diff --git a/apps/desktop/vite.config.ts b/apps/desktop/vite.config.ts
index 0512c6c759e80..0bfc6bb2b1d35 100644
--- a/apps/desktop/vite.config.ts
+++ b/apps/desktop/vite.config.ts
@@ -6,6 +6,9 @@ import path from 'path'
 export default defineConfig({
   base: './',
   plugins: [react(), tailwindcss()],
+  test: {
+    environment: 'jsdom'
+  },
   build: {
     // Keep desktop packaging stable: Shiki ships many dynamic chunks by
     // default, and electron-builder can OOM scanning thousands of files.
diff --git a/run_agent.py b/run_agent.py
index 7e3a211caff12..ac0a33cc9158a 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -5324,7 +5324,13 @@ def chat(self, message: str, stream_callback: Optional[callable] = None) -> str:
             str: Final assistant response
         """
         result = self.run_conversation(message, stream_callback=stream_callback)
-        return result["final_response"]
+        # run_conversation has early-return / error paths (interrupted, retries
+        # exhausted, policy/billing bail) that omit "final_response". Honor the
+        # documented `-> str` contract instead of crashing with KeyError, so a
+        # run that never produced a final turn is recorded as a clean empty
+        # result (treated as a failed run upstream) rather than a harness Python
+        # traceback that gets misclassified as a launcher/harness fault.
+        return result.get("final_response") or ""
 
     def _run_codex_app_server_turn(
         self,
@@ -5517,10 +5523,11 @@ def main(
     print(f"📞 API Calls: {result['api_calls']}")
     print(f"💬 Messages: {len(result['messages'])}")
     
-    if result['final_response']:
+    _final_response = result.get('final_response')
+    if _final_response:
         print("\n🎯 FINAL RESPONSE:")
         print("-" * 30)
-        print(result['final_response'])
+        print(_final_response)
     
     # Save sample trajectory to UUID-named file if requested
     if save_sample:
diff --git a/tests/test_chat_final_response_guard.py b/tests/test_chat_final_response_guard.py
new file mode 100644
index 0000000000000..0f84a042081ca
--- /dev/null
+++ b/tests/test_chat_final_response_guard.py
@@ -0,0 +1,37 @@
+"""Tests for AIAgent.chat() final_response guard.
+
+run_agent.py:chat() must honor its ``-> str`` contract even when
+run_conversation returns an early-exit / error dict that omits
+``"final_response"`` (interrupted, retries exhausted, policy/billing bail).
+Before the guard this raised ``KeyError: 'final_response'``, which crashed the
+dispatched ``hermes -z`` goal worker and was misclassified upstream as a
+harness fault instead of a clean "no final response → failed run".
+"""
+
+from unittest.mock import patch
+
+from run_agent import AIAgent
+
+
+def _agent_without_init() -> AIAgent:
+    # Bypass __init__ (heavy: providers/config/IO). We only exercise chat()'s
+    # handling of the run_conversation return dict.
+    return AIAgent.__new__(AIAgent)
+
+
+def test_chat_returns_empty_string_when_final_response_key_missing():
+    agent = _agent_without_init()
+    with patch.object(agent, "run_conversation", return_value={"completed": False, "api_calls": 1}):
+        assert agent.chat("hi") == ""
+
+
+def test_chat_returns_empty_string_when_final_response_is_none():
+    agent = _agent_without_init()
+    with patch.object(agent, "run_conversation", return_value={"final_response": None}):
+        assert agent.chat("hi") == ""
+
+
+def test_chat_returns_final_response_when_present():
+    agent = _agent_without_init()
+    with patch.object(agent, "run_conversation", return_value={"final_response": "the answer"}):
+        assert agent.chat("hi") == "the answer"