fix(llm): merge extra system messages for vllm compatibility

backend/app/services/llm/caller.py

@@ -452,9 +452,26 @@ async def call_llm(
         tools_for_llm = await get_agent_tools_for_llm(agent_id) if agent_id else AGENT_TOOLS
     allowed_tool_names = _allowed_tool_names(tools_for_llm)
 
-    # Convert messages to LLMMessage format
-    api_messages = [LLMMessage(role="system", content=static_prompt, dynamic_content=dynamic_prompt)]
+    # Convert messages to LLMMessage format. Some OpenAI-compatible local
+    # providers only allow one leading system message, so fold caller-supplied
+    # system instructions into the base agent context.
+    conversation_messages: list[dict] = []
+    extra_system_parts: list[str] = []
     for msg in messages:
+        if msg.get("role") == "system":
+            content = msg.get("content")
+            if isinstance(content, str):
+                extra_system_parts.append(content)
+            elif content:
+                extra_system_parts.append(json.dumps(content, ensure_ascii=False))
+            continue
+        conversation_messages.append(msg)
+    if extra_system_parts:
+        extra_system = "\n\n".join(part for part in extra_system_parts if part)
+        dynamic_prompt = f"{dynamic_prompt}\n\n{extra_system}" if dynamic_prompt else extra_system
+
+    api_messages = [LLMMessage(role="system", content=static_prompt, dynamic_content=dynamic_prompt)]
+    for msg in conversation_messages:
         api_messages.append(LLMMessage(
             role=msg.get("role", "user"),
             content=msg.get("content"),

backend/tests/test_finish_protocol.py

@@ -266,6 +266,41 @@ async def test_skip_tools_still_exposes_finish(monkeypatch):
     assert tool_names == ["finish"]
 
 
+@pytest.mark.asyncio
+async def test_call_llm_merges_extra_system_messages(monkeypatch):
+    from app.services.llm import caller
+
+    fake_client = FakeStreamClient([_finish_response("Hello.")])
+
+    monkeypatch.setattr(caller, "_get_agent_config", lambda _agent_id: _async_return((1, None)))
+    monkeypatch.setattr(caller, "_get_user_name", lambda _user_id: _async_return("Ray"))
+    monkeypatch.setattr(
+        "app.services.agent_context.build_agent_context",
+        lambda *_args, **_kwargs: _async_return(("static", "dynamic")),
+    )
+    monkeypatch.setattr(caller, "create_llm_client", lambda **_kwargs: fake_client)
+    monkeypatch.setattr(caller, "record_token_usage", lambda *_args, **_kwargs: _async_return(None))
+
+    result = await caller.call_llm(
+        _model(),
+        [
+            {"role": "system", "content": "onboarding instructions"},
+            {"role": "user", "content": "Please begin the onboarding."},
+        ],
+        "Agent",
+        "",
+        agent_id=uuid.uuid4(),
+        user_id=uuid.uuid4(),
+        skip_tools=True,
+    )
+
+    assert result == "Hello."
+    first_round_messages = fake_client.messages_seen[0]
+    assert [msg.role for msg in first_round_messages] == ["system", "user"]
+    assert first_round_messages[0].content == "static"
+    assert first_round_messages[0].dynamic_content == "dynamic\n\nonboarding instructions"
+
+
 @pytest.mark.asyncio
 async def test_execute_tool_finish_is_noop_control_signal(monkeypatch):
     from app.services import agent_tools