diff --git a/changelog.d/fixes/10171-instrumentation-hook-boot-fatal-log.md b/changelog.d/fixes/10171-instrumentation-hook-boot-fatal-log.md
new file mode 100644
index 0000000000..3f2c0fb028
--- /dev/null
+++ b/changelog.d/fixes/10171-instrumentation-hook-boot-fatal-log.md
@@ -0,0 +1 @@
+- fix(cli): guarantee a non-empty `[STARTUP] Fatal:` log line for any instrumentation-hook boot throw, not just DB-driver init failures (#10171)
diff --git a/src/instrumentation.ts b/src/instrumentation.ts
index 5a92447398..87348b2741 100644
--- a/src/instrumentation.ts
+++ b/src/instrumentation.ts
@@ -8,12 +8,35 @@
  * @see https://nextjs.org/docs/app/building-your-application/optimizing/instrumentation
  */
 
-export async function register() {
+/**
+ * `registerNodejsFn` is only for tests to inject a fake without module-mocking
+ * (`node:test` does not support `mock.module` reliably here) — mirrors the
+ * same injection pattern as `ensureDbReadyForBoot` in `./instrumentation-node`.
+ */
+export async function register(registerNodejsFn?: () => Promise<void>) {
   if (process.env.NEXT_RUNTIME === "nodejs") {
-    // Literal path so Webpack emits the chunk (computed string breaks dev:
-    // MODULE_NOT_FOUND for ./instrumentation-node at runtime).
-    // Turbopack may still avoid tracing this into Edge when guarded by NEXT_RUNTIME.
-    const { registerNodejs } = await import("./instrumentation-node");
-    await registerNodejs();
+    try {
+      // Literal path so Webpack emits the chunk (computed string breaks dev:
+      // MODULE_NOT_FOUND for ./instrumentation-node at runtime).
+      // Turbopack may still avoid tracing this into Edge when guarded by NEXT_RUNTIME.
+      const registerNodejs =
+        registerNodejsFn ?? (await import("./instrumentation-node")).registerNodejs;
+      await registerNodejs();
+    } catch (err: unknown) {
+      // Outermost boot boundary (#10171): any throw during module-load of
+      // ./instrumentation-node OR anywhere inside registerNodejs() runs
+      // BEFORE initConsoleInterceptor() is wired up. ensureDbReadyForBoot's
+      // own #7773 guard already logs one specific failure class (DB driver
+      // init), but it does not cover a throw that happens before it is even
+      // reached (e.g. a module-load-time error importing ./instrumentation-node
+      // itself, as reported on native Windows/WSL2 boots). Without an
+      // unconditional log line here, the process can keep its HTTP listener
+      // up while every DB-touching route 500s forever with a permanently
+      // empty app.log. This guarantees stdout/app.log is never silently
+      // empty on a failed boot, regardless of platform or which step threw.
+      const message = err instanceof Error ? err.message : String(err);
+      console.error("[STARTUP] Fatal: instrumentation hook failed during boot:", message);
+      throw err;
+    }
   }
 }
diff --git a/tests/unit/instrumentation-hook-boot-fatal-log-10171.test.ts b/tests/unit/instrumentation-hook-boot-fatal-log-10171.test.ts
new file mode 100644
index 0000000000..9242043d0d
--- /dev/null
+++ b/tests/unit/instrumentation-hook-boot-fatal-log-10171.test.ts
@@ -0,0 +1,114 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { register } from "../../src/instrumentation";
+
+// Regression guard for #10171: on native Windows/WSL2 boots, the reported
+// symptom is a bare HTTP 500 on every DB-touching route with `app.log`
+// staying completely empty, even though the CLI prints "OmniRoute is
+// running!". ensureDbReadyForBoot() (#7773/#7828) already guarantees a
+// non-empty [STARTUP] Fatal: log line for ONE specific failure class
+// (database driver init), but nothing previously guaranteed a fatal throw
+// ANYWHERE during instrumentation-hook boot (including a throw before
+// ensureDbReadyForBoot is even reached) produces a diagnostic line. This
+// test proves register() now catches any such throw at the outermost
+// boundary and unconditionally logs a `[STARTUP] Fatal:` line to stdout
+// before rethrowing, so app.log is never silently empty on a failed boot.
+
+function captureConsoleError(): { captured: string[]; restore: () => void } {
+  const originalError = console.error;
+  const captured: string[] = [];
+  console.error = (...args: unknown[]) => {
+    captured.push(args.map((arg) => String(arg)).join(" "));
+  };
+  return {
+    captured,
+    restore: () => {
+      console.error = originalError;
+    },
+  };
+}
+
+test("#10171: any instrumentation-hook boot throw is logged with a non-empty [STARTUP] Fatal: line before rethrow", async () => {
+  const { captured, restore } = captureConsoleError();
+  const previousRuntime = process.env.NEXT_RUNTIME;
+  process.env.NEXT_RUNTIME = "nodejs";
+
+  // Simulates a module-load-time failure reaching the instrumentation hook
+  // BEFORE ensureDbReadyForBoot's own #7773 guard is reached — the class of
+  // failure the reporter's empty app.log suggests on native Windows/WSL2.
+  const bootFailureMessage = "Cannot find native binding for platform=win32-x64";
+  const fakeRegisterNodejs = async () => {
+    throw new Error(bootFailureMessage);
+  };
+
+  try {
+    await assert.rejects(
+      () => register(fakeRegisterNodejs),
+      (err: Error) => err.message === bootFailureMessage
+    );
+
+    const fatalLine = captured.find(
+      (line) => line.includes("[STARTUP] Fatal:") && line.includes(bootFailureMessage)
+    );
+    assert.ok(
+      fatalLine,
+      "Expected a non-empty '[STARTUP] Fatal:' console.error line containing the boot " +
+        "failure before it propagates, so app.log is never silently empty on a failed " +
+        "boot (#10171). None was logged."
+    );
+  } finally {
+    if (previousRuntime === undefined) {
+      delete process.env.NEXT_RUNTIME;
+    } else {
+      process.env.NEXT_RUNTIME = previousRuntime;
+    }
+    restore();
+  }
+});
+
+test("#10171: a non-Error throw during instrumentation-hook boot is still logged with a non-empty [STARTUP] Fatal: line", async () => {
+  const { captured, restore } = captureConsoleError();
+  const previousRuntime = process.env.NEXT_RUNTIME;
+  process.env.NEXT_RUNTIME = "nodejs";
+
+  // Mirrors real-world non-Error throws (e.g. sql.js's bare `throw "Database closed"`,
+  // #6560) reaching this outermost boundary before normalization.
+  const fakeRegisterNodejs = async () => {
+    throw "raw string boot failure";
+  };
+
+  try {
+    await assert.rejects(() => register(fakeRegisterNodejs));
+
+    const fatalLine = captured.find(
+      (line) => line.includes("[STARTUP] Fatal:") && line.includes("raw string boot failure")
+    );
+    assert.ok(fatalLine, "Expected a non-empty '[STARTUP] Fatal:' line for a non-Error throw too.");
+  } finally {
+    if (previousRuntime === undefined) {
+      delete process.env.NEXT_RUNTIME;
+    } else {
+      process.env.NEXT_RUNTIME = previousRuntime;
+    }
+    restore();
+  }
+});
+
+test("register() does not log anything on a clean successful boot", async () => {
+  const { captured, restore } = captureConsoleError();
+  const previousRuntime = process.env.NEXT_RUNTIME;
+  process.env.NEXT_RUNTIME = "nodejs";
+  const fakeRegisterNodejs = async () => {};
+
+  try {
+    await assert.doesNotReject(register(fakeRegisterNodejs));
+    assert.equal(captured.length, 0, "a successful boot must not emit any fatal startup log lines");
+  } finally {
+    if (previousRuntime === undefined) {
+      delete process.env.NEXT_RUNTIME;
+    } else {
+      process.env.NEXT_RUNTIME = previousRuntime;
+    }
+    restore();
+  }
+});