needed. It emits the alert content as
+step outputs; feed those directly into a `notify` step in the same job.
+
+### Why a composite action rather than a reusable workflow?
+
+The guard needs to fire before any release/build/deploy job and, when it fires,
+stand those jobs down. Both packaging choices can do that; the composite ships
+with less caller boilerplate:
+
+- **One job, not two.** A reusable-workflow guard forces the caller to add a
+  second `*-notify` job so the environment-scoped Slack webhook resolves. A
+  composite runs inside the caller's job, so the guard and its alert share the
+  same `environment:` block.
+- **Step outputs, not cross-job outputs.** The alert step reads
+  `steps.<id>.outputs.header` directly; no `needs.<x>.outputs.*` plumbing.
+- **One node in the pipeline graph, not two.** The visual dependency tree
+  stays compact.
+
+Same underlying revert/edit script; only the packaging differs.
+
+| Input | Type | Required | Default | Description |
+|-------|------|----------|---------|-------------|
+| `label` | string | no | `Service` | Short name shown in the alert header the caller renders (e.g. `Backend`, `Web`). |
+| `github-token` | string | no | `${{ github.token }}` | Token used to read the PR, push the revert, and edit the PR body. Override with a GitHub-App token for cross-org. |
+
+| Output | Description |
+|--------|-------------|
+| `block` | `'true'` when the merge was reverted and the caller's release must stand down. |
+| `outcome` | Empty when nothing was done; else `reverted` \| `push_failed` \| `conflict`. Gate the notify step on `outcome != ''`. |
+| `header` | Alert header line for the caller's Slack card. |
+| `color` | Alert colour bar (hex) for the caller's Slack card. |
+| `reason` | Alert status line for the caller's Slack card. |
+
+Grant `contents: write` + `pull-requests: write` on the calling job. To surface
+the alert on Slack, add a follow-up step in the same job (the environment-scoped
+webhook resolves directly there) that renders the card via `notify` from the
+guard's outputs. Expose `block` as a job-level output so downstream jobs can
+gate on it.
+
+```yaml
+jobs:
+  guard:
+    if: github.event_name == 'push' && github.ref_name == 'dev'
+    runs-on: ubuntu-latest
+    environment: dev                              # so ${{ secrets.SLACK_WEBHOOK_URL }} resolves
+    permissions: { contents: write, pull-requests: write }
+    outputs:
+      block: ${{ steps.g.outputs.block }}
+    steps:
+      - id: g
+        uses: nurdsoft/ci-workflows/actions/enforce@v3
+        with:
+          label: Backend
+
+      # Alert on Slack when the guard acted; on a clean merge this step skips.
+      # Best-effort — a missing/failed webhook never fails the job.
+      - if: ${{ steps.g.outputs.outcome != '' }}
+        uses: nurdsoft/ci-workflows/actions/notify@v3
+        with:
+          result: failure
+          webhook-url: ${{ secrets.SLACK_WEBHOOK_URL }}
+          label: Backend
+          header: ${{ steps.g.outputs.header }}
+          color:  ${{ steps.g.outputs.color }}
+          status: ${{ steps.g.outputs.reason }}
+
+  release:
+    needs: [guard]
+    # !cancelled() so a skipped guard (e.g. non-dev push) doesn't skip release;
+    # block=true stands it down; a guard that *failed* fails closed.
+    if: ${{ !cancelled() && needs.guard.result != 'failure' && needs.guard.outputs.block != 'true' }}
+    # ...
+```
+
 ## notify — inputs
 
 Posts a rich Slack card for a pipeline result: a green/red header
@@ -287,12 +378,22 @@ local-timezone message timestamp.
 Best-effort: an empty `webhook-url` is a no-op that still succeeds, and a failed
 post never fails the pipeline.
 
+The `header`, `color`, and `status` inputs turn the same card into a general
+**alert** (e.g. a revert or guard notice) instead of a deploy result: supply a
+custom title and color bar, and the first field becomes **Status** instead of
+**Version**. The PR/commit resolution, body, and context line are unchanged, so
+the card still shows the PR (or commit) that `target-sha` traces to. Omit all
+three and the card is exactly the deploy-result card above.
+
 | Input | Required | Default | Description |
 |-------|----------|---------|-------------|
 | `result` | yes | — | `success` / `failure` — e.g. a deploy job's `result`. Anything other than `success` renders as failed. |
 | `webhook-url` | no | `''` | Slack incoming webhook URL. Empty makes the action a no-op. |
 | `label` | no | `Deployment` | Short label for the card header (app / component name). |
 | `version` | no | `''` | Version string shown in the Version field (`n/a` if empty). |
+| `header` | no | `''` | Full header line, verbatim, overriding the default `<emoji> <label> — <status>`. May contain Slack emoji shortcodes. |
+| `color` | no | `''` | Attachment bar color (hex) overriding the result-derived green/red. |
+| `status` | no | `''` | When set, the first field renders as **Status** with this text in place of the **Version** field. |
 | `channel` | no | `slack` | Chat channel; only `slack` is implemented today. |
 | `github-token` | no | `${{ github.token }}` | Token used to read the PR/commit. The default job token covers a same-repo deploy; for a cross-org deploy pass a token minted in the caller from a GitHub App with read access to `target-repo`. |
 | `target-repo` | no | `${{ github.repository }}` | `owner/repo` whose PR/commit the card describes. Override for a cross-repo deploy. |
diff --git a/actions/enforce/action.yml b/actions/enforce/action.yml
new file mode 100644
index 0000000..a7b37f4
--- /dev/null
+++ b/actions/enforce/action.yml
@@ -0,0 +1,169 @@
+name: Enforce PR description
+description: >-
+  Merge-time gate. On a push, resolves the PR behind github.sha and, if its
+  body is empty/whitespace-only, reverts the merge, edits the PR to explain,
+  and returns block=true so the caller can stand its release/build/deploy
+  jobs down (we don't ship code we're reverting). This is the merge-time
+  backstop for enforcing PR descriptions.
+
+  The caller decides when to invoke this action (its own step-level `if:`,
+  e.g. only on protected-branch pushes) and grants the token scopes; the
+  action operates on whatever branch the push targeted (github.ref_name)
+  and resolves the PR from github.sha. Only a GitHub token with contents
+  and pull-requests write scopes is needed (revert + PR edit) — no external
+  secrets to pass, so this is safe to call across an org boundary.
+
+  What gets reverted depends on how the PR landed:
+    - merge commit (>=2 parents) -> `git revert -m 1` undoes the whole PR.
+    - squash (1 commit) / rebase (N commits) -> revert every commit this push
+      introduced (`before..HEAD`), so a multi-commit rebase is fully undone.
+
+  Fail behaviour:
+    - PR-fetch API error -> fail SAFE: skip (block=false), never false-revert.
+    - revert push rejected / conflicts -> block stays true, outcome reports
+      "revert by hand", and the caller (which gates on this action's outputs)
+      stands down.
+
+  Slack: this action does NOT post to Slack. It emits the alert content
+  (outcome/header/color/reason) as outputs; the caller renders the card in
+  a subsequent step within the same job (typically via actions/notify),
+  where any environment-scoped webhook is already resolved.
+
+inputs:
+  label:
+    description: >-
+      Short name shown in the alert output the caller renders
+      (e.g. "Backend", "Web").
+    required: false
+    default: Service
+  github-token:
+    description: >-
+      Token used to read the PR, push the revert, and edit the PR body.
+      Defaults to the calling workflow's github.token; for a cross-org call
+      pass a token minted in the caller from a GitHub App with the
+      corresponding write scopes on the target repo.
+    required: false
+    default: ${{ github.token }}
+
+outputs:
+  block:
+    description: "'true' when the merge was reverted and the caller's release must stand down."
+    value: ${{ steps.check.outputs.block }}
+  outcome:
+    description: "Empty when nothing was done; else reverted | push_failed | conflict."
+    value: ${{ steps.check.outputs.outcome }}
+  header:
+    description: "Alert header text for the caller's Slack card (empty unless the guard acted)."
+    value: ${{ steps.check.outputs.header }}
+  color:
+    description: "Alert colour bar (hex) for the caller's Slack card."
+    value: ${{ steps.check.outputs.color }}
+  reason:
+    description: "Alert status line for the caller's Slack card."
+    value: ${{ steps.check.outputs.reason }}
+
+runs:
+  using: composite
+  steps:
+    - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd  # v6.0.2
+      with:
+        ref: ${{ github.ref_name }}
+        fetch-depth: 0   # full history so the merge commit is revertable
+
+    - id: check
+      shell: bash
+      env:
+        GH_TOKEN:  ${{ inputs.github-token }}
+        REPO:      ${{ github.repository }}
+        SHA:       ${{ github.sha }}
+        BEFORE:    ${{ github.event.before }}
+        BASE_REF:  ${{ github.ref_name }}
+        LABEL:     ${{ inputs.label }}
+      run: |
+        set -euo pipefail
+        git config user.name "github-actions[bot]"
+        git config user.email "github-actions[bot]@users.noreply.github.com"
+
+        block=false
+
+        # Find the PR that introduced this commit. Empty for a direct push
+        # (no PR) — that's branch-protection's job, not ours.
+        pr_json="$(gh api "repos/$REPO/commits/$SHA/pulls" \
+            -H "Accept: application/vnd.github+json" 2>/dev/null || echo '[]')"
+        pr_number="$(jq -r 'map(select(.merged_at != null)) | (.[0].number // empty)' <<<"$pr_json")"
+
+        if [ -z "$pr_number" ]; then
+          echo "No merged PR associated with $SHA (direct push?); nothing to enforce."
+        else
+          # Fetch the PR body in one call. Fail SAFE: if the API call fails
+          # (rate limit, network), do NOT treat it as an empty description —
+          # that would revert a legitimate merge. Skip the guard and proceed.
+          pr="$(gh pr view "$pr_number" --repo "$REPO" --json body 2>/dev/null)" || {
+            echo "::warning::Could not fetch PR #$pr_number; skipping description guard for $SHA."
+            echo "block=false" >> "$GITHUB_OUTPUT"
+            exit 0
+          }
+          pr_body="$(jq -r '.body // ""' <<<"$pr")"
+
+          if [ -n "$(printf '%s' "$pr_body" | tr -d '[:space:]')" ]; then
+            echo "PR #$pr_number has a description; proceeding."
+          else
+            echo "PR #$pr_number merged with an empty description — reverting $SHA."
+            block=true
+            # Persist block early so a later failure in this branch can't fail
+            # open and let the reverted code ship (last write to the file wins).
+            echo "block=true" >> "$GITHUB_OUTPUT"
+
+            # merge commit (>=3 tokens: self + 2 parents) → revert via -m 1;
+            # squash/rebase → revert the whole before..HEAD range.
+            parents="$(git rev-list --parents -n 1 "$SHA" | wc -w)"
+            if [ "$parents" -ge 3 ]; then
+              revert_target="$SHA"; margs="-m 1"
+            else
+              revert_target="${BEFORE}..${SHA}"; margs=""
+            fi
+
+            outcome=""
+            # shellcheck disable=SC2086
+            if git revert --no-edit $margs $revert_target; then
+              if git push origin "HEAD:$BASE_REF"; then
+                outcome="reverted"
+                rsha="$(git rev-parse HEAD)"
+                # Write the revert notice into the PR description itself (not a
+                # comment) so it's visible at the top of the PR. The body was
+                # empty (that's what tripped the guard), so setting it is safe —
+                # nothing to preserve. Editing a merged PR's body is allowed.
+                gh pr edit "$pr_number" --repo "$REPO" \
+                  --body "> ⚠️ **Auto-reverted on \`$BASE_REF\` (${rsha:0:7})** — this PR merged with an empty description, so the release was stood down and the merge reverted. Add a summary of *what* changed and *why*, then open a new PR." || true
+                echo "::warning::Reverted $SHA on $BASE_REF as ${rsha:0:7}; skipping this release."
+              else
+                outcome="push_failed"
+                echo "::error::Revert commit created but push to $BASE_REF was rejected (branch protection?); manual revert needed."
+              fi
+            else
+              git revert --abort || true
+              outcome="conflict"
+              echo "::error::Auto-revert of $SHA conflicts with later commits; manual revert needed."
+            fi
+
+            # Per-outcome alert content, emitted as outputs for the caller to
+            # render (it holds the env-scoped webhook). The shared notify action
+            # resolves the PR from github.sha for the title/link/author, so we
+            # only supply the header, color bar, and status text here.
+            case "$outcome" in
+              reverted)    color="#F9A825"; header=":leftwards_arrow_with_hook: ${LABEL} — merge reverted (empty description)"; reason="Merge reverted on \`${BASE_REF}\` (${rsha:0:7}); release skipped" ;;
+              push_failed) color="#D32F2F"; header=":rotating_light: ${LABEL} — revert blocked (empty description)"; reason="Revert push to \`${BASE_REF}\` rejected (branch protection); revert by hand" ;;
+              conflict)    color="#D32F2F"; header=":rotating_light: ${LABEL} — MANUAL revert needed (empty description)"; reason="Auto-revert conflicted on \`${BASE_REF}\`; revert by hand" ;;
+              *)           color="#D32F2F"; header=":rotating_light: ${LABEL} — empty-description merge"; reason="Revert outcome unknown; check the run" ;;
+            esac
+
+            {
+              echo "outcome=$outcome"
+              echo "color=$color"
+              echo "header=$header"
+              echo "reason=$reason"
+            } >> "$GITHUB_OUTPUT"
+          fi
+        fi
+
+        echo "block=$block" >> "$GITHUB_OUTPUT"
diff --git a/actions/notify/action.yml b/actions/notify/action.yml
index b6f7c36..9439774 100644
--- a/actions/notify/action.yml
+++ b/actions/notify/action.yml
@@ -23,6 +23,26 @@ inputs:
     description: Version string shown in the message.
     required: false
     default: ""
+  header:
+    description: >-
+      Full header line, verbatim. When set, overrides the default
+      "<emoji> <label> — <succeeded|failed>" header — for alert cards that need
+      a custom title (e.g. a revert notice). May contain Slack emoji shortcodes.
+    required: false
+    default: ""
+  color:
+    description: >-
+      Attachment bar color (hex, e.g. "#F9A825"). When set, overrides the
+      result-derived green/red — lets alert callers signal a state that isn't a
+      clean pass/fail.
+    required: false
+    default: ""
+  status:
+    description: >-
+      When set, the first card field renders as "*Status*\n<status>" in place of
+      "*Version*\n<version>". For alert cards where a version is meaningless.
+    required: false
+    default: ""
   channel:
     description: "Chat channel: slack (only slack implemented today)."
     required: false
@@ -58,6 +78,9 @@ runs:
         RESULT: ${{ inputs.result }}
         LABEL: ${{ inputs.label }}
         VERSION: ${{ inputs.version }}
+        HEADER_IN: ${{ inputs.header }}
+        COLOR_IN: ${{ inputs.color }}
+        STATUS_IN: ${{ inputs.status }}
         WEBHOOK: ${{ inputs.webhook-url }}
         TARGET_REPO: ${{ inputs.target-repo }}
         TARGET_SHA: ${{ inputs.target-sha }}
@@ -78,6 +101,18 @@ runs:
         version="${VERSION:-}"; [ -z "$version" ] && version="n/a"
         short_sha="${TARGET_SHA:0:7}"
 
+        # Card chrome. Alert callers can override the header line and color bar,
+        # and swap the first field from Version to Status. With none of these
+        # inputs set, the result-derived defaults reproduce the original card
+        # verbatim, so existing deploy callers are unaffected.
+        header="${HEADER_IN:-${emoji} ${LABEL} — ${status}}"
+        [ -n "${COLOR_IN:-}" ] && color="$COLOR_IN"
+        if [ -n "${STATUS_IN:-}" ]; then
+          field1_label="Status"; field1_value="$STATUS_IN"
+        else
+          field1_label="Version"; field1_value="$version"
+        fi
+
         # Build date in UTC (YYYY.MM.DD), shown on the card so the deploy date is
         # unambiguous rather than inferred from Slack's local-timezone timestamp.
         built_utc="$(date -u +%Y.%m.%d)"
@@ -95,15 +130,16 @@ runs:
           pr_url="$(jq -r '.html_url // ""' <<<"$sel")"
           pr_user="$(jq -r '.user.login // "unknown"' <<<"$sel")"
           payload="$(jq -n \
-            --arg color "$color" --arg header "${emoji} ${LABEL} — ${status}" \
-            --arg version "$version" --arg prlabel "${TARGET_REPO}#${pr_num}" --arg title "$pr_title" \
+            --arg color "$color" --arg header "$header" \
+            --arg f1label "$field1_label" --arg f1value "$field1_value" \
+            --arg prlabel "${TARGET_REPO}#${pr_num}" --arg title "$pr_title" \
             --arg user "$pr_user" --arg runurl "$RUN_URL" --arg prurl "$pr_url" \
             --arg built "$built_utc" '
             def esc: gsub("&";"&amp;") | gsub("<";"&lt;") | gsub(">";"&gt;");
             { attachments: [{ color: $color, blocks: [
                 { type:"header", text:{ type:"plain_text", text:$header, emoji:true } },
                 { type:"section", fields:[
-                    { type:"mrkdwn", text:("*Version*\n" + $version) },
+                    { type:"mrkdwn", text:("*" + $f1label + "*\n" + $f1value) },
                     { type:"mrkdwn", text:("*PR*\n" + (if $prurl != "" then "<" + $prurl + "|" + $prlabel + ">" else $prlabel end)) } ]},
                 { type:"section", text:{ type:"mrkdwn", text:("*" + ($title|esc) + "*") } },
                 { type:"context", elements:[
@@ -119,15 +155,16 @@ runs:
           if [ -n "$c_subject" ]; then
             # Commit card: subject + short SHA (linked) + author.
             payload="$(jq -n \
-              --arg color "$color" --arg header "${emoji} ${LABEL} — ${status}" \
-              --arg version "$version" --arg short "$short_sha" --arg cmurl "$c_url" \
+              --arg color "$color" --arg header "$header" \
+              --arg f1label "$field1_label" --arg f1value "$field1_value" \
+              --arg short "$short_sha" --arg cmurl "$c_url" \
               --arg subject "$c_subject" --arg user "$c_author" --arg runurl "$RUN_URL" \
               --arg built "$built_utc" '
               def esc: gsub("&";"&amp;") | gsub("<";"&lt;") | gsub(">";"&gt;");
               { attachments: [{ color: $color, blocks: [
                   { type:"header", text:{ type:"plain_text", text:$header, emoji:true } },
                   { type:"section", fields:[
-                      { type:"mrkdwn", text:("*Version*\n" + $version) },
+                      { type:"mrkdwn", text:("*" + $f1label + "*\n" + $f1value) },
                       { type:"mrkdwn", text:("*Commit*\n" + (if $cmurl != "" then "<" + $cmurl + "|" + $short + ">" else "`" + $short + "`" end)) } ]},
                   { type:"section", text:{ type:"mrkdwn", text:("*" + ($subject|esc) + "*") } },
                   { type:"context", elements:[