diff --git a/src/cli/operations/dev/__tests__/dev-server.test.ts b/src/cli/operations/dev/__tests__/dev-server.test.ts
index b6ca6085b..3e00e0c14 100644
--- a/src/cli/operations/dev/__tests__/dev-server.test.ts
+++ b/src/cli/operations/dev/__tests__/dev-server.test.ts
@@ -1,7 +1,7 @@
 import type { DevConfig } from '../config.js';
 import { DevServer, type DevServerCallbacks, type DevServerOptions, type SpawnConfig } from '../dev-server.js';
 import { EventEmitter } from 'events';
-import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { type MockInstance, afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 
 const mockSpawn = vi.fn();
 vi.mock('child_process', () => ({
@@ -56,6 +56,8 @@ describe('DevServer', () => {
   let options: DevServerOptions;
   let server: TestDevServer;
   let mockChild: ReturnType<typeof createMockChildProcess>;
+  let onceSpy: MockInstance;
+  let removeListenerSpy: MockInstance;
 
   beforeEach(() => {
     onLog = vi.fn<DevServerCallbacks['onLog']>();
@@ -65,12 +67,20 @@ describe('DevServer', () => {
     server = new TestDevServer(config, options);
     mockChild = createMockChildProcess();
     mockSpawn.mockReturnValue(mockChild);
+    onceSpy = vi.spyOn(process, 'once').mockReturnValue(process);
+    removeListenerSpy = vi.spyOn(process, 'removeListener').mockReturnValue(process);
   });
 
   afterEach(() => {
+    vi.restoreAllMocks();
     vi.clearAllMocks();
   });
 
+  function getRegisteredExitHandler(): (() => void) | undefined {
+    const call = onceSpy.mock.calls.find(([event]) => event === 'exit');
+    return call?.[1] as (() => void) | undefined;
+  }
+
   describe('start()', () => {
     it('calls spawn with correct cmd, args, cwd, env, and stdio when prepare succeeds', async () => {
       await server.start();
@@ -173,6 +183,47 @@ describe('DevServer', () => {
     });
   });
 
+  describe('exit cleanup', () => {
+    it('reaps the detached process group on process exit', async () => {
+      mockChild.pid = 4242;
+      const processKillSpy = vi.spyOn(process, 'kill').mockImplementation(() => true);
+
+      await server.start();
+      const exitHandler = getRegisteredExitHandler();
+      expect(exitHandler).toBeDefined();
+
+      exitHandler!();
+      expect(processKillSpy).toHaveBeenCalledWith(-4242, 'SIGKILL');
+    });
+
+    it('does not register an exit reaper when the child has no pid', async () => {
+      await server.start();
+      expect(getRegisteredExitHandler()).toBeUndefined();
+    });
+
+    it('removes the exit reaper once the child exits on its own', async () => {
+      mockChild.pid = 4242;
+
+      await server.start();
+      const exitHandler = getRegisteredExitHandler();
+      mockChild.emit('exit', 0);
+
+      expect(removeListenerSpy).toHaveBeenCalledWith('exit', exitHandler);
+    });
+
+    it('swallows errors when the process group is already gone', async () => {
+      mockChild.pid = 4242;
+      vi.spyOn(process, 'kill').mockImplementation(() => {
+        throw new Error('kill ESRCH');
+      });
+
+      await server.start();
+      const exitHandler = getRegisteredExitHandler();
+
+      expect(() => exitHandler!()).not.toThrow();
+    });
+  });
+
   describe('output routing', () => {
     it('forwards stdout lines to onLog at info level', async () => {
       await server.start();
diff --git a/src/cli/operations/dev/dev-server.ts b/src/cli/operations/dev/dev-server.ts
index d44d4c765..3d208eb51 100644
--- a/src/cli/operations/dev/dev-server.ts
+++ b/src/cli/operations/dev/dev-server.ts
@@ -48,6 +48,7 @@ function isInternalFrame(line: string): boolean {
 
 export abstract class DevServer {
   protected child: ChildProcess | null = null;
+  private onProcessExit: (() => void) | null = null;
   private recentStderr: string[] = [];
   private inTraceback = false;
   private tracebackBuffer: string[] = [];
@@ -77,9 +78,38 @@ export abstract class DevServer {
     });
 
     this.attachHandlers();
+    this.registerExitCleanup();
     return this.child;
   }
 
+  /**
+   * Reap the detached child's process group when this process exits.
+   *
+   * The dev command's SIGINT/SIGTERM handler (which calls kill()) is registered after the
+   * global handler in setupAltScreenCleanup, which runs first and calls process.exit(0) — so
+   * kill() never runs and the child (its own process group since it is spawned detached) is
+   * orphaned, holding the port. The 'exit' event always fires on process.exit(), so use it as
+   * a last-resort reaper. POSIX only: on Windows the child is not detached and dies with the parent.
+   */
+  private registerExitCleanup(): void {
+    const pid = this.child?.pid;
+    if (isWindows || !pid) return;
+    this.onProcessExit = () => {
+      try {
+        process.kill(-pid, 'SIGKILL');
+      } catch {
+        // Group already gone — nothing to reap.
+      }
+    };
+    process.once('exit', this.onProcessExit);
+  }
+
+  private clearExitCleanup(): void {
+    if (!this.onProcessExit) return;
+    process.removeListener('exit', this.onProcessExit);
+    this.onProcessExit = null;
+  }
+
   /** Kill the dev server process tree. Sends SIGTERM to the process group, then SIGKILL after 2 seconds. */
   kill(): void {
     if (!this.child || this.child.killed) return;
@@ -203,6 +233,7 @@ export abstract class DevServer {
     });
 
     this.child?.on('exit', code => {
+      this.clearExitCleanup();
       if (code !== 0 && code !== null && this.recentStderr.length > 0) {
         for (const line of this.recentStderr) {
           onLog('error', line);