diff --git a/src/backend/app.py b/src/backend/app.py
index 550b1e1a3..4685b7fd1 100644
--- a/src/backend/app.py
+++ b/src/backend/app.py
@@ -161,6 +161,14 @@ async def handle_chat():
     """Unified chat endpoint - routes messages to appropriate handlers based on intent."""
     data = await request.get_json()
 
+    if not data:
+        return jsonify({
+            "action_type": "error",
+            "message": "Request body is required",
+            "data": {},
+            "conversation_id": ""
+        }), 400
+
     # Extract request fields
     conversation_id = data.get("conversation_id") or str(uuid.uuid4())
     user_id = data.get("user_id", "anonymous")
@@ -168,6 +176,15 @@ async def handle_chat():
     action = data.get("action")
     payload = data.get("payload", {})
 
+    # Validate that message content is not empty when no action is specified
+    if not action and not message.strip():
+        return jsonify({
+            "action_type": "error",
+            "message": "Message content must not be empty",
+            "data": {},
+            "conversation_id": conversation_id
+        }), 400
+
     selected_products = data.get("selected_products", [])
     brief_data = data.get("brief", {})
 
diff --git a/src/tests/test_app.py b/src/tests/test_app.py
index 57d9237d1..7882a72ae 100644
--- a/src/tests/test_app.py
+++ b/src/tests/test_app.py
@@ -63,7 +63,46 @@ async def test_health_check_api(client):
 
 @pytest.mark.asyncio
 async def test_chat_missing_message(client):
-    """Test chat endpoint with missing message still returns response (no validation)."""
+    """Test chat endpoint rejects missing/empty message with 400."""
+    response = await client.post(
+        "/api/chat",
+        json={"conversation_id": "test-conv"}
+    )
+
+    assert response.status_code == 400
+    data = await response.get_json()
+    assert data["action_type"] == "error"
+    assert "empty" in data["message"].lower()
+
+
+@pytest.mark.asyncio
+async def test_chat_empty_body(client):
+    """Test chat endpoint rejects empty request body with 400."""
+    response = await client.post(
+        "/api/chat",
+        data="",
+        headers={"Content-Type": "application/json"}
+    )
+
+    assert response.status_code == 400
+
+
+@pytest.mark.asyncio
+async def test_chat_whitespace_message(client):
+    """Test chat endpoint rejects whitespace-only message with 400."""
+    response = await client.post(
+        "/api/chat",
+        json={"conversation_id": "test-conv", "message": "   "}
+    )
+
+    assert response.status_code == 400
+    data = await response.get_json()
+    assert data["action_type"] == "error"
+
+
+@pytest.mark.asyncio
+async def test_chat_empty_message_with_action_allowed(client):
+    """Test chat endpoint allows empty message when action is specified."""
     with patch("app.get_routing_service") as mock_routing, \
          patch("app.get_cosmos_service") as mock_cosmos, \
          patch("app.get_orchestrator") as mock_orch:
@@ -88,10 +127,10 @@ async def test_chat_missing_message(client):
 
         response = await client.post(
             "/api/chat",
-            json={"conversation_id": "test-conv"}
+            json={"conversation_id": "test-conv", "action": "confirm_brief", "message": ""}
         )
 
-        # API doesn't validate missing message - routes to handler with empty message
+        # Action-based requests bypass message validation
         assert response.status_code in [200, 500]
 
 
@@ -187,36 +226,15 @@ async def test_chat_cosmos_failure(client):
 
 @pytest.mark.asyncio
 async def test_parse_brief_missing_text(client):
-    """Test chat endpoint with missing message still processes (no validation)."""
-    with patch("app.get_routing_service") as mock_routing, \
-         patch("app.get_cosmos_service") as mock_cosmos, \
-         patch("app.get_orchestrator") as mock_orch:
-
-        from services.routing_service import Intent, RoutingResult, ConversationState
-        mock_routing_service = MagicMock()
-        mock_routing_service.classify_intent = MagicMock(return_value=RoutingResult(
-            intent=Intent.PARSE_BRIEF,
-            confidence=0.5
-        ))
-        mock_routing_service.derive_state_from_conversation = MagicMock(return_value=ConversationState())
-        mock_routing.return_value = mock_routing_service
-
-        mock_cosmos_service = AsyncMock()
-        mock_cosmos_service.get_conversation = AsyncMock(return_value=None)
-        mock_cosmos_service.add_message_to_conversation = AsyncMock()
-        mock_cosmos.return_value = mock_cosmos_service
-
-        mock_orchestrator = AsyncMock()
-        mock_orchestrator.parse_brief = AsyncMock(return_value=(MagicMock(model_dump=lambda: {}), None, False))
-        mock_orch.return_value = mock_orchestrator
-
-        response = await client.post(
-            "/api/chat",
-            json={"conversation_id": "test-conv"}
-        )
+    """Test chat endpoint rejects missing message with 400."""
+    response = await client.post(
+        "/api/chat",
+        json={"conversation_id": "test-conv"}
+    )
 
-        # API doesn't validate missing message - routes to handler with empty message
-        assert response.status_code in [200, 500]
+    assert response.status_code == 400
+    data = await response.get_json()
+    assert data["action_type"] == "error"
 
 
 @pytest.mark.asyncio
@@ -864,39 +882,17 @@ async def test_regenerate_content_success(client, sample_creative_brief_dict):
 
 @pytest.mark.asyncio
 async def test_regenerate_content_missing_modification_request(client, sample_creative_brief_dict):
-    """Test regeneration without message still routes (no validation)."""
-    with patch("app.get_routing_service") as mock_routing, \
-         patch("app.get_cosmos_service") as mock_cosmos, \
-         patch("app.get_orchestrator") as mock_orch:
-
-        from services.routing_service import Intent, RoutingResult, ConversationState
-        mock_routing_service = MagicMock()
-        mock_routing_service.classify_intent = MagicMock(return_value=RoutingResult(
-            intent=Intent.PARSE_BRIEF,
-            confidence=0.5
-        ))
-        mock_routing_service.derive_state_from_conversation = MagicMock(return_value=ConversationState())
-        mock_routing.return_value = mock_routing_service
-
-        mock_cosmos_service = AsyncMock()
-        mock_cosmos_service.get_conversation = AsyncMock(return_value=None)
-        mock_cosmos_service.add_message_to_conversation = AsyncMock()
-        mock_cosmos.return_value = mock_cosmos_service
-
-        mock_orchestrator = AsyncMock()
-        mock_orchestrator.parse_brief = AsyncMock(return_value=(MagicMock(model_dump=lambda: {}), None, False))
-        mock_orch.return_value = mock_orchestrator
-
-        response = await client.post(
-            "/api/chat",
-            json={
-                "conversation_id": "test-conv"
-                # Missing message - no validation in backend
-            }
-        )
+    """Test regeneration rejects missing message with 400."""
+    response = await client.post(
+        "/api/chat",
+        json={
+            "conversation_id": "test-conv"
+        }
+    )
 
-        # Backend doesn't validate missing message
-        assert response.status_code in [200, 500]
+    assert response.status_code == 400
+    data = await response.get_json()
+    assert data["action_type"] == "error"
 
 
 @pytest.mark.asyncio