diff --git a/.github/workflows/cerberus-notify.yml b/.github/workflows/cerberus-notify.yml
new file mode 100644
index 00000000..4f68cc76
--- /dev/null
+++ b/.github/workflows/cerberus-notify.yml
@@ -0,0 +1,105 @@
+name: Cerberus notify
+
+# Ported from the `notify-on-master-fail` job in the retired .gitlab-ci.yml as
+# part of the GitLab -> GitHub migration (STAC-25142 / STAC-25519). The GitHub
+# migration in STAC-25420 dropped it, which left a failing master completely
+# silent -- image publishing broke on 2026-07-23 and went unnoticed for 12 days
+# (STAC-25510).
+#
+# Called by ci.yml as a terminal job so a failed master build reaches the team's
+# Slack CI channel. Cerberus is the internal notify/block Lambda (source:
+# https://gitlab.com/stackvista/devops/cerberus); the calling convention here
+# follows `cerberus-block-on-master-fail` in StackVista/stackstate's
+# .github/workflows/ci.yml, with `platform: github` in the context so Cerberus
+# builds GitHub pipeline/commit URLs rather than GitLab ones.
+#
+# `action: notify`, never `action: block`. Policy for migrated repos is notify by
+# default; blocking locks the branch (`lock_branch`) on every master failure,
+# additionally requires the Cerberus GitHub App to be installed on the repo, and
+# mutates Pulumi-managed branch protection out from under it.
+#
+# The repo's own .cerberus/cerberus_notify_failure.sh is not reused -- it was
+# deleted with the GitLab files, was built around GitLab's CI_* variables, and
+# predates the `platform` field.
+#
+# Prerequisite: CERBERUS_LAMBDA_URL must reach this repo as a REPO-level secret.
+# The org-level copy is visibility=private, which excludes this PUBLIC repo, and
+# widening it is not an option: the Cerberus endpoint is unauthenticated, so the
+# URL is the whole capability. Until the pulumi-infra change applies, this
+# workflow warns and exits 0 rather than adding a second red job to an already
+# failed run -- the annotation is the signal.
+#
+# The Slack channel is deliberately not sent. Cerberus resolves it as
+# `util.GetOrDefault(req.Context, "channel", s.Channel)`, and GetOrDefault treats
+# an empty or whitespace value as absent, so omitting `channel` falls back to the
+# Lambda's own SLACK_CHANNEL.
+
+on:
+  workflow_call:
+    inputs:
+      suite:
+        description: Suite label shown in the Slack message, e.g. build.
+        required: true
+        type: string
+    secrets:
+      # `required: false`. A caller passing `${{ secrets.X }}` for a secret the
+      # repo does not hold yields an empty string, which GitHub rejects as "not
+      # provided" against a required secret and fails the call before the run
+      # step's guard can warn -- the failure mode this workflow exists to avoid.
+      CERBERUS_LAMBDA_URL:
+        required: false
+
+# Nothing here reads the repository; the payload is built entirely from the
+# github context.
+permissions: {}
+
+jobs:
+  notify:
+    name: Notify Slack via Cerberus
+    runs-on: ubuntu-24.04
+    timeout-minutes: 5
+    steps:
+      - name: Post the failure to Cerberus
+        env:
+          CERBERUS_LAMBDA_URL: ${{ secrets.CERBERUS_LAMBDA_URL }}
+          REPOSITORY: ${{ github.repository }}
+          BRANCH: ${{ github.ref_name }}
+          PIPELINE: ${{ github.run_id }}
+          COMMIT_SHA: ${{ github.sha }}
+          COMMIT_MESSAGE: ${{ github.event.head_commit.message }}
+          SUITE: ${{ inputs.suite }}
+        run: |
+          set -euo pipefail
+
+          if [ -z "${CERBERUS_LAMBDA_URL}" ]; then
+            echo "::warning title=Cerberus not configured::CERBERUS_LAMBDA_URL is not visible to this repo, so the ${SUITE} failure was not reported to Slack. Needs the repo-level secret from pulumi-infra (STAC-25519)."
+            exit 0
+          fi
+
+          COMMIT_TITLE=$(printf '%s' "${COMMIT_MESSAGE}" | head -n1)
+
+          curl --verbose --fail \
+            -X POST "${CERBERUS_LAMBDA_URL}" \
+            -H "Content-Type: application/json" \
+            -d "$(jq -n \
+              --arg repo "${REPOSITORY}" \
+              --arg branch "${BRANCH}" \
+              --arg pipeline "${PIPELINE}" \
+              --arg sha "${COMMIT_SHA}" \
+              --arg title "${COMMIT_TITLE}" \
+              --arg suite "${SUITE}" \
+              '{
+                action: "notify",
+                context: {
+                  platform: "github",
+                  "project.id": $repo,
+                  "project.slug": $repo,
+                  "project.name": "StackState Process Agent",
+                  branch: $branch,
+                  pipeline: $pipeline,
+                  "commit.sha": $sha,
+                  "commit.title": $title,
+                  suite: $suite
+                }
+              }'
+            )"
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index fbf5b60a..6879fa09 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -331,3 +331,20 @@ jobs:
             exit 1
           fi
           echo "All required process-agent jobs passed."
+
+  # Terminal job, master only. ci-success aggregates every other job, so hanging
+  # the notification off it means one funnel for all failures rather than a
+  # notify job per pipeline job. Replaces `notify-on-master-fail` from the
+  # retired .gitlab-ci.yml (STAC-25519).
+  cerberus-notify:
+    name: Report failure to Slack (Cerberus)
+    needs: ci-success
+    if: >-
+      failure() &&
+      github.ref == 'refs/heads/master' &&
+      github.event_name == 'push'
+    uses: ./.github/workflows/cerberus-notify.yml
+    with:
+      suite: build
+    secrets:
+      CERBERUS_LAMBDA_URL: ${{ secrets.CERBERUS_LAMBDA_URL }}