diff --git a/src/brain/prompt_optimizer.py b/src/brain/prompt_optimizer.py
index 7dc2c17..fdc0d99 100644
--- a/src/brain/prompt_optimizer.py
+++ b/src/brain/prompt_optimizer.py
@@ -179,8 +179,8 @@ class PromptOptimizer:
             # Minimal instructions
             prompt = (
                 f"{market_name} trader. Analyze:\n{data_str}\n\n"
-                'Return JSON: {"act":"BUY"|"SELL"|"HOLD","conf":<0-100>,"reason":"<text>"}\n'
-                "Rules: act=BUY/SELL/HOLD, conf=0-100, reason=concise. No markdown."
+                'Return JSON: {"action":"BUY"|"SELL"|"HOLD","confidence":<0-100>,"rationale":"<text>"}\n'
+                "Rules: action=BUY/SELL/HOLD, confidence=0-100, rationale=concise. No markdown."
             )
         else:
             # Data only (for cached contexts where instructions are known)
diff --git a/src/broker/overseas.py b/src/broker/overseas.py
index 84b0fdb..13f3903 100644
--- a/src/broker/overseas.py
+++ b/src/broker/overseas.py
@@ -133,7 +133,7 @@ class OverseasBroker:
                 "AUTH": "",
                 "EXCD": ranking_excd,
                 "NDAY": "0",
-                "GUBN": "1",
+                "GUBN": "0",  # 0=전체(상승+하락), 1=상승만 — 변동성 스캐너는 전체 필요
                 "VOL_RANG": "0",
             }
 
diff --git a/tests/test_overseas_broker.py b/tests/test_overseas_broker.py
index 65d6d60..3f346f6 100644
--- a/tests/test_overseas_broker.py
+++ b/tests/test_overseas_broker.py
@@ -124,7 +124,7 @@ class TestFetchOverseasRankings:
         assert "/uapi/overseas-stock/v1/ranking/updown-rate" in url
         assert params["EXCD"] == "NAS"
         assert params["NDAY"] == "0"
-        assert params["GUBN"] == "1"
+        assert params["GUBN"] == "0"  # 0=전체(상승+하락), 변동성 스캐너에 필요
         assert params["VOL_RANG"] == "0"
 
         overseas_broker._broker._auth_headers.assert_called_with("HHDFS76290000")
diff --git a/tests/test_token_efficiency.py b/tests/test_token_efficiency.py
index 96bcbfc..c049d1c 100644
--- a/tests/test_token_efficiency.py
+++ b/tests/test_token_efficiency.py
@@ -124,6 +124,10 @@ class TestPromptOptimizer:
         assert len(prompt) < 300
         assert "005930" in prompt
         assert "75000" in prompt
+        # Keys must match parse_response expectations (#242)
+        assert '"action"' in prompt
+        assert '"confidence"' in prompt
+        assert '"rationale"' in prompt
 
     def test_build_compressed_prompt_no_instructions(self):
         """Test compressed prompt without instructions."""