diff --git a/.github/workflows/pr-auto-review.yml b/.github/workflows/pr-auto-review.yml
index 485ac64b..b484dd03 100644
--- a/.github/workflows/pr-auto-review.yml
+++ b/.github/workflows/pr-auto-review.yml
@@ -44,6 +44,29 @@ on:
 
 permissions: {}
 
+# Deduplicate ONLY the default-branch-context triggers (issue #1126). On GitHub
+# Free (20 concurrent jobs, org-wide) this stub fanned out per trigger. Its runs
+# split by context:
+#   • check_suite / workflow_run — head_branch=main, the check does NOT attach to
+#     the PR head, so superseding an in-flight run is SAFE. Both events collapse
+#     onto one group per PR (`…-pr-<number>`) with cancel-in-progress: true.
+#   • pull_request / pull_request_review — run on the PR head; a cancelled run
+#     would leave a cancelled `pr-auto-review / check-and-dispatch` check on the
+#     head. They get a unique-per-run group and never cancel, so they behave
+#     exactly as before.
+# A default-branch trigger with no associated PR (fork / no PR) also falls back to
+# the unique-per-run group so unrelated PRs never collapse into one group.
+concurrency:
+  group: >-
+    ${{
+    (github.event_name == 'check_suite' && github.event.check_suite.pull_requests[0].number)
+    && format('pr-auto-review-ready-check-pr-{0}', github.event.check_suite.pull_requests[0].number)
+    || (github.event_name == 'workflow_run' && github.event.workflow_run.pull_requests[0].number)
+    && format('pr-auto-review-ready-check-pr-{0}', github.event.workflow_run.pull_requests[0].number)
+    || format('pr-auto-review-ready-check-unique-{0}', github.run_id)
+    }}
+  cancel-in-progress: ${{ github.event_name == 'check_suite' || github.event_name == 'workflow_run' }}
+
 jobs:
   pr-auto-review:
     permissions:
diff --git a/tests/pr-auto-review-concurrency.bats b/tests/pr-auto-review-concurrency.bats
new file mode 100644
index 00000000..7aea1c78
--- /dev/null
+++ b/tests/pr-auto-review-concurrency.bats
@@ -0,0 +1,100 @@
+#!/usr/bin/env bats
+# Compliance regression guard for the pr-auto-review thin caller stub's
+# `concurrency:` surface (issue #209).
+#
+# .github/workflows/pr-auto-review.yml is a THIN CALLER STUB. Its `on:` triggers,
+# `permissions:` grants, and `concurrency:` block are owned centrally by
+# petry-projects/.github → standards/workflows/pr-auto-review.yml and are NOT
+# repo-adjustable (only the documented `with:` inputs and the tier channel pin
+# may differ per repo). This guard pins the centrally-owned `concurrency:`
+# surface so it cannot silently drift back out of sync.
+#
+# The canonical block deduplicates ONLY the default-branch-context triggers
+# (issue #1126):
+#   • check_suite / workflow_run — head_branch=main, the check does NOT attach to
+#     the PR head, so superseding an in-flight run is SAFE. Both events collapse
+#     onto one group per PR (`…-pr-<number>`) with cancel-in-progress: true.
+#   • pull_request / pull_request_review — run on the PR head; a cancelled run
+#     would leave a cancelled run, so they get a unique-per-run group (keyed by
+#     github.run_id) and never cancel.
+# A default-branch trigger with no associated PR also falls back to the
+# unique-per-run group so unrelated PRs never collapse into one group.
+
+STUB="${BATS_TEST_DIRNAME}/../.github/workflows/pr-auto-review.yml"
+
+# Semantic parser to extract concurrency fields from the YAML stub.
+# This avoids fragile regex/awk parsing of YAML and handles multi-line folded
+# strings, comments, and key-ordering changes. PyYAML is already a project test
+# dependency (see pr-auto-review-stub.bats).
+get_concurrency_val() {
+  python3 - "$STUB" "$1" <<'PYEOF'
+import yaml, sys
+with open(sys.argv[1], encoding="utf-8") as f:
+    stub = yaml.safe_load(f)
+concurrency = stub.get('concurrency') or {}
+val = concurrency.get(sys.argv[2])
+if val is None:
+    sys.exit(1)
+if isinstance(val, str):
+    print(" ".join(val.split()))
+else:
+    print(val)
+PYEOF
+}
+
+@test "pr-auto-review stub exists" {
+  [ -f "$STUB" ]
+}
+
+@test "stub declares a top-level concurrency block" {
+  grep -qE '^concurrency:[[:space:]]*(#.*)?$' "$STUB" \
+    || { echo "pr-auto-review.yml is missing its centrally-owned top-level concurrency block"; return 1; }
+}
+
+@test "concurrency block defines a group" {
+  get_concurrency_val "group" >/dev/null \
+    || { echo "concurrency block has no 'group:' key"; return 1; }
+}
+
+@test "group collapses check_suite runs onto one per-PR slot" {
+  local group
+  group="$(get_concurrency_val "group")" || return 1
+  [[ "$group" == '${{'* && "$group" == *'}}' ]] \
+    || { echo "group expression must be wrapped in \${{ ... }}"; return 1; }
+  printf '%s\n' "$group" | grep -qF "github.event.check_suite.pull_requests[0].number" \
+    || { echo "group must key check_suite runs by their PR number"; return 1; }
+  printf '%s\n' "$group" | grep -qF "format('pr-auto-review-ready-check-pr-{0}', github.event.check_suite.pull_requests[0].number)" \
+    || { echo "group must map a check_suite PR to the shared 'pr-auto-review-ready-check-pr-<n>' slot"; return 1; }
+}
+
+@test "group collapses workflow_run runs onto one per-PR slot" {
+  local group
+  group="$(get_concurrency_val "group")" || return 1
+  [[ "$group" == '${{'* && "$group" == *'}}' ]] \
+    || { echo "group expression must be wrapped in \${{ ... }}"; return 1; }
+  printf '%s\n' "$group" | grep -qF "github.event.workflow_run.pull_requests[0].number" \
+    || { echo "group must key workflow_run runs by their PR number"; return 1; }
+  printf '%s\n' "$group" | grep -qF "format('pr-auto-review-ready-check-pr-{0}', github.event.workflow_run.pull_requests[0].number)" \
+    || { echo "group must map a workflow_run PR to the shared 'pr-auto-review-ready-check-pr-<n>' slot"; return 1; }
+}
+
+@test "group falls back to a unique-per-run slot keyed by github.run_id" {
+  local group
+  group="$(get_concurrency_val "group")" || return 1
+  [[ "$group" == '${{'* && "$group" == *'}}' ]] \
+    || { echo "group expression must be wrapped in \${{ ... }}"; return 1; }
+  # PR-head triggers (and default-branch triggers with no PR) must get their own
+  # slot so a cancellation never leaves a cancelled check on a PR head.
+  printf '%s\n' "$group" | grep -qF "format('pr-auto-review-ready-check-unique-{0}', github.run_id)" \
+    || { echo "group must fall back to a unique-per-run slot ('pr-auto-review-ready-check-unique-<run_id>')"; return 1; }
+}
+
+@test "cancel-in-progress is gated to check_suite and workflow_run only" {
+  local cancel
+  cancel="$(get_concurrency_val "cancel-in-progress")" || return 1
+  [[ "$cancel" == '${{'* && "$cancel" == *'}}' ]] \
+    || { echo "cancel-in-progress expression must be wrapped in \${{ ... }}"; return 1; }
+  # It must NOT be a bare `true`: PR-head triggers must never be cancelled.
+  printf '%s\n' "$cancel" | grep -qF "github.event_name == 'check_suite' || github.event_name == 'workflow_run'" \
+    || { echo "cancel-in-progress must be gated to check_suite/workflow_run only, not a bare true"; return 1; }
+}