diff --git a/agent/conversation_loop.py b/agent/conversation_loop.py
index fae6ff2e6950..0c0660e9cdcc 100644
--- a/agent/conversation_loop.py
+++ b/agent/conversation_loop.py
@@ -559,6 +559,20 @@ def _stored_prompt_matches_runtime(agent, prompt: str) -> bool:
     if stored_provider and current_provider and stored_provider != current_provider:
         return False
 
+    # Detect cwd drift: if the stored prompt was built in a different working
+    # directory, reuse would silently inject a stale path into the prefix cache.
+    stored_cwd = line_value("Current working directory")
+    if stored_cwd:
+        import os as _os
+        if stored_cwd != _os.getcwd():
+            return False
+
+    # Detect runtime surface drift: desktop GUI vs non-desktop.
+    has_desktop_marker = "Runtime surface: you're running inside the Hermes desktop GUI app." in prompt
+    in_desktop = (os.environ.get("HERMES_DESKTOP", "").strip().lower() in {"1", "true", "yes"})
+    if has_desktop_marker != in_desktop:
+        return False
+
     return True
 
 
diff --git a/tests/run_agent/test_compression_persistence.py b/tests/run_agent/test_compression_persistence.py
index 9f0f6e70fa5f..4532b84de9f3 100644
--- a/tests/run_agent/test_compression_persistence.py
+++ b/tests/run_agent/test_compression_persistence.py
@@ -437,3 +437,51 @@ class TestGatewayHistoryOffsetAfterSplit:
         assert len(new_messages) == 0, (
             "Expected 0 messages with stale offset=200 (demonstrates the bug)"
         )
+
+
+class TestStoredPromptCwdDrift:
+    """Verify that stored system prompts are rejected when cwd changed."""
+
+    def _make_agent(self, model="test/model", provider="openrouter"):
+        class _Agent:
+            pass
+
+        agent = _Agent()
+        agent.model = model
+        agent.provider = provider
+        return agent
+
+    def test_stored_prompt_stale_when_cwd_differs(self):
+        """Different cwd should force a prompt rebuild."""
+        from unittest.mock import patch
+        from agent.conversation_loop import _stored_prompt_matches_runtime
+
+        agent = self._make_agent()
+        stored_prompt = (
+            "Current working directory: /project/old\n"
+            "Model: test/model\n"
+            "Provider: openrouter\n"
+        )
+
+        with patch("os.getcwd", return_value="/project/new"):
+            assert _stored_prompt_matches_runtime(agent, stored_prompt) is False, (
+                "Expected False when stored cwd differs from current cwd"
+            )
+
+    def test_stored_prompt_fresh_when_cwd_matches(self):
+        """Matching cwd should allow prompt reuse."""
+        from unittest.mock import patch
+        from agent.conversation_loop import _stored_prompt_matches_runtime
+
+        agent = self._make_agent()
+        current_cwd = "/project/current"
+        stored_prompt = (
+            f"Current working directory: {current_cwd}\n"
+            "Model: test/model\n"
+            "Provider: openrouter\n"
+        )
+
+        with patch("os.getcwd", return_value=current_cwd):
+            assert _stored_prompt_matches_runtime(agent, stored_prompt) is True, (
+                "Expected True when stored cwd matches current cwd"
+            )