diff --git a/backend/__tests__/unit/services/pgBootService.sentryWrap.test.js b/backend/__tests__/unit/services/pgBootService.sentryWrap.test.js
new file mode 100644
index 000000000..61f020834
--- /dev/null
+++ b/backend/__tests__/unit/services/pgBootService.sentryWrap.test.js
@@ -0,0 +1,130 @@
+/**
+ * TASK-172 — the PREMISE behind #1958's wrap clause, checked against the real
+ * library instead of a stand-in.
+ *
+ * THE GAP THIS CLOSES. `pgBootService.test.js` proves the wrap clause works:
+ * it hands `routerIsMounted` a layer whose handle it has wrapped by hand
+ * (`wrapLikeSentry`). It cannot do better, because jest owns module loading and
+ * `@sentry/node` patches express through a require-hook — so in-suite
+ * `layer.handle === router` is TRUE even with Sentry initialised. The
+ * consequence is that the suite pins the FIX and never the REASON: if a Sentry
+ * upgrade stops exposing `.stack` on the wrapper, every case stays green and
+ * production goes back to refusing a pod that has the PG routes mounted, which
+ * is the rollout hang of 2026-09-27 (deploy `4e60f240`: `deployment.apps/backend`
+ * did not become ready within 8m, `/api/pg/messages` 401 on a pod whose
+ * `/ready` said "not mounted").
+ *
+ * HOW IT ESCAPES THE TIER. The probe is a spawned plain-`node` child
+ * (`__tests__/utils/sentryLayerWrapProbe.js`), so the module registry is the
+ * real one: `Sentry.init()` runs before `require('express')` and the patch
+ * lands. `process.execPath` is used, so the child runs whatever node the suite
+ * runs — the premise is checked under the interpreter that would run it.
+ *
+ * WHY THREE ASSERTIONS AND NOT ONE. "It reads true" can be true for the wrong
+ * reason, so the probe reports the shape of the instrumentation as well as the
+ * answer, and the suite pins both directions of the failure:
+ *
+ *   - `wrapped` false under Sentry  → the check is VACUOUS. Identity alone
+ *     answers the question, so the suite would pass with the clause deleted and
+ *     a future reader would be told the clause is load-bearing when it is not.
+ *     (Or the instrumentation failed to install in the child, which is worth
+ *     knowing too — the message names both.)
+ *   - `wrapped` true, `handleHasStack` false → the exact production break: the
+ *     clause can no longer match a mounted router, so readiness refuses a pod
+ *     that has chat. Red here is a deploy-blocking regression caught pre-merge
+ *     instead of at rollout.
+ *   - no-wrong-answer direction: an unmounted router must be false, and a
+ *     router mounted on a DIFFERENT app must be false. The clause WIDENS a
+ *     match (it accepts `handle.stack === routerStack`), so this is the
+ *     direction that would silently start reporting "mounted" for a router the
+ *     app does not serve.
+ *
+ * The `PROBE_SENTRY=0` run is the positive control: identical construction with
+ * no instrumentation. `wrapped` must be false there, which is what makes the
+ * instrumented run's `wrapped: true` attributable to Sentry rather than to the
+ * express version — and what proves the probe can see a difference rather than
+ * reporting a blind null.
+ */
+const { spawnSync } = require('child_process');
+const path = require('path');
+
+const PROBE = path.join(__dirname, '..', '..', 'utils', 'sentryLayerWrapProbe.js');
+const BACKEND_ROOT = path.join(__dirname, '..', '..', '..');
+
+const runProbe = (withSentry) => {
+  const proc = spawnSync(process.execPath, [PROBE], {
+    cwd: BACKEND_ROOT,
+    encoding: 'utf8',
+    timeout: 120000,
+    env: { ...process.env, PROBE_SENTRY: withSentry ? '1' : '0' },
+  });
+
+  const lines = (proc.stdout || '').split('\n').map((l) => l.trim()).filter(Boolean);
+  let result = null;
+  try {
+    result = JSON.parse(lines[lines.length - 1]);
+  } catch (error) {
+    result = null;
+  }
+
+  // Reported rather than asserted here, so each case can name what it expected
+  // and the reader gets the child's own output when the probe could not run.
+  return {
+    result,
+    status: proc.status,
+    signal: proc.signal,
+    stderr: (proc.stderr || '').trim(),
+    stdout: (proc.stdout || '').trim(),
+  };
+};
+
+const describeProbe = (label, run) => {
+  const detail = `[${label}] exit=${run.status} signal=${run.signal} stdout=${run.stdout} stderr=${run.stderr}`;
+  if (!run.result) throw new Error(`the probe produced no JSON result. ${detail}`);
+  if (run.result.error) {
+    throw new Error(`the probe threw in the child: ${run.result.error}. ${detail}`);
+  }
+  return run.result;
+};
+
+describe('routerIsMounted under real @sentry/node instrumentation (TASK-172)', () => {
+  let plain;
+  let instrumented;
+
+  beforeAll(() => {
+    plain = runProbe(false);
+    instrumented = runProbe(true);
+  });
+
+  it('control: with no instrumentation the layer handle IS the router, and the answer is true', () => {
+    const result = describeProbe('control', plain);
+    expect(result.sentry).toBe(false);
+    // The control is what gives the instrumented run meaning: if this were true
+    // there too, the probe would be blind to the patch rather than measuring it.
+    expect(result.wrapped).toBe(false);
+    expect(result.self).toBe(true);
+  });
+
+  it('instrumentation really replaces the layer handle, so this suite is not vacuous', () => {
+    const result = describeProbe('instrumented', instrumented);
+    expect(result.sentry).toBe(true);
+    expect(result.layerFound).toBe(true);
+    expect(result.wrapped).toBe(true);
+    expect(result.handleName).not.toBe('router');
+    // The property #1958's clause keys on. If this goes false, the clause
+    // cannot match and readiness refuses a pod that has the routes mounted —
+    // the production shape this file exists to catch before a rollout.
+    expect(result.handleHasStack).toBe(true);
+  });
+
+  it('answers true for a mounted router while instrumentation has wrapped its layer', () => {
+    const result = describeProbe('instrumented', instrumented);
+    expect(result.self).toBe(true);
+  });
+
+  it('answers false for a router that is not mounted, and for one mounted on another app', () => {
+    const result = describeProbe('instrumented', instrumented);
+    expect(result.unmounted).toBe(false);
+    expect(result.foreign).toBe(false);
+  });
+});
diff --git a/backend/__tests__/utils/sentryLayerWrapProbe.js b/backend/__tests__/utils/sentryLayerWrapProbe.js
new file mode 100644
index 000000000..c0839b63a
--- /dev/null
+++ b/backend/__tests__/utils/sentryLayerWrapProbe.js
@@ -0,0 +1,103 @@
+/**
+ * TASK-172 — the real-Sentry proof of the premise behind #1958's wrap clause.
+ *
+ * WHY THIS IS A SPAWNED CHILD AND NOT A SUITE. Jest cannot host this check.
+ * `@sentry/node` patches `express` through OpenTelemetry's require-hook, which
+ * installs itself on `Module._load` when `Sentry.init()` runs; jest owns module
+ * loading instead, so express is never patched in-suite and
+ * `layer.handle === router` stays TRUE there. That is exactly why
+ * `pgBootService.test.js` has to fake the wrapper with `wrapLikeSentry` — and
+ * exactly why the faked wrapper pins the FIX and can never pin the PREMISE.
+ * The premise is a fact about the real library, so it needs a real `node`.
+ *
+ * It lives under `__tests__/utils/` because jest's `testMatch` collects
+ * everything under `__tests__/` and would otherwise fail this directory-shaped
+ * way ("your test suite must contain at least one test" — the same reason
+ * `utils/` is already ignored in `backend/jest.config.js`). This file is data
+ * for a test, never a test.
+ *
+ * WHAT IT MEASURES. Three facts, because "the guard reads true" alone can be
+ * true for the wrong reason:
+ *   1. `wrapped` — did instrumentation actually replace the layer handle with a
+ *      wrapper? If this is false, the suite is vacuous: identity alone answers
+ *      the question, and a future Sentry that stops wrapping would silently
+ *      retire the premise while this probe kept reporting `self: true`.
+ *   2. `handleHasStack` — does the wrapper expose the router's own stack array?
+ *      That is the property #1958's clause keys on.
+ *   3. `self` / `unmounted` / `foreign` — the real `routerIsMounted` answering
+ *      true for the mounted router, false for a router that is not mounted, and
+ *      false for an app that mounted a DIFFERENT router (the false-positive
+ *      direction, which matters because the clause widens a match).
+ *
+ * `PROBE_SENTRY=0` runs the identical construction with no instrumentation, as
+ * the positive control: `wrapped` must be false there, so the difference in
+ * `self` is attributable to Sentry rather than to the express version.
+ *
+ * Prints one JSON line and always exits 0; the suite asserts on the fields, so
+ * a crash is reported as a missing result with stderr attached rather than as
+ * an opaque non-zero status.
+ */
+const withSentry = process.env.PROBE_SENTRY === '1';
+const out = { sentry: withSentry };
+
+try {
+  // ts-node first: nothing else may load express before Sentry.init().
+  require('ts-node').register({
+    transpileOnly: true,
+    skipProject: true,
+    compilerOptions: { module: 'commonjs', target: 'es2020', esModuleInterop: true },
+  });
+
+  if (withSentry) {
+    const Sentry = require('@sentry/node');
+    Sentry.init({
+      // A syntactically valid DSN with a non-resolving host: init() must run for
+      // the express instrumentation to install, and no event should ever leave.
+      dsn: 'https://0123456789abcdef0123456789abcdef@example.invalid/1',
+      tracesSampleRate: 0,
+    });
+  }
+
+  const express = require('express');
+  // A literal path, both so eslint can see the target and so ts-node's hook is
+  // the only thing that has to understand the extension.
+  const { routerIsMounted } = require('../../services/pgBootService.ts');
+
+  const app = express();
+  const router = express.Router();
+  const other = express.Router();
+  const neverMounted = express.Router();
+  router.get('/', (req, res) => res.json({ ok: true }));
+  other.get('/', (req, res) => res.json({ ok: true }));
+  app.use('/api/pg/messages', router);
+  app.use('/api/other', other);
+
+  // A second app that mounts only the other router: asking it about `router`
+  // must be false, whether or not the layers are wrapped.
+  const foreignApp = express();
+  foreignApp.use('/api/other', other);
+
+  const stackOf = (a) => (a._router && a._router.stack) || (a.router && a.router.stack) || [];
+  const layerFor = (a, target) => stackOf(a).find(
+    (l) => l.handle === target || (l.handle && l.handle.stack === target.stack),
+  );
+
+  const layer = layerFor(app, router);
+  const foreignLayer = layerFor(foreignApp, other);
+
+  out.layerFound = Boolean(layer);
+  out.wrapped = Boolean(layer) && layer.handle !== router;
+  out.handleType = layer ? typeof layer.handle : null;
+  out.handleName = (layer && layer.handle && layer.handle.name) || null;
+  out.handleHasStack = Boolean(layer && layer.handle && layer.handle.stack);
+  out.foreignWrapped = Boolean(foreignLayer) && foreignLayer.handle !== other;
+  out.self = routerIsMounted(app, router);
+  out.unmounted = routerIsMounted(app, neverMounted);
+  out.foreign = routerIsMounted(foreignApp, router);
+} catch (error) {
+  out.error = (error && error.message) || String(error);
+  out.stack = (error && error.stack) || null;
+}
+
+process.stdout.write(`${JSON.stringify(out)}\n`);
+process.exit(0);