prompting: let Ctrl+C cancel poll-based prompt reads

The wait tool's prompt runs off-loop on a worker thread, but its read polls
select/msvcrt, so it can act on a cancel: the SIGINT target now sets a flag the
reader checks between polls and raises PromptCancelledError with, which the tool
reports as "cancelled by user" while the turn continues. run_prompt_async /
run_cancellable_prompt_async hand the matching hook to the CLI guard; a stale
cancel never leaks into the next prompt read.

src/plyngent/prompting.py

@@ -4,6 +4,7 @@ import asyncio
 import contextlib
 import select
 import sys
+import threading
 import time
 from contextlib import AbstractContextManager
 from dataclasses import dataclass
@@ -19,6 +20,30 @@ class NonInteractiveError(RuntimeError):
     """Raised when a prompt is required but no interactive backend is available."""
 
 
+class PromptCancelledError(RuntimeError):
+    """Raised by a poll-based prompt read when the human cancels it (Ctrl+C).
+
+    Only poll-based reads (``read_line_with_timeout``) can observe the cancel:
+    they check :func:`prompt_cancelled` between polls, so the reader thread
+    returns and nothing is left blocked on stdin.
+    """
+
+
+# Set by the SIGINT target registered for a poll-based prompt read (see
+# cli.interrupt.off_loop_prompt); cleared around each off-loop prompt.
+_prompt_cancel = threading.Event()
+
+
+def prompt_cancelled() -> bool:
+    """Whether the in-flight poll-based prompt read was cancelled by SIGINT."""
+    return _prompt_cancel.is_set()
+
+
+def cancel_prompt_read() -> None:
+    """Ask the in-flight poll-based prompt read to return (thread-safe)."""
+    _prompt_cancel.set()
+
+
 @dataclass(frozen=True, slots=True)
 class ChoiceOption:
     """One selectable option for :func:`choose`."""
@@ -117,6 +142,8 @@ def _read_line_with_timeout_posix(
 
     Returns the line (without trailing newline), ``""`` for an empty Enter, or
     ``None`` when the timeout elapses before any line is complete (EOF too).
+    Raises :class:`PromptCancelledError` when :func:`prompt_cancelled` turns true
+    while waiting (the poll loop notices within one interval).
     """
     _ = sys.stdout.write(prompt)
     _ = sys.stdout.flush()
@@ -125,6 +152,8 @@ def _read_line_with_timeout_posix(
         remaining = deadline - time.monotonic()
         if remaining <= 0:
             return None
+        if prompt_cancelled():
+            raise PromptCancelledError
         ready, _, _ = poll([stream], [], [], min(remaining, 0.1))
         if not ready:
             continue
@@ -143,6 +172,8 @@ def _read_line_with_timeout_windows(prompt: str, timeout: float) -> str | None:
     deadline = time.monotonic() + timeout
     buf: list[str] = []
     while time.monotonic() < deadline:
+        if prompt_cancelled():
+            raise PromptCancelledError
         if msvcrt.kbhit():
             ch = msvcrt.getwch()
             if ch in ("\r", "\n"):
@@ -171,7 +202,9 @@ def read_line_with_timeout(prompt: str, timeout: float) -> str | None:
 
     Returns the typed line (without trailing newline), ``""`` for an empty
     Enter, or ``None`` when the timeout elapses first (EOF too). Ctrl+C
-    propagates as :class:`KeyboardInterrupt`. POSIX polls stdin via ``select``;
+    propagates as :class:`KeyboardInterrupt`; :func:`cancel_prompt_read` (the
+    SIGINT target of an off-loop prompt read) raises
+    :class:`PromptCancelledError` instead. POSIX polls stdin via ``select``;
     Windows uses ``msvcrt`` char polling (best-effort).
     """
     if timeout < 0:
@@ -263,16 +296,18 @@ class NonInteractiveBackend:
 
 
 _backend: PromptBackend = ClickPromptBackend()
-_pause_factory: Callable[[], AbstractContextManager[None]] | None = None
+# CLI hook: context manager around an off-loop prompt. It receives the read's
+# cancel hook (poll-based reads) or None (reads that cannot be interrupted).
+_pause_factory: Callable[[Callable[[], None] | None], AbstractContextManager[None]] | None = None
 _prompt_lock = asyncio.Lock()
 
 
 def configure_prompting(
     *,
     backend: PromptBackend | None = None,
-    pause_factory: Callable[[], AbstractContextManager[None]] | None = None,
+    pause_factory: Callable[[Callable[[], None] | None], AbstractContextManager[None]] | None = None,
 ) -> None:
-    """Install process-wide prompt backend and optional cancel-pause context."""
+    """Install process-wide prompt backend and optional SIGINT guard for prompts."""
     global _backend, _pause_factory  # noqa: PLW0603
     if backend is not None:
         _backend = backend
@@ -284,10 +319,11 @@ def get_prompt_backend() -> PromptBackend:
 
 
 def reset_prompting() -> None:
-    """Restore default Click backend and clear pause hook (tests)."""
+    """Restore default Click backend, clear the pause hook and cancel flag (tests)."""
     global _backend, _pause_factory  # noqa: PLW0603
     _backend = ClickPromptBackend()
     _pause_factory = None
+    _prompt_cancel.clear()
 
 
 def _normalize_options(
@@ -464,13 +500,45 @@ def form(
         backend.echo("Starting over…")
 
 
-async def run_prompt_async[**P, R](func: Callable[P, R], *args: P.args, **kwargs: P.kwargs) -> R:
-    """Run a blocking prompt off the event loop, serialized, with optional SIGINT pause."""
+async def _prompt_off_loop[R](
+    func: Callable[..., R],
+    *args: object,
+    cancel_hook: Callable[[], None] | None,
+    **kwargs: object,
+) -> R:
+    """Shared body: run *func* off the event loop under the CLI's SIGINT guard."""
     async with _prompt_lock:
-        if _pause_factory is not None:
-            with _pause_factory():
-                return await asyncio.to_thread(func, *args, **kwargs)
-        return await asyncio.to_thread(func, *args, **kwargs)
+        _prompt_cancel.clear()
+        try:
+            if _pause_factory is not None:
+                with _pause_factory(cancel_hook):
+                    return await asyncio.to_thread(func, *args, **kwargs)
+            return await asyncio.to_thread(func, *args, **kwargs)
+        finally:
+            _prompt_cancel.clear()
+
+
+async def run_prompt_async[**P, R](func: Callable[P, R], *args: P.args, **kwargs: P.kwargs) -> R:
+    """Run a blocking prompt off the event loop, serialized, SIGINT-guarded.
+
+    While the prompt waits, a Ctrl+C must not cancel the enclosing turn: the CLI
+    guard registers the prompt as the SIGINT target. readline-based reads cannot
+    be interrupted without abandoning the reader thread on stdin, so they ignore
+    the interrupt with a one-line hint (see :func:`run_cancellable_prompt_async`
+    for poll-based reads).
+    """
+    return await _prompt_off_loop(func, *args, cancel_hook=None, **kwargs)
+
+
+async def run_cancellable_prompt_async[**P, R](func: Callable[P, R], *args: P.args, **kwargs: P.kwargs) -> R:
+    """Run a *poll-based* prompt read off the event loop; Ctrl+C cancels it.
+
+    Only for reads that observe :func:`prompt_cancelled`
+    (``read_line_with_timeout``): the SIGINT raises :class:`PromptCancelledError`
+    in the reader thread, so it returns to the caller instead of leaving a thread
+    blocked on stdin (and without cancelling the enclosing turn).
+    """
+    return await _prompt_off_loop(func, *args, cancel_hook=cancel_prompt_read, **kwargs)
 
 
 async def ask_async(prompt: str, *, default: str | None = None) -> str:

src/plyngent/tools/chat/wait.py

@@ -7,7 +7,12 @@ import sys
 from click import style
 
 from plyngent.agent import ToolTag, tool
-from plyngent.prompting import get_prompt_backend, read_line_with_timeout, run_prompt_async
+from plyngent.prompting import (
+    PromptCancelledError,
+    get_prompt_backend,
+    read_line_with_timeout,
+    run_cancellable_prompt_async,
+)
 from plyngent.tools.chat.shape import first_error, number_arg, string_error
 
 _ANSI_ESCAPE_RE = re.compile(r"\x1b\[[0-9;]*m")
@@ -36,7 +41,10 @@ async def _interactive_wait(seconds: float, reason: str | None) -> str:
     """Show the two-line wait prompt and report how the wait ended."""
     label = f"{seconds:g}"
     prompt = _wait_prompt(seconds, reason=reason)
-    line = await run_prompt_async(read_line_with_timeout, prompt, seconds)
+    try:
+        line = await run_cancellable_prompt_async(read_line_with_timeout, prompt, seconds)
+    except PromptCancelledError:
+        return "cancelled by user"
     if line is None:
         return f"waited {label}s"
     text = line.strip()
@@ -51,8 +59,9 @@ async def wait(duration: int, *, reason: str | None = None) -> str:
 
     Interactive sessions show a two-line prompt: a status line, then an
     optional-reason input — pressing Enter (optionally after typing a reason)
-    "disturbs" the wait so the turn continues immediately. Non-interactive runs
-    simply sleep the full duration.
+    "disturbs" the wait so the turn continues immediately, and Ctrl+C cancels
+    the wait (the turn continues). Non-interactive runs simply sleep the full
+    duration.
     """
     seconds, duration_error = number_arg("wait", "duration", duration, example="5")
     reason_error = None if reason is None else string_error("wait", "reason", reason)

tests/test_cli/test_interrupt.py

@@ -21,7 +21,13 @@ from plyngent.cli.interrupt import (
 )
 from plyngent.cli.limits import prompt_continue_limit
 from plyngent.cli.retry import run_cancellable, sleep_cancellable
-from plyngent.prompting import temporary_backend
+from plyngent.prompting import (
+    PromptCancelledError,
+    configure_prompting,
+    prompt_cancelled,
+    run_cancellable_prompt_async,
+    temporary_backend,
+)
 from tests.test_prompting import ScriptedBackend
 
 if TYPE_CHECKING:
@@ -224,6 +230,37 @@ async def test_off_loop_prompt_cancel_hook_is_preferred(router: None) -> None:
     assert turn_calls == []
 
 
+async def test_sigint_cancels_off_loop_prompt_read(router: None) -> None:
+    """Wiring: SIGINT aborts a poll-based read (not the turn) and clears the flag."""
+    del router
+    turn_calls: list[str] = []
+    seen: list[str] = []
+
+    def _poll_read(prompt: str, timeout: float) -> str:
+        """Stand-in for ``read_line_with_timeout``: polls the cancel flag."""
+        del prompt
+        deadline = time.monotonic() + timeout
+        while time.monotonic() < deadline:
+            if prompt_cancelled():
+                seen.append("cancelled")
+                raise PromptCancelledError
+            time.sleep(0.01)
+        seen.append("timed out")
+        return ""
+
+    with temporary_backend(ScriptedBackend([])):
+        configure_prompting(pause_factory=off_loop_prompt)
+        with sigint_cancels(FakeTarget(turn_calls, "turn")):
+            reader = asyncio.create_task(run_cancellable_prompt_async(_poll_read, "p> ", 30.0))
+            await asyncio.sleep(0.05)
+            os.kill(os.getpid(), signal.SIGINT)
+            with pytest.raises(PromptCancelledError):
+                await asyncio.wait_for(reader, timeout=5)
+    assert seen == ["cancelled"]
+    assert turn_calls == []
+    assert not prompt_cancelled()  # cleared for the next prompt
+
+
 async def test_pause_task_cancel_for_prompt_swaps_sigint(router: None) -> None:
     """A main-thread blocking read takes SIGINT as a KeyboardInterrupt."""
     del router

tests/test_prompting.py

@@ -1,6 +1,8 @@
 from __future__ import annotations
 
+import contextlib
 import io
+from typing import TYPE_CHECKING
 
 import pytest
 
@@ -9,15 +11,33 @@ from plyngent.prompting import (
     FormField,
     NonInteractiveBackend,
     NonInteractiveError,
+    PromptCancelledError,
     _read_line_with_timeout_posix,
     ask,
+    cancel_prompt_read,
     choose,
+    configure_prompting,
     confirm,
     form,
+    prompt_cancelled,
     read_line_with_timeout,
+    reset_prompting,
+    run_cancellable_prompt_async,
+    run_prompt_async,
     temporary_backend,
 )
 
+if TYPE_CHECKING:
+    from collections.abc import Generator, Iterator
+
+
+@pytest.fixture(autouse=True)
+def reset_prompting_state() -> Iterator[None]:
+    """Prompting state (backend, SIGINT guard, cancel flag) is process-global."""
+    reset_prompting()
+    yield
+    reset_prompting()
+
 
 def _ready_poll(r, w, x, t):
     return (r, w, x)
@@ -51,6 +71,43 @@ def test_read_line_with_timeout_negative_timeout() -> None:
     assert read_line_with_timeout("p> ", -1) is None
 
 
+def test_read_line_with_timeout_cancelled() -> None:
+    """A cancel request (SIGINT during an off-loop prompt) ends the read."""
+    stream = io.StringIO()
+    cancel_prompt_read()
+    try:
+        assert prompt_cancelled()
+        with pytest.raises(PromptCancelledError):
+            _read_line_with_timeout_posix("p> ", 5.0, stream=stream, poll=_never_poll)
+    finally:
+        stream.close()
+
+
+async def test_run_prompt_async_clears_cancel_flag() -> None:
+    """A stale cancel request must not leak into the next prompt read."""
+    cancel_prompt_read()
+    with temporary_backend(NonInteractiveBackend()):
+        assert await run_prompt_async(lambda: "ok") == "ok"
+    assert not prompt_cancelled()
+
+
+async def test_run_prompt_async_passes_cancel_hook_only_when_cancellable() -> None:
+    """The CLI guard gets the read's cancel hook for poll-based reads only."""
+    seen: list[object] = []
+
+    @contextlib.contextmanager
+    def _guard(cancel: object) -> Generator[None]:
+        seen.append(cancel)
+        yield
+
+    with temporary_backend(NonInteractiveBackend()):
+        # temporary_backend resets the guard, so install it inside.
+        configure_prompting(pause_factory=_guard)
+        assert await run_cancellable_prompt_async(lambda: "a") == "a"
+        assert await run_prompt_async(lambda: "b") == "b"
+    assert seen == [cancel_prompt_read, None]
+
+
 class ScriptedBackend:
     """Deterministic backend for unit tests."""
 

tests/test_tools/test_chat_tools.py

@@ -3,8 +3,10 @@ from __future__ import annotations
 import importlib
 import json
 
+import pytest
+
 from plyngent.agent import ToolRegistry
-from plyngent.prompting import NonInteractiveBackend, temporary_backend
+from plyngent.prompting import NonInteractiveBackend, PromptCancelledError, temporary_backend
 from plyngent.tools.chat import CHAT_TOOLS, ask_user, choose_user, form_user, wait
 from tests.test_prompting import ScriptedBackend
 
@@ -146,6 +148,20 @@ async def test_wait_tool_times_out(monkeypatch) -> None:
     assert out == "waited 3s"
 
 
+async def test_wait_tool_cancelled_by_user(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Ctrl+C during the wait prompt cancels the wait; the turn continues."""
+
+    def _cancelled(prompt: str, timeout: float) -> str:
+        del prompt, timeout
+        raise PromptCancelledError
+
+    monkeypatch.setattr(wait_module, "read_line_with_timeout", _cancelled)
+    with temporary_backend(ScriptedBackend([])):
+        registry = ToolRegistry([wait])
+        out = await registry.execute("wait", '{"duration": 30}')
+    assert out == "cancelled by user"
+
+
 async def test_wait_tool_prompt_two_lines_with_reason(monkeypatch) -> None:
     prompts: list[str] = []
     monkeypatch.setattr(