diff --git a/src/package.json b/src/package.json
index e60c37065..13b969d25 100644
--- a/src/package.json
+++ b/src/package.json
@@ -143,7 +143,7 @@
   },
   "scripts": {
     "lint": "eslint .",
-    "test": "cross-env NODE_ENV=production mocha --import=tsx --timeout 120000 --recursive tests/backend/specs/**.ts ../node_modules/ep_*/static/tests/backend/specs/**",
+    "test": "cross-env NODE_ENV=production mocha --import=tsx --require ./tests/backend/diagnostics.ts --timeout 120000 --recursive tests/backend/specs/**.ts ../node_modules/ep_*/static/tests/backend/specs/**",
     "test-utils": "cross-env NODE_ENV=production mocha --import=tsx --timeout 5000 --recursive tests/backend/specs/*utils.ts",
     "test-container": "mocha --import=tsx --timeout 5000 tests/container/specs/api",
     "dev": "cross-env NODE_ENV=development  node --require tsx/cjs node/server.ts",
diff --git a/src/tests/backend/diagnostics.ts b/src/tests/backend/diagnostics.ts
new file mode 100644
index 000000000..ac29ab198
--- /dev/null
+++ b/src/tests/backend/diagnostics.ts
@@ -0,0 +1,96 @@
+'use strict';
+
+// Diagnostic-only mocha bootstrap, loaded via `mocha --require ./tests/backend/diagnostics.ts`.
+//
+// PR #7663 added unhandledRejection / uncaughtException handlers in
+// tests/backend/common.ts to surface the silent ~22% backend-test flake.
+// The next failure (run 25279692065, Windows without plugins, Node 24)
+// showed mocha exit with code 1 mid-suite, 261ms after the last passing
+// test, with NEITHER handler firing. This means the process was killed
+// in a way that bypassed JS handlers — SIGKILL, OOM, or a fatal native
+// error — OR mocha itself called process.exit before the handlers ran.
+//
+// This file:
+//   1. Registers handlers UNCONDITIONALLY at mocha startup (common.ts is
+//      only imported by ~27 of 47 specs, so its handlers may register
+//      late or after a death-causing event).
+//   2. Writes via fs.writeSync(2, ...) — synchronous stderr writes that
+//      complete before the kernel returns from the syscall, so the line
+//      lands in the runner log even if the process is killed
+//      milliseconds later.
+//   3. Tracks the last-seen test via a mocha root afterEach hook so the
+//      death point is identified.
+//   4. Logs exit-related events so we can discriminate:
+//        beforeExit + exit  -> clean event-loop drain
+//        only exit          -> process.exit() called somewhere
+//        neither            -> hard kill (SIGKILL/OOM/runner)
+//        signal lines       -> SIGTERM / SIGINT / SIGBREAK received
+//
+// Drop this file once the flake's root cause is identified and fixed.
+
+import {writeSync} from 'node:fs';
+
+const t0 = Date.now();
+let lastSeenTest = '<no test seen yet>';
+
+const diag = (msg: string): void => {
+  const line = `[diag +${Date.now() - t0}ms] ${msg}\n`;
+  try {
+    writeSync(2, line);
+  } catch (_) {
+    // Best-effort: if stderr is closed there is nothing we can do.
+  }
+};
+
+diag('diagnostics loaded');
+
+process.on('unhandledRejection', (reason: any) => {
+  diag(`unhandledRejection: ${
+    reason && reason.stack ? reason.stack : String(reason)
+  } (lastTest="${lastSeenTest}")`);
+  // Re-throw so existing common.ts handlers / mocha behavior is preserved.
+  throw reason;
+});
+
+process.on('uncaughtException', (err: any) => {
+  diag(`uncaughtException: ${
+    err && err.stack ? err.stack : String(err)
+  } (lastTest="${lastSeenTest}")`);
+  // Force fail-fast. Specs that don't import common.ts only have THIS handler,
+  // and Node won't exit on its own once an uncaughtException listener is
+  // registered. Without the explicit exit a fatal error would be swallowed.
+  // common.ts has the same process.exit(1); whichever handler runs first wins.
+  process.exit(1);
+});
+
+process.on('beforeExit', (code: number) => {
+  diag(`beforeExit code=${code} exitCode=${process.exitCode} ` +
+    `lastTest="${lastSeenTest}"`);
+});
+
+process.on('exit', (code: number) => {
+  diag(`exit code=${code} lastTest="${lastSeenTest}"`);
+});
+
+for (const sig of ['SIGINT', 'SIGTERM', 'SIGHUP', 'SIGBREAK'] as const) {
+  // SIGHUP / SIGBREAK don't exist on every platform; ignore registration errors.
+  try {
+    process.on(sig as any, () => {
+      diag(`received ${sig} (lastTest="${lastSeenTest}")`);
+      // Let the default behavior (exit) happen.
+      process.exit(128);
+    });
+  } catch (_) {
+    // ignore
+  }
+}
+
+// Mocha root hook — only registered if mocha picks up this file via --require.
+// We track the most recently-finished test so the death point is visible.
+export const mochaHooks = {
+  afterEach(this: any) {
+    if (this.currentTest) {
+      lastSeenTest = this.currentTest.fullTitle();
+    }
+  },
+};