diff --git a/.github/workflows/ci-cache-receipts.yml b/.github/workflows/ci-cache-receipts.yml
index 05385e947d32..8ebd3005f985 100644
--- a/.github/workflows/ci-cache-receipts.yml
+++ b/.github/workflows/ci-cache-receipts.yml
@@ -30,6 +30,10 @@ on:
 permissions:
   contents: read
 
+concurrency:
+  group: ci-cache-receipts-${{ github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
 jobs:
   receipt-contract:
     runs-on: ${{ vars.LINUX_RUNNER || 'blacksmith-4vcpu-ubuntu-2404' }}
diff --git a/.github/workflows/ci-guards.yml b/.github/workflows/ci-guards.yml
index f1fa9ac66108..8f9de9db9d3f 100644
--- a/.github/workflows/ci-guards.yml
+++ b/.github/workflows/ci-guards.yml
@@ -291,6 +291,10 @@ jobs:
         if: ${{ matrix.group == 'ci' }}
         run: python3 tests/test_ci_workflow_guards_are_wired.py
 
+      - name: Validate required checks are bounded
+        if: ${{ matrix.group == 'ci' }}
+        run: python3 tests/test_ci_required_checks_are_bounded.py
+
       - name: Validate triage radar scoring
         if: ${{ matrix.group == 'ci' }}
         run: python3 tests/test_triage_radar.py
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 04d233125467..7f210d5b13c4 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -951,6 +951,10 @@ jobs:
       - tests
     if: ${{ always() }}
     runs-on: ${{ vars.LINUX_RUNNER || 'blacksmith-4vcpu-ubuntu-2404' }}
+    # One step that reads `needs` and decides. Its siblings are bounded at 5
+    # to 20 minutes; this one was inheriting the six-hour default, as the
+    # required check every pull request waits on.
+    timeout-minutes: 5
     steps:
       - name: Check routed CI jobs
         env:
diff --git a/.github/workflows/cmux-cloud-cli.yml b/.github/workflows/cmux-cloud-cli.yml
index aeffa94d5421..fcaf74fb8278 100644
--- a/.github/workflows/cmux-cloud-cli.yml
+++ b/.github/workflows/cmux-cloud-cli.yml
@@ -14,6 +14,10 @@ on:
   workflow_dispatch:
 permissions:
   contents: read
+concurrency:
+  group: cmux-cloud-cli-${{ github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
 jobs:
   test:
     runs-on: blacksmith-4vcpu-ubuntu-2404
diff --git a/.github/workflows/indexnow-tests.yml b/.github/workflows/indexnow-tests.yml
index b812bf8f296a..6f7c9ed7b475 100644
--- a/.github/workflows/indexnow-tests.yml
+++ b/.github/workflows/indexnow-tests.yml
@@ -13,6 +13,10 @@ on:
 permissions:
   contents: read
 
+concurrency:
+  group: indexnow-tests-${{ github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
 jobs:
   notification-tests:
     runs-on: ${{ vars.LINUX_RUNNER || 'blacksmith-4vcpu-ubuntu-2404' }}
diff --git a/.github/workflows/localization-catalog.yml b/.github/workflows/localization-catalog.yml
index 44b4cc6f77be..86c16e870573 100644
--- a/.github/workflows/localization-catalog.yml
+++ b/.github/workflows/localization-catalog.yml
@@ -17,6 +17,10 @@ on:
 permissions:
   contents: read
 
+concurrency:
+  group: localization-catalog-${{ github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
 jobs:
   catalog-structure:
     runs-on: ${{ vars.LINUX_RUNNER || 'blacksmith-4vcpu-ubuntu-2404' }}
diff --git a/.github/workflows/r2-upload-tests.yml b/.github/workflows/r2-upload-tests.yml
index 20a63becaeac..cc823c420115 100644
--- a/.github/workflows/r2-upload-tests.yml
+++ b/.github/workflows/r2-upload-tests.yml
@@ -17,6 +17,10 @@ on:
 permissions:
   contents: read
 
+concurrency:
+  group: r2-upload-tests-${{ github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
 jobs:
   r2-upload-tests:
     runs-on: ${{ vars.LINUX_RUNNER || 'blacksmith-4vcpu-ubuntu-2404' }}
diff --git a/tests/test-execution.toml b/tests/test-execution.toml
index 4eba6badc3ca..9b48d598a1a1 100644
--- a/tests/test-execution.toml
+++ b/tests/test-execution.toml
@@ -614,6 +614,10 @@ lane = "linux-guard"
 path = "tests/test_ci_release_helper_archs.py"
 lane = "linux-guard"
 
+[[test]]
+path = "tests/test_ci_required_checks_are_bounded.py"
+lane = "linux-guard"
+
 [[test]]
 path = "tests/test_ci_selective_layer_wiring.py"
 lane = "linux-guard"
diff --git a/tests/test_ci_required_checks_are_bounded.py b/tests/test_ci_required_checks_are_bounded.py
new file mode 100755
index 000000000000..ef41034a4d49
--- /dev/null
+++ b/tests/test_ci_required_checks_are_bounded.py
@@ -0,0 +1,123 @@
+#!/usr/bin/env python3
+"""A required check must be bounded, and a superseded PR run must be cancelled.
+
+Two properties, both of which CI is structurally unable to complain about
+because neither one makes anything fail.
+
+A job with no `timeout-minutes` inherits GitHub's six-hour default. Every job
+in `ci.yml` declares one between 5 and 20 minutes except `ci-status`, which is
+the required aggregate: the check every pull request in the repository is
+blocked on. A hang there blocks the whole repository for six hours and reads
+as "still running" the entire time.
+
+A `pull_request` workflow with no concurrency group starts a fresh run per
+push and lets the superseded ones finish. `ci.yml` and most of its neighbours
+already cancel; five path-filtered workflows had no group at all.
+
+Both rules stay derived rather than listed. The required contexts come from
+the repository ruleset and are matched to jobs by name, and a context that
+stops matching any job fails this test rather than quietly guarding nothing.
+"""
+
+from __future__ import annotations
+
+import sys
+from pathlib import Path
+
+import yaml
+
+
+ROOT = Path(__file__).resolve().parents[1]
+WORKFLOWS = ROOT / ".github/workflows"
+
+# `gh api repos/manaflow-ai/cmux/rules/branches/main`, the
+# required_status_checks rule. Update alongside the ruleset.
+REQUIRED_CONTEXTS = (
+    "CLA Assistant",
+    "CLA policy guard",
+    "ci-status",
+    "Web complexity",
+    "web-validation",
+)
+
+
+def load(path: Path) -> dict:
+    parsed = yaml.safe_load(path.read_text(encoding="utf-8"))
+    return parsed if isinstance(parsed, dict) else {}
+
+
+def triggers(workflow: dict) -> set[str]:
+    # PyYAML resolves a bare `on:` key to the boolean True.
+    on = workflow.get("on", workflow.get(True, {}))
+    if isinstance(on, dict):
+        return set(on)
+    if isinstance(on, str):
+        return {on}
+    return set(on or ())
+
+
+def context_of(job_id: str, job: dict) -> str:
+    return job.get("name") or job_id
+
+
+def main() -> int:
+    workflows = {p: load(p) for p in sorted(WORKFLOWS.glob("*.y*ml"))}
+    failures: list[str] = []
+
+    # Which workflows own a required check, by what their jobs are called.
+    owners: dict[str, Path] = {}
+    for path, workflow in workflows.items():
+        for job_id, job in (workflow.get("jobs") or {}).items():
+            if not isinstance(job, dict):
+                continue
+            context = context_of(job_id, job)
+            if context in REQUIRED_CONTEXTS:
+                owners[context] = path
+
+    unmatched = [c for c in REQUIRED_CONTEXTS if c not in owners]
+    if unmatched:
+        failures.append(
+            "required contexts match no job; the ruleset and this list have "
+            f"drifted apart: {', '.join(unmatched)}"
+        )
+
+    for path in sorted(set(owners.values())):
+        for job_id, job in (workflows[path].get("jobs") or {}).items():
+            if not isinstance(job, dict) or "uses" in job:
+                continue
+            if "timeout-minutes" not in job:
+                failures.append(
+                    f"{path.relative_to(ROOT)}:{job_id} has no "
+                    "timeout-minutes, so it inherits GitHub's six-hour "
+                    "default -- in a workflow that owns a required check"
+                )
+
+    for path, workflow in workflows.items():
+        if "pull_request" not in triggers(workflow):
+            continue
+        # A group is the requirement; what it does with a superseded run is
+        # the workflow's call. repair-nightly-appcast-content-types.yml sets
+        # cancel-in-progress: false on purpose because it writes to R2, and
+        # serializing is as good an answer as cancelling.
+        if not workflow.get("concurrency"):
+            failures.append(
+                f"{path.relative_to(ROOT)} runs on pull_request with no "
+                "concurrency group, so every push leaves the superseded run "
+                "running"
+            )
+
+    if failures:
+        print("unbounded CI:", file=sys.stderr)
+        for failure in failures:
+            print(f"  - {failure}", file=sys.stderr)
+        return 1
+
+    print(
+        f"{len(REQUIRED_CONTEXTS)} required checks are bounded; "
+        "every pull_request workflow has a concurrency group"
+    )
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())