fix(rag): avoid empty system messages that crash Vertex AI

agent_cli/rag/engine.py

@@ -134,7 +134,8 @@ def _convert_messages(
 
     for m in history_msgs:
         if m.role == "system":
-            pyd_messages.append(ModelRequest(parts=[SystemPromptPart(content=m.content)]))
+            if m.content:  # Skip empty system messages (rejected by some providers)
+                pyd_messages.append(ModelRequest(parts=[SystemPromptPart(content=m.content)]))
         elif m.role == "user":
             pyd_messages.append(ModelRequest(parts=[UserPromptPart(content=m.content)]))
         elif m.role == "assistant":
@@ -228,13 +229,11 @@ def read_full_document(file_path: str) -> str:
     # - If CLI flag `enable_rag_tools` is False, tools are disabled globally.
     # - If CLI flag is True, check request.rag_enable_tools (default True).
     tools_allowed = enable_rag_tools and (request.rag_enable_tools is not False)
-
+    system_prompt: str | tuple[()] = ()  # No system prompt by default
     if retrieval and retrieval.context:
         truncated = truncate_context(retrieval.context)
         template = RAG_PROMPT_WITH_TOOLS if tools_allowed else RAG_PROMPT_NO_TOOLS
         system_prompt = template.format(context=truncated)
-    else:
-        system_prompt = ""
 
     # 4. Setup Agent
     from pydantic_ai import Agent  # noqa: PLC0415

tests/rag/test_engine.py

@@ -6,7 +6,7 @@
 import pytest
 
 from agent_cli.rag import engine
-from agent_cli.rag.engine import _is_path_safe, truncate_context
+from agent_cli.rag.engine import _convert_messages, _is_path_safe, truncate_context
 from agent_cli.rag.models import ChatRequest, Message
 
 
@@ -82,6 +82,35 @@ def test_is_path_safe_symlink_escape(tmp_path: Path) -> None:
         pass
 
 
+def test_convert_messages_skips_empty_system() -> None:
+    """Test that empty system messages are filtered out."""
+    messages = [
+        Message(role="system", content=""),
+        Message(role="user", content="Hello"),
+        Message(role="user", content="Question"),
+    ]
+
+    history, user_prompt = _convert_messages(messages)
+
+    # Empty system message should be skipped
+    assert user_prompt == "Question"
+    # Only the first user message should be in history (system was skipped)
+    assert len(history) == 1
+
+
+def test_convert_messages_keeps_nonempty_system() -> None:
+    """Test that non-empty system messages are preserved."""
+    messages = [
+        Message(role="system", content="You are helpful."),
+        Message(role="user", content="Question"),
+    ]
+
+    history, user_prompt = _convert_messages(messages)
+
+    assert user_prompt == "Question"
+    assert len(history) == 1
+
+
 def test_retrieve_context_direct() -> None:
     """Test direct usage of _retrieve_context without async/HTTP."""
     mock_collection = MagicMock()
@@ -132,6 +161,7 @@ async def test_process_chat_request_no_rag(tmp_path: Path) -> None:
     with (
         patch("pydantic_ai.Agent.run", new_callable=AsyncMock) as mock_run,
         patch("agent_cli.rag.engine.search_context") as mock_search,
+        patch("pydantic_ai.Agent.__init__", return_value=None) as mock_agent_init,
     ):
         mock_run.return_value = mock_run_result
         # Mock retrieval to return empty
@@ -151,11 +181,14 @@ async def test_process_chat_request_no_rag(tmp_path: Path) -> None:
         )
 
         assert resp["choices"][0]["message"]["content"] == "Response"
-        # Should check if search was called
         mock_search.assert_called_once()
-        # Verify Agent.run was called
         mock_run.assert_called_once()
 
+        # Verify system_prompt is an empty tuple, not an empty string
+        # (empty strings cause errors on providers like Vertex AI)
+        init_kwargs = mock_agent_init.call_args
+        assert init_kwargs.kwargs["system_prompt"] == ()
+
 
 @pytest.mark.asyncio
 async def test_process_chat_request_with_rag(tmp_path: Path) -> None: