diff --git a/.changeset/test-core-stall-guard.md b/.changeset/test-core-stall-guard.md
new file mode 100644
index 0000000000..ef8ac652b1
--- /dev/null
+++ b/.changeset/test-core-stall-guard.md
@@ -0,0 +1,4 @@
+---
+---
+
+ci: Test Core mid-suite stalls become labeled fast failures via an output stall guard (#4250). CI-only — releases nothing.
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index e32065a72f..a666cd7ebc 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -52,7 +52,12 @@ jobs:
     needs: filter
     if: needs.filter.outputs.core == 'true'
     runs-on: ubuntu-latest
-    timeout-minutes: 45
+    # Backstop only — the stall guard on the test steps is the primary
+    # detector for a #4250-style hang and fires well before this. 30 min is
+    # 2.5-3× a normal run (main ~9.5 min, PR ~12 min), with margin for a cold
+    # Turbo cache; the old 45 left a hung job "running" for half an hour past
+    # any plausible healthy finish.
+    timeout-minutes: 30
     permissions:
       contents: read
 
@@ -117,18 +122,22 @@ jobs:
       # in parallel, and it dominated this job's critical path. The exclusion
       # subtracts from the affected set (verified: turbo unions inclusive
       # filters, then applies `!` negations to the result).
-      # `set -o pipefail` is load-bearing: GitHub runs these with `bash -e`, which
-      # does NOT set it, so `turbo … | tee` would report TEE's status and a
-      # failing suite would go green. That is the same class of bug the tee is
-      # here to catch, so it must not be introduced by the catching.
+      # run-with-stall-guard replaces the old `… 2>&1 | tee $log` +
+      # `set -o pipefail` idiom: the guard tees combined output to the log
+      # itself and propagates the suite's real exit status, so there is no
+      # pipe whose status tee could mask (do not reintroduce `| tee`). Its
+      # actual job is #4250: a run whose output freezes mid-suite while the
+      # job sits in_progress. Silence past --stall-minutes is declared a
+      # stall — a labeled red naming the last output line — instead of a
+      # 20-minute wait for a human (or the job timeout) to notice. 10 min is
+      # ~5× the longest healthy quiet gap and still under half a normal run.
       - name: Run affected tests (PR)
         if: github.event_name == 'pull_request'
         env:
           TURBO_SCM_BASE: ${{ github.event.pull_request.base.sha }}
         run: |
-          set -o pipefail
-          pnpm turbo run test --affected --filter=!@objectstack/dogfood --concurrency=4 \
-            2>&1 | tee "$RUNNER_TEMP/test-core.log"
+          node scripts/run-with-stall-guard.mjs --log "$RUNNER_TEMP/test-core.log" --stall-minutes 10 -- \
+            pnpm turbo run test --affected --filter=!@objectstack/dogfood --concurrency=4
 
       # Push to main: full run. Spec's suite runs here plain (uninstrumented);
       # the coverage-instrumented pass moved to the nightly Spec Coverage
@@ -139,9 +148,8 @@ jobs:
       - name: Run all tests (push)
         if: github.event_name == 'push'
         run: |
-          set -o pipefail
-          pnpm turbo run test --filter=!@objectstack/dogfood --concurrency=4 \
-            2>&1 | tee "$RUNNER_TEMP/test-core.log"
+          node scripts/run-with-stall-guard.mjs --log "$RUNNER_TEMP/test-core.log" --stall-minutes 10 -- \
+            pnpm turbo run test --filter=!@objectstack/dogfood --concurrency=4
 
       # Runs even when the suite failed — that is when it earns its keep. A red
       # suite plus a GREEN completeness check means real test failures; a red
diff --git a/scripts/run-with-stall-guard.mjs b/scripts/run-with-stall-guard.mjs
new file mode 100644
index 0000000000..6f83cecf65
--- /dev/null
+++ b/scripts/run-with-stall-guard.mjs
@@ -0,0 +1,154 @@
+#!/usr/bin/env node
+// Copyright (c) 2025 ObjectStack. Licensed under the Apache-2.0 license.
+//
+// run-with-stall-guard -- a stalled test run must SAY it stalled, not sit
+// in_progress until someone reads log timestamps.
+//
+// Test Core has hung mid-suite with frozen output -- three times in one day
+// (#4250). The signature is mechanical: a healthy run prints continuously and
+// finishes in ~9-12 minutes; a stalled one stops emitting bytes entirely (two
+// log fetches 9 minutes apart returned byte-identical content) while the job
+// stays in_progress until the job-level timeout or a human cancels. A job
+// timeout cannot tell "slow" from "stopped" -- only output progress can. So
+// this wrapper watches exactly that: if the wrapped command emits nothing for
+// --stall-minutes, it declares a stall, prints the last line seen and how long
+// ago it was seen, kills the command's process group, and exits 75
+// (EX_TEMPFAIL: the sanctioned response is a rerun -- every #4250 occurrence
+// passed on rerun of the same commit).
+//
+//   node scripts/run-with-stall-guard.mjs --log <file> [--stall-minutes N] -- <command...>
+//
+// It also owns the log tee: combined stdout+stderr is forwarded to this
+// process's stdout AND appended to --log (which check-test-completeness.mjs
+// reads afterwards). The old `cmd 2>&1 | tee $log` pattern needed
+// `set -o pipefail` or tee's exit status would mask a red suite; here the
+// child's real exit status is propagated by construction, so there is no pipe
+// to guard. Do not reintroduce `| tee`.
+//
+// Exit status: the child's own code when it finishes; 75 on a declared stall;
+// 1 when the child dies on a signal this guard did not send.
+
+import { spawn } from 'node:child_process';
+import { createWriteStream } from 'node:fs';
+
+const STALL_EXIT_CODE = 75; // EX_TEMPFAIL
+const CHECK_INTERVAL_MS = 5_000;
+const SIGKILL_GRACE_MS = 10_000;
+
+const argv = process.argv.slice(2);
+let logPath = '';
+let stallMinutes = 10;
+let command = [];
+
+for (let i = 0; i < argv.length; i++) {
+  if (argv[i] === '--log') logPath = argv[++i] ?? '';
+  else if (argv[i] === '--stall-minutes') stallMinutes = Number(argv[++i]);
+  else if (argv[i] === '--') {
+    command = argv.slice(i + 1);
+    break;
+  } else {
+    console.error(`run-with-stall-guard: unknown option ${argv[i]}`);
+    process.exit(1);
+  }
+}
+
+if (!logPath || command.length === 0 || !Number.isFinite(stallMinutes) || stallMinutes <= 0) {
+  console.error(
+    'run-with-stall-guard: usage: run-with-stall-guard.mjs --log <file> [--stall-minutes N] -- <command...>',
+  );
+  process.exit(1);
+}
+
+const stallMs = stallMinutes * 60_000;
+const log = createWriteStream(logPath, { flags: 'w' });
+
+// detached: own process group, so a stall verdict can kill pnpm -> turbo -> the
+// per-package vitest processes together, not just the top of the tree.
+const child = spawn(command[0], command.slice(1), {
+  detached: true,
+  stdio: ['ignore', 'pipe', 'pipe'],
+});
+
+let lastOutputAt = Date.now();
+let lastLine = '(no output yet)';
+let carry = '';
+let stalled = false;
+
+function onChunk(chunk) {
+  lastOutputAt = Date.now();
+  process.stdout.write(chunk);
+  log.write(chunk);
+
+  // Remember the last complete non-blank line for the stall verdict. ANSI is
+  // stripped so the verdict quotes text, not colour codes.
+  carry = (carry + chunk.toString('utf8')).slice(-8192);
+  const nl = carry.lastIndexOf('\n');
+  if (nl === -1) return;
+  for (const line of carry.slice(0, nl).split('\n').reverse()) {
+    const clean = line.replace(/\x1B\[[0-9;]*m/g, '').trim();
+    if (clean) {
+      lastLine = clean;
+      break;
+    }
+  }
+  carry = carry.slice(nl + 1);
+}
+
+child.stdout.on('data', onChunk);
+child.stderr.on('data', onChunk);
+
+function killGroup(signal) {
+  try {
+    process.kill(-child.pid, signal);
+  } catch {
+    // Group already gone -- the 'exit' handler finishes up.
+  }
+}
+
+const watchdog = setInterval(() => {
+  const silentMs = Date.now() - lastOutputAt;
+  if (silentMs < stallMs) return;
+
+  stalled = true;
+  clearInterval(watchdog);
+  const banner = `
+${'═'.repeat(72)}
+⛔ STALL: no test output for ${(silentMs / 60_000).toFixed(1)} minutes (limit: ${stallMinutes}m).
+
+   frozen since : ${new Date(lastOutputAt).toISOString()}
+   last line    : ${lastLine}
+
+   This is the #4250 failure mode -- the suite is STOPPED, not slow. A
+   healthy run prints continuously; only a hang goes silent this long.
+   Killing the test process group and failing the step now, instead of
+   sitting in_progress until the job timeout.
+
+   Triage: every #4250 stall so far passed on a plain rerun of the same
+   commit -- rerun this job before suspecting the diff. If it stalls twice
+   at the same test file, add that occurrence to #4250.
+${'═'.repeat(72)}
+`;
+  process.stdout.write(banner);
+  log.write(banner);
+  killGroup('SIGTERM');
+  setTimeout(() => killGroup('SIGKILL'), SIGKILL_GRACE_MS).unref();
+}, CHECK_INTERVAL_MS);
+
+child.on('error', (err) => {
+  clearInterval(watchdog);
+  console.error(`run-with-stall-guard: failed to start ${command[0]} -- ${err.message}`);
+  log.end(() => process.exit(1));
+});
+
+child.on('exit', (code, signal) => {
+  clearInterval(watchdog);
+  const finish = (status) => log.end(() => process.exit(status));
+  if (stalled) {
+    finish(STALL_EXIT_CODE);
+  } else if (signal) {
+    console.error(`run-with-stall-guard: command killed by ${signal}`);
+    finish(1);
+  } else {
+    finish(code ?? 1);
+  }
+});