diff --git a/test/fixtures/runtime-default-boot.ts b/test/fixtures/runtime-default-boot.ts
new file mode 100644
index 0000000..913c589
--- /dev/null
+++ b/test/fixtures/runtime-default-boot.ts
@@ -0,0 +1,7 @@
+import { runtime } from 'moroutine';
+import { double } from './math.ts';
+
+// No registerRuntime() — first dispatch boots the DEFAULT definition.
+const viaRun = await runtime.run(double(4));
+console.log('SIZE ' + runtime.workers.length);
+console.log('DONE ' + viaRun);
diff --git a/test/fixtures/runtime-natural-exit.ts b/test/fixtures/runtime-natural-exit.ts
new file mode 100644
index 0000000..074703b
--- /dev/null
+++ b/test/fixtures/runtime-natural-exit.ts
@@ -0,0 +1,8 @@
+import { runtime, registerRuntime } from 'moroutine';
+import { tinyRuntime } from './runtime-def.ts';
+import { busy } from './load-balancing.ts';
+
+registerRuntime(tinyRuntime);
+const r = await runtime.run(busy(20));
+console.log('DONE ' + r);
+// No shutdown() call — the unref'd runtime must let the process exit here.
diff --git a/test/fixtures/runtime-shutdown.ts b/test/fixtures/runtime-shutdown.ts
new file mode 100644
index 0000000..04881b4
--- /dev/null
+++ b/test/fixtures/runtime-shutdown.ts
@@ -0,0 +1,20 @@
+import { runtime, registerRuntime } from 'moroutine';
+import { tinyRuntime } from './runtime-def.ts';
+import { double } from './math.ts';
+
+registerRuntime(tinyRuntime);
+await runtime.run(double(1));
+
+let signalFired = false;
+runtime.signal.addEventListener('abort', () => {
+  signalFired = true;
+});
+await runtime.shutdown();
+console.log('SIGNAL ' + signalFired);
+
+try {
+  await runtime.run(double(2));
+  console.log('POST-SHUTDOWN-DISPATCH no-throw');
+} catch (err) {
+  console.log('POST-SHUTDOWN-DISPATCH threw');
+}
diff --git a/test/runtime-lifecycle.test.ts b/test/runtime-lifecycle.test.ts
new file mode 100644
index 0000000..4a20002
--- /dev/null
+++ b/test/runtime-lifecycle.test.ts
@@ -0,0 +1,35 @@
+import { describe, it } from 'node:test';
+import assert from 'node:assert/strict';
+import { execFile } from 'node:child_process';
+import { promisify } from 'node:util';
+import { fileURLToPath } from 'node:url';
+import { join } from 'node:path';
+import { availableParallelism } from 'node:os';
+
+const exec = promisify(execFile);
+const fixturesDir = join(fileURLToPath(import.meta.url), '..', 'fixtures');
+
+describe('global runtime lifecycle', () => {
+  it('first dispatch lazily boots the default definition', async () => {
+    const { stdout } = await exec(process.execPath, ['--no-warnings', join(fixturesDir, 'runtime-default-boot.ts')], {
+      timeout: 20000,
+    });
+    assert.ok(stdout.includes(`SIZE ${availableParallelism()}`));
+    assert.ok(stdout.includes('DONE 8'));
+  });
+
+  it('process exits naturally without shutdown()', async () => {
+    const { stdout } = await exec(process.execPath, ['--no-warnings', join(fixturesDir, 'runtime-natural-exit.ts')], {
+      timeout: 15000,
+    });
+    assert.match(stdout, /DONE done/);
+  });
+
+  it('shutdown() fires signal, drains, and locks out further dispatch', async () => {
+    const { stdout } = await exec(process.execPath, ['--no-warnings', join(fixturesDir, 'runtime-shutdown.ts')], {
+      timeout: 15000,
+    });
+    assert.ok(stdout.includes('SIGNAL true'));
+    assert.ok(stdout.includes('POST-SHUTDOWN-DISPATCH threw'));
+  });
+});