diff --git a/agent/transports/codex_app_server_session.py b/agent/transports/codex_app_server_session.py
index a72599ae719..d9ee92dfbf5 100644
--- a/agent/transports/codex_app_server_session.py
+++ b/agent/transports/codex_app_server_session.py
@@ -404,7 +404,7 @@ class CodexAppServerSession:
             return result
 
         result.turn_id = (ts.get("turn") or {}).get("id")
-        deadline = time.time() + turn_timeout
+        deadline = time.monotonic() + turn_timeout
         turn_complete = False
         # Post-tool watchdog state. last_tool_completion_at is set whenever
         # a tool-shaped item completes; if no further notification arrives
@@ -412,7 +412,7 @@ class CodexAppServerSession:
         # fast-fail and retire the session.
         last_tool_completion_at: Optional[float] = None
 
-        while time.time() < deadline and not turn_complete:
+        while time.monotonic() < deadline and not turn_complete:
             if self._interrupt_event.is_set():
                 self._issue_interrupt(result.turn_id)
                 result.interrupted = True
@@ -440,7 +440,7 @@ class CodexAppServerSession:
             # up on this turn instead of waiting for the outer deadline.
             if (
                 last_tool_completion_at is not None
-                and (time.time() - last_tool_completion_at)
+                and (time.monotonic() - last_tool_completion_at)
                     > post_tool_quiet_timeout
             ):
                 self._issue_interrupt(result.turn_id)
@@ -471,7 +471,7 @@ class CodexAppServerSession:
                         result.projected_messages.extend(proj.messages)
                     if proj.is_tool_iteration:
                         result.tool_iterations += 1
-                        last_tool_completion_at = time.time()
+                        last_tool_completion_at = time.monotonic()
                     if proj.final_text is not None:
                         result.final_text = proj.final_text
                         if _has_turn_aborted_marker(proj.final_text):
@@ -514,7 +514,7 @@ class CodexAppServerSession:
                 result.tool_iterations += 1
                 # Arm/refresh the post-tool quiet watchdog whenever a
                 # tool-shaped item completes.
-                last_tool_completion_at = time.time()
+                last_tool_completion_at = time.monotonic()
             else:
                 # Any non-tool projected activity (assistant message,
                 # status update, etc.) means codex is still producing
diff --git a/tests/agent/transports/test_codex_app_server_session.py b/tests/agent/transports/test_codex_app_server_session.py
index f51996dd067..b192d64e1c8 100644
--- a/tests/agent/transports/test_codex_app_server_session.py
+++ b/tests/agent/transports/test_codex_app_server_session.py
@@ -9,10 +9,12 @@ from __future__ import annotations
 
 import threading
 import time
+from unittest.mock import patch
 from typing import Any, Optional
 
 import pytest
 
+import agent.transports.codex_app_server_session as session_mod
 from agent.transports.codex_app_server_session import (
     CodexAppServerSession,
     TurnResult,
@@ -344,6 +346,23 @@ class TestRunTurn:
         assert r.interrupted is True
         assert r.error and "timed out" in r.error
 
+    def test_deadline_uses_monotonic_clock(self):
+        client = FakeClient()
+        s = make_session(client)
+        monotonic_values = iter([1000.0, 999.0, 999.0, 1001.0])
+        with patch.object(
+            session_mod.time,
+            "monotonic",
+            side_effect=lambda: next(monotonic_values),
+        ):
+            r = s.run_turn(
+                "never finishes",
+                turn_timeout=0.1,
+                notification_poll_timeout=0.0,
+            )
+        assert r.interrupted is True
+        assert r.error and "timed out" in r.error
+
     def test_failed_turn_records_error_from_turn_completed(self):
         client = FakeClient()
         client.queue_notification(
@@ -666,6 +685,35 @@ class TestSessionRetirement:
         # Confirm we issued turn/interrupt to free codex compute
         assert any(method == "turn/interrupt" for (method, _) in client.requests)
 
+    def test_post_tool_watchdog_uses_monotonic_clock(self):
+        client = FakeClient()
+        client.queue_notification(
+            "item/completed",
+            item={
+                "type": "commandExecution", "id": "ex1",
+                "command": "echo hi", "cwd": "/tmp",
+                "status": "completed", "aggregatedOutput": "hi",
+                "exitCode": 0, "commandActions": [],
+            },
+            threadId="t", turnId="tu1",
+        )
+        s = make_session(client)
+        monotonic_values = iter([1000.0, 999.0, 999.0, 999.0, 1000.2])
+        with patch.object(
+            session_mod.time,
+            "monotonic",
+            side_effect=lambda: next(monotonic_values),
+        ):
+            r = s.run_turn(
+                "tool then silence",
+                turn_timeout=5.0,
+                notification_poll_timeout=0.0,
+                post_tool_quiet_timeout=0.15,
+            )
+        assert r.interrupted is True
+        assert r.should_retire is True
+        assert r.error and "silent" in r.error
+
     def test_post_tool_watchdog_resets_on_further_activity(self):
         """A tool completion followed by an agent message should NOT trip
         the watchdog — further activity = codex still alive."""