feat(tools): optional reject_yield hook before agent finish

cecli/tools/_yield.py

@@ -1,7 +1,6 @@
 import asyncio
 import logging
 
-from cecli.helpers.threading import ThreadSafeEvent
 from cecli.tools.utils.base_tool import BaseTool
 from cecli.tools.utils.helpers import ToolError
 from cecli.tools.utils.output import color_markers, tool_footer, tool_header
@@ -17,10 +16,7 @@ class Tool(BaseTool):
         "type": "function",
         "function": {
             "name": "Yield",
-            "description": (
-                "Yield control to subagents, to await their results or back to the user,"
-                " indicating all sub-goals are complete."
-            ),
+            "description": "Yield control back to the user, indicating all sub-goals are complete.",
             "parameters": {
                 "type": "object",
                 "properties": {
@@ -50,6 +46,12 @@ async def execute(cls, coder, **kwargs):
         cls.clear_invocation_cache()
 
         if coder:
+            reject = getattr(coder, "reject_yield", None)
+            if callable(reject):
+                blocked = reject(coder, **kwargs)
+                if blocked:
+                    return blocked
+
             # Check for active child sub-agents and await their tasks before finishing
             try:
                 agent_service = AgentService.get_instance(coder)
@@ -69,7 +71,7 @@ async def execute(cls, coder, **kwargs):
                     # the interrupt event, avoiding nested asyncio.wait() calls.
                     interrupt_event = coder.interrupt_event
                     if interrupt_event is None:
-                        interrupt_event = ThreadSafeEvent()
+                        interrupt_event = asyncio.Event()
 
                     interrupt_task = asyncio.create_task(interrupt_event.wait())
                     pending = set(active_tasks) | {interrupt_task}

tests/tools/test_yield_guard.py

@@ -0,0 +1,32 @@
+"""Yield guard on implement turns (reject_yield hook)."""
+
+from __future__ import annotations
+
+import asyncio
+import unittest
+
+from cecli.tools._yield import Tool
+
+
+class _CoderStub:
+    def __init__(self, *, reject_message: str | None = None):
+        self.reject_yield = (
+            (lambda _c, **_k: reject_message) if reject_message is not None else None
+        )
+        self.agent_finished = False
+
+
+class TestYieldGuard(unittest.TestCase):
+    def test_yield_rejected_when_hook_blocks(self):
+        coder = _CoderStub(
+            reject_message="Yield rejected: no file edits saved this implement turn."
+        )
+
+        result = asyncio.run(Tool.execute(coder, summary="done"))
+
+        self.assertIn("Yield rejected", result)
+        self.assertFalse(coder.agent_finished)
+
+
+if __name__ == "__main__":
+    unittest.main()