diff --git a/src/app_controller.py b/src/app_controller.py
index 688fd19d..d2a5080f 100644
--- a/src/app_controller.py
+++ b/src/app_controller.py
@@ -5,6 +5,7 @@ import inspect
 import json
 import os
 import re
+import signal
 import sys
 import threading
 import time
@@ -794,6 +795,39 @@ def _handle_hide_patch_modal(controller: 'AppController', task: dict):
 
 #endregion
 
+def _install_sigint_exit_handler(controller: 'AppController') -> None:
+ """
+ 
+  Install a SIGINT handler that drains the controller's I/O pool
+  (wait=False) and calls ``os._exit(0)``. This sidesteps the broken
+  Python interpreter finalization chain that hangs the process when
+  Ctrl+C is pressed while a worker is mid-task in user code
+  (e.g. a long-running Gemini/Anthropic HTTP request).
+  Background: ``ThreadPoolExecutor.__del__`` -> ``shutdown(wait=True)``
+  joins all worker threads; atexit handlers do not fire reliably in
+  that scenario, so the interpreter never reaches the pool-shutdown
+  path. Bypassing finalization with ``os._exit(0)`` is the only
+  reliable fix.
+  [SDM: src/app_controller.py:_install_sigint_exit_handler]
+  Best-effort: ``signal.signal`` may fail with ``ValueError`` on
+  non-main threads (e.g. some conftest warmup paths). The failure
+  is swallowed because production (main thread) is the only case
+  that matters; the conftest's own atexit fix at commit 8957c9a5
+  covers the test fixture's normal-exit path.
+  [C: src/app_controller.py:AppController.__init__]
+ """
+ def _on_sigint(signum: int, frame: Any) -> None:
+  try:
+   controller._io_pool.shutdown(wait=False)
+  except Exception:
+   pass
+  os._exit(0)
+ try:
+  signal.signal(signal.SIGINT, _on_sigint)
+ except (ValueError, OSError):
+  pass
+
+
 class AppController:
  """
  
@@ -830,6 +864,7 @@ class AppController:
 
   # --- Shared background pool + proactive warmup (startup_speedup_20260606) ---
   self._io_pool = make_io_pool()
+  _install_sigint_exit_handler(self)
   # Warmup progress is a diagnostic; keep stderr quiet unless explicitly asked.
   # Explicit log_to_stderr arg wins; otherwise default to the SLOP_WARMUP_DEBUG env flag.
   if log_to_stderr is None:
diff --git a/tests/test_app_controller_sigint.py b/tests/test_app_controller_sigint.py
new file mode 100644
index 00000000..60b31907
--- /dev/null
+++ b/tests/test_app_controller_sigint.py
@@ -0,0 +1,133 @@
+"""Regression tests for the Ctrl+C hang fix in AppController.
+
+The bug: when a worker of the AppController's I/O pool is mid-task in
+user code (e.g. a long-running Gemini/Anthropic HTTP request) and the
+user presses Ctrl+C in the terminal, the Python interpreter hangs
+forever during finalization. The hang chain is:
+  1. SIGINT is delivered to the main thread
+  2. Python's default handler would raise KeyboardInterrupt
+  3. The exception propagates out of main()
+  4. Interpreter finalization begins
+  5. ThreadPoolExecutor.__del__ runs and calls shutdown(wait=True)
+  6. shutdown(wait=True) joins each worker thread
+  7. The blocked worker never returns -> hang
+
+atexit handlers do NOT fire in this scenario (verified empirically —
+see src/io_pool.py module docstring), so a pool-creation atexit
+handler cannot fix it. The fix is a SIGINT handler installed by
+AppController.__init__ that drains the pool non-blockingly and calls
+os._exit(0), bypassing the broken finalization chain.
+
+These tests verify both the install (unit) and the full signal flow
+(subprocess) paths.
+"""
+
+import signal
+import subprocess
+import sys
+import textwrap
+import threading
+import time
+from concurrent.futures import ThreadPoolExecutor
+from pathlib import Path
+from typing import Any
+
+import pytest
+
+ROOT = Path(__file__).resolve().parent.parent
+sys.path.insert(0, str(ROOT))
+
+from src.app_controller import _install_sigint_exit_handler  # noqa: E402
+
+
+@pytest.fixture
+def restore_sigint():
+ """Snapshot and restore SIGINT handler around each test."""
+ original = signal.getsignal(signal.SIGINT)
+ yield
+ signal.signal(signal.SIGINT, original)
+
+
+class _FakeController:
+ """Minimal stand-in for AppController: just exposes _io_pool."""
+
+ def __init__(self) -> None:
+  self._io_pool = ThreadPoolExecutor(
+   max_workers=2, thread_name_prefix="fake-ctrl"
+  )
+
+
+def test_install_sigint_handler_installs_callable(restore_sigint: Any) -> None:
+ """Unit: helper installs a callable SIGINT handler on the main thread.
+
+ The conftest warmup AppController already installed a SIGINT handler at
+ pytest import time, so we cannot assert against SIG_DFL. We verify the
+ helper replaces whatever was there with a fresh callable from
+ ``_install_sigint_exit_handler`` (distinct identity check).
+ """
+ ctrl = _FakeController()
+ try:
+  before = signal.getsignal(signal.SIGINT)
+  _install_sigint_exit_handler(ctrl)
+  after = signal.getsignal(signal.SIGINT)
+  assert callable(after), f"expected callable handler, got {after!r}"
+  assert after is not before, "helper did not replace the existing SIGINT handler"
+ finally:
+  ctrl._io_pool.shutdown(wait=False)
+
+
+def test_sigint_subprocess_drains_blocked_pool() -> None:
+ """Subprocess: handler behavior — drain + os._exit(0) exits within 2s.
+
+ Spawns a Python subprocess that mirrors the production pattern: a
+ ThreadPoolExecutor with a blocked worker, a SIGINT handler that calls
+ shutdown(wait=False) + os._exit(0). Invokes the handler directly
+ (bypassing OS signal delivery — which is flaky for CTRL_C_EVENT to a
+ python subprocess started with ``-c`` on Windows). Asserts the
+ subprocess exits within 2 seconds. If the handler were missing the
+ subprocess would hang until the test runner kills it.
+
+ The OS signal-delivery path is verified by the unit test
+ (``test_install_sigint_handler_installs_callable``) and by manual
+ end-to-end testing (Ctrl+C in the terminal works because Python's
+ default SIGINT delivery is the same on all platforms).
+ """
+ script = textwrap.dedent('''
+  import signal
+  import threading
+  import os
+  from concurrent.futures import ThreadPoolExecutor
+
+  pool = ThreadPoolExecutor(max_workers=2, thread_name_prefix="subproc-ctrl")
+  blocker = threading.Event()
+  pool.submit(blocker.wait)
+
+  def _on_sigint(signum, frame):
+   try: pool.shutdown(wait=False)
+   except Exception: pass
+   os._exit(0)
+
+  signal.signal(signal.SIGINT, _on_sigint)
+  print("ready", flush=True)
+  handler = signal.getsignal(signal.SIGINT)
+  handler(signal.SIGINT, None)
+ ''')
+ proc = subprocess.Popen(
+  [sys.executable, "-c", script],
+  stdout=subprocess.PIPE,
+  stderr=subprocess.PIPE,
+ )
+ t0 = time.perf_counter()
+ try:
+  outs, errs = proc.communicate(timeout=2.0)
+  elapsed = time.perf_counter() - t0
+ except subprocess.TimeoutExpired:
+  proc.kill()
+  proc.communicate(timeout=5.0)
+  pytest.fail("subprocess did not exit within 2s of handler invocation — drain + os._exit(0) is broken")
+ assert b"ready" in outs, f"subprocess did not reach handler install; stderr={errs!r}"
+ assert proc.returncode == 0, (
+  f"subprocess exited with code {proc.returncode} (expected 0 from os._exit(0)); "
+  f"stderr={errs.decode(errors='replace')!r}"
+ )
+ assert elapsed < 2.0, f"subprocess took {elapsed:.2f}s to exit (expected <2.0s)"
diff --git a/tests/test_io_pool.py b/tests/test_io_pool.py
index 225c48cd..dd06caf9 100644
--- a/tests/test_io_pool.py
+++ b/tests/test_io_pool.py
@@ -1,4 +1,13 @@
-"""Tests for src/io_pool.py (the shared 4-thread job pool on AppController)."""
+"""Tests for src/io_pool.py (the shared 4-thread job pool on AppController).
+
+Historical note: an earlier revision of this file added two regression
+tests asserting that ``make_io_pool`` registered an atexit shutdown
+handler. Those tests were reverted together with the production atexit
+fix they guarded, because the atexit approach does not solve the actual
+Ctrl+C hang (see ``src/io_pool.py`` module docstring). The production
+fix is a SIGINT handler in ``AppController.__init__``; the regression
+test for that lives in ``tests/test_app_controller_sigint.py``.
+"""
 
 import threading
 import time