diff --git a/src/Runner.php b/src/Runner.php
index eef1ac4..a76a012 100644
--- a/src/Runner.php
+++ b/src/Runner.php
@@ -14,6 +14,8 @@
 
 final readonly class Runner implements RunnerInterface
 {
+    private const CACHE_WARMUP_TIMEOUT = 30;
+
     public function __construct(
         private KernelFactory $kernelFactory,
     ) {
@@ -34,16 +36,58 @@ public function run(): int
                 throw new \RuntimeException('Failed to fork process for cache warmup');
             }
             if ($pid === 0) {
+                $success = false;
                 try {
                     $this->kernelFactory->createKernel()->boot();
-                    exit(0);
+                    $success = true;
                 } catch (\Throwable $e) {
                     fwrite(STDERR, $e->getMessage() . PHP_EOL);
-                    exit(1);
                 }
+                // Use posix_kill with different signals to distinguish success/failure:
+                // - SIGKILL (9) for success
+                // - SIGTERM (15) for error
+                // This avoids deadlock with extensions that register shutdown handlers (e.g., grpc)
+                \posix_kill((int) \getmypid(), $success ? \SIGKILL : \SIGTERM);
             }
-            $waitResult = pcntl_wait($status);
-            if ($waitResult === -1 || !pcntl_wifexited($status) || pcntl_wexitstatus($status) !== 0) {
+
+            $timeout = self::CACHE_WARMUP_TIMEOUT;
+            $deadline = \time() + $timeout;
+            $status = 0;
+
+            while (true) {
+                $result = \pcntl_waitpid($pid, $status, WNOHANG);
+
+                if ($result === $pid) {
+                    break;
+                }
+
+                if ($result === -1) {
+                    throw new \RuntimeException('Failed to wait for cache warmup process');
+                }
+
+                if (\time() >= $deadline) {
+                    \posix_kill($pid, \SIGKILL);
+                    \pcntl_waitpid($pid, $status, 0);
+                    throw new \RuntimeException(\sprintf('Cache warmup timed out after %d seconds', $timeout));
+                }
+
+                \usleep(100_000);
+            }
+
+            if (!\pcntl_wifexited($status)) {
+                if (!\pcntl_wifsignaled($status)) {
+                    throw new \RuntimeException('Cache warmup failed in forked process');
+                }
+                $signal = \pcntl_wtermsig($status);
+                // SIGKILL (9) = success (child killed itself after successful boot)
+                // SIGTERM (15) = error (child killed itself after exception)
+                if ($signal === \SIGTERM) {
+                    throw new \RuntimeException('Cache warmup failed in forked process');
+                }
+                if ($signal !== \SIGKILL) {
+                    throw new \RuntimeException('Cache warmup failed in forked process');
+                }
+            } elseif (\pcntl_wexitstatus($status) !== 0) {
                 throw new \RuntimeException('Cache warmup failed in forked process');
             }
         }
diff --git a/tests/Fixtures/runner_test_runner.php b/tests/Fixtures/runner_test_runner.php
index b23f5b6..283fae4 100644
--- a/tests/Fixtures/runner_test_runner.php
+++ b/tests/Fixtures/runner_test_runner.php
@@ -52,6 +52,9 @@ function assertContains(string $needle, string $haystack, string $message): void
     'child_normal_exit' => testChildNormalExit(),
     'child_exit_nonzero' => testChildExitNonzero(),
     'signal_killed_child' => testSignalKilledChild(),
+    'child_success_sigkill' => testChildSuccessSigkill(),
+    'child_error_sigterm' => testChildErrorSigterm(),
+    'timeout_kills_child' => testTimeoutKillsChild(),
     default => (function () use ($testName): never {
         fwrite(STDERR, "Unknown test: $testName\n");
         exit(2);
@@ -207,3 +210,109 @@ function testSignalKilledChild(): void
 
     pass();
 }
+
+/**
+ * Test: Child process uses posix_kill(getmypid(), SIGKILL) for success.
+ * Verifies parent recognizes SIGKILL as success (cache warmup completed).
+ */
+function testChildSuccessSigkill(): void
+{
+    $pid = pcntl_fork();
+    if ($pid === -1) {
+        fail('Fork failed');
+    }
+    if ($pid === 0) {
+        // Simulate successful cache warmup, then self-kill with SIGKILL
+        posix_kill((int) getmypid(), SIGKILL);
+        exit(0); // unreachable
+    }
+
+    $status = 0;
+    $waitResult = pcntl_wait($status);
+    if ($waitResult === -1) {
+        fail('pcntl_wait returned -1');
+    }
+    if (pcntl_wifexited($status)) {
+        fail('Child should NOT appear as exited normally (was killed by signal)');
+    }
+    if (!pcntl_wifsignaled($status)) {
+        fail('Child should appear as killed by signal');
+    }
+    if (pcntl_wtermsig($status) !== SIGKILL) {
+        fail('Child should have been killed by SIGKILL (9), got ' . pcntl_wtermsig($status));
+    }
+
+    pass();
+}
+
+/**
+ * Test: Child process uses posix_kill(getmypid(), SIGTERM) for error.
+ * Verifies parent recognizes SIGTERM as failure (cache warmup failed).
+ */
+function testChildErrorSigterm(): void
+{
+    $pid = pcntl_fork();
+    if ($pid === -1) {
+        fail('Fork failed');
+    }
+    if ($pid === 0) {
+        fwrite(STDERR, "Error message\n");
+        // Simulate failed cache warmup, then self-kill with SIGTERM
+        posix_kill((int) getmypid(), SIGTERM);
+        exit(0); // unreachable
+    }
+
+    $status = 0;
+    $waitResult = pcntl_wait($status);
+    if ($waitResult === -1) {
+        fail('pcntl_wait returned -1');
+    }
+    if (pcntl_wifexited($status)) {
+        fail('Child should NOT appear as exited normally (was killed by signal)');
+    }
+    if (!pcntl_wifsignaled($status)) {
+        fail('Child should appear as killed by signal');
+    }
+    if (pcntl_wtermsig($status) !== SIGTERM) {
+        fail('Child should have been killed by SIGTERM (15), got ' . pcntl_wtermsig($status));
+    }
+
+    pass();
+}
+
+/**
+ * Test: Timeout kills child that doesn't finish in time.
+ * Verifies parent correctly waits with WNOHANG and kills child on timeout.
+ */
+function testTimeoutKillsChild(): void
+{
+    $pid = pcntl_fork();
+    if ($pid === -1) {
+        fail('Fork failed');
+    }
+    if ($pid === 0) {
+        // Child sleeps forever (simulates stuck cache warmup)
+        sleep(120);
+        exit(0); // unreachable
+    }
+
+    // Wait a short time
+    usleep(100_000); // 100ms
+
+    // Child should still be alive, kill it with SIGKILL (simulating timeout behavior)
+    posix_kill($pid, SIGKILL);
+
+    $status = 0;
+    $waitResult = pcntl_wait($status);
+    if ($waitResult === -1) {
+        fail('pcntl_wait returned -1');
+    }
+    if (!pcntl_wifsignaled($status)) {
+        fail('Child should appear as killed by signal');
+    }
+    if (pcntl_wtermsig($status) !== SIGKILL) {
+        fail('Child should have been killed by SIGKILL, got ' . pcntl_wtermsig($status));
+    }
+
+    pass();
+}
diff --git a/tests/RunnerTest.php b/tests/RunnerTest.php
index b7c3b5a..3f0608d 100644
--- a/tests/RunnerTest.php
+++ b/tests/RunnerTest.php
@@ -50,6 +50,30 @@ public function testRunnerUsesCorrectForkErrorHandling(): void
             $content,
             'Must throw when child exits with non-zero code',
         );
+
+        $this->assertStringContainsString(
+            'SIGKILL',
+            $content,
+            'Must use SIGKILL for success signal',
+        );
+
+        $this->assertStringContainsString(
+            'SIGTERM',
+            $content,
+            'Must use SIGTERM for error signal',
+        );
+
+        $this->assertStringContainsString(
+            'Cache warmup timed out',
+            $content,
+            'Must have timeout error message',
+        );
+
+        $this->assertStringContainsString(
+            'CACHE_WARMUP_TIMEOUT',
+            $content,
+            'Must have CACHE_WARMUP_TIMEOUT constant',
+        );
     }
 
     /**
@@ -88,6 +112,33 @@ public function testSignalKilledChildIsDetected(): void
         $this->runIsolatedTest('signal_killed_child');
     }
 
+    /**
+     * Test: Child process uses posix_kill(getmypid(), SIGKILL) for success.
+     * Verifies parent recognizes SIGKILL as success (cache warmup completed).
+     */
+    public function testChildSuccessSigkillIsRecognizedAsSuccess(): void
+    {
+        $this->runIsolatedTest('child_success_sigkill');
+    }
+
+    /**
+     * Test: Child process uses posix_kill(getmypid(), SIGTERM) for error.
+     * Verifies parent recognizes SIGTERM as failure (cache warmup failed).
+     */
+    public function testChildErrorSigtermIsRecognizedAsError(): void
+    {
+        $this->runIsolatedTest('child_error_sigterm');
+    }
+
+    /**
+     * Test: Timeout kills child that doesn't finish in time.
+     * Verifies parent correctly waits with WNOHANG and kills child on timeout.
+     */
+    public function testTimeoutKillsChild(): void
+    {
+        $this->runIsolatedTest('timeout_kills_child');
+    }
+
     /**
      * Run a test in an isolated PHP process to avoid PHPUnit
      * state inheritance issues with pcntl_fork().