core/cli: fix Ctrl+C after mid-turn prompts (SIGINT cancel)

asyncio signal handlers freeze ContextVars at install time; reinstalling
while allow_task_cancel was still False left cancel permanently disabled.
Use a process-level prompt depth counter and reinstall only after depth=0.

src/plyngent/cli/interrupt.py

@@ -4,7 +4,6 @@ import asyncio
 import contextlib
 import signal
 from contextlib import contextmanager
-from contextvars import ContextVar
 from typing import TYPE_CHECKING
 
 if TYPE_CHECKING:
@@ -13,13 +12,16 @@ if TYPE_CHECKING:
 
     type SigHandler = Callable[[int, FrameType | None], None] | int | None
 
-_allow_task_cancel: ContextVar[bool] = ContextVar("allow_task_cancel", default=True)
+# Depth of nested pause_task_cancel_for_prompt. Must be a plain int (not ContextVar):
+# asyncio.add_signal_handler freezes the ContextVar snapshot at install time, so a
+# ContextVar reset after reinstall would leave SIGINT permanently non-cancelling.
+_prompt_depth: int = 0
 _reinstall_holder: list[Callable[[], None] | None] = [None]
 
 
 def allow_task_cancel() -> bool:
     """Whether the CLI SIGINT handler should cancel the in-flight turn task."""
-    return _allow_task_cancel.get()
+    return _prompt_depth == 0
 
 
 def set_sigint_reinstall(callback: Callable[[], None] | None) -> None:
@@ -34,8 +36,14 @@ def pause_task_cancel_for_prompt() -> Generator[None]:
     Must run on the main thread (signal handlers are main-thread only).
     Restores the default SIGINT handler so ``click.confirm`` can receive
     KeyboardInterrupt / Abort instead of the asyncio turn being cancelled.
+
+    Nested prompts are supported via a depth counter. The asyncio SIGINT
+    handler is reinstalled only after depth returns to 0, and only after
+    cancel is re-enabled, so the handler never freezes ``allow=False``.
     """
-    token = _allow_task_cancel.set(False)
+
+    global _prompt_depth  # noqa: PLW0603
+    _prompt_depth += 1
     loop_handler_removed = False
     previous: SigHandler = signal.SIG_DFL
     try:
@@ -56,11 +64,15 @@ def pause_task_cancel_for_prompt() -> Generator[None]:
     finally:
         with contextlib.suppress(ValueError):
             _ = signal.signal(signal.SIGINT, previous)
-        reinstall = _reinstall_holder[0]
-        if loop_handler_removed and reinstall is not None:
-            with contextlib.suppress(RuntimeError, NotImplementedError, ValueError):
-                reinstall()
-        _allow_task_cancel.reset(token)
+        _prompt_depth = max(0, _prompt_depth - 1)
+        # Reinstall only when fully out of prompts and cancel is allowed again.
+        # Order matters: decrement depth first so allow_task_cancel() is True
+        # before reinstall (and handlers never freeze allow=False).
+        if _prompt_depth == 0 and loop_handler_removed:
+            reinstall = _reinstall_holder[0]
+            if reinstall is not None:
+                with contextlib.suppress(RuntimeError, NotImplementedError, ValueError):
+                    reinstall()
 
 
 async def run_in_prompt_thread[**P, R](func: Callable[P, R], *args: P.args, **kwargs: P.kwargs) -> R:

src/plyngent/cli/retry.py

@@ -66,6 +66,9 @@ async def run_cancellable[T](coro: Coroutine[object, object, T]) -> T:
     installed = False
 
     def _on_sigint() -> None:
+        # allow_task_cancel() uses a process-level depth counter (not ContextVar)
+        # so this remains correct even if the handler was installed under a
+        # frozen context (asyncio signal handles capture contextvars).
         if allow_task_cancel() and not task.done():
             _ = task.cancel()
 

tests/test_cli/test_interrupt.py

@@ -1,16 +1,22 @@
 from __future__ import annotations
 
+import asyncio
+import signal
 from typing import TYPE_CHECKING
 
+import pytest
+
 from plyngent.cli.interrupt import (
     allow_task_cancel,
     pause_task_cancel_for_prompt,
     run_in_prompt_thread,
+    set_sigint_reinstall,
 )
 from plyngent.cli.limits import prompt_continue_limit, prompt_continue_limit_async
+from plyngent.cli.retry import run_cancellable
 
 if TYPE_CHECKING:
-    import pytest
+    pass
 
 
 def test_pause_task_cancel_for_prompt() -> None:
@@ -20,6 +26,16 @@ def test_pause_task_cancel_for_prompt() -> None:
     assert allow_task_cancel() is True
 
 
+def test_nested_pause_depth() -> None:
+    assert allow_task_cancel() is True
+    with pause_task_cancel_for_prompt():
+        assert allow_task_cancel() is False
+        with pause_task_cancel_for_prompt():
+            assert allow_task_cancel() is False
+        assert allow_task_cancel() is False
+    assert allow_task_cancel() is True
+
+
 def test_prompt_continue_limit_under_pause(monkeypatch: pytest.MonkeyPatch) -> None:
     def _confirm(*_a: object, **_k: object) -> bool:
         assert allow_task_cancel() is False
@@ -36,7 +52,6 @@ async def test_run_in_prompt_thread_pauses_cancel() -> None:
     def work() -> str:
         return "ok"
 
-    # ContextVar may not propagate to worker threads; assert pause around the call.
     result = await run_in_prompt_thread(work)
     assert result == "ok"
     assert allow_task_cancel() is True
@@ -48,3 +63,53 @@ async def test_prompt_continue_limit_async(monkeypatch: pytest.MonkeyPatch) -> N
 
     monkeypatch.setattr("click.confirm", _confirm)
     assert await prompt_continue_limit_async("too many rounds") is True
+
+
+async def test_sigint_cancels_after_prompt_pause() -> None:
+    """Regression: after a mid-turn prompt, SIGINT must still cancel the turn.
+
+    Previously, reinstalling the asyncio SIGINT handler while
+    allow_task_cancel was still False froze that value into the callback
+    forever (ContextVar snapshot at add_signal_handler).
+    """
+    loop = asyncio.get_running_loop()
+    try:
+        loop.add_signal_handler(signal.SIGINT, lambda: None)
+        loop.remove_signal_handler(signal.SIGINT)
+    except NotImplementedError, RuntimeError, ValueError:
+        pytest.skip("asyncio signal handlers not available on this platform")
+
+    started = asyncio.Event()
+    cancelled = asyncio.Event()
+
+    async def hang() -> None:
+        started.set()
+        try:
+            await asyncio.sleep(3600)
+        except asyncio.CancelledError:
+            cancelled.set()
+            raise
+
+    async def turn() -> None:
+        # Simulate soft-confirm pause mid-turn, then keep streaming.
+        with pause_task_cancel_for_prompt():
+            assert allow_task_cancel() is False
+        assert allow_task_cancel() is True
+        await hang()
+
+    task = asyncio.create_task(run_cancellable(turn()))
+    await started.wait()
+    # Give run_cancellable a tick to install the handler after the pause reinstall.
+    await asyncio.sleep(0)
+    assert allow_task_cancel() is True
+    # Deliver SIGINT the same way Ctrl+C does under asyncio.
+    loop.call_soon(lambda: None)  # ensure loop is processing
+    # Invoke the process signal: raise SIGINT to this process.
+    import os
+
+    os.kill(os.getpid(), signal.SIGINT)
+    with pytest.raises(asyncio.CancelledError):
+        await task
+    assert cancelled.is_set()
+    # Cleanup any leftover reinstall hook from run_cancellable
+    set_sigint_reinstall(None)