Fix assistant-first provider history

Omit orphaned leading assistant messages from model history so Claude-compatible providers receive a user-first conversation.

Add regression coverage for the initial UI greeting path.

helpers/history.py

@@ -724,6 +724,8 @@ def output_langchain(messages: list[OutputMessage]):
             result.append(HumanMessage(content))  # type: ignore
     # ensure message type alternation
     result = group_messages_abab(result)
+    while result and isinstance(result[0], AIMessage):
+        result.pop(0)
     return result
 
 

helpers/history.py.dox.md

@@ -80,6 +80,7 @@
 - Update this file whenever public functions, classes, persistence behavior, path/security assumptions, side effects, or cross-module contracts change.
 - `clear_responses_provider_state(agent)` removes the active provider continuation IDs after local history rewrites while preserving stored response ID lists for later cleanup.
 - `Message.from_dict()` normalizes legacy AI Responses metadata through `LLMResult.metadata()` so loaded chats shed transient payloads while unrelated metadata and non-AI tool-result inputs remain intact.
+- `output_langchain()` removes leading assistant messages after grouping so provider histories always begin with a user turn; the WebUI greeting remains persisted and displayed but is not sent as an orphaned assistant message.
 - Observed side-effect areas: filesystem writes, filesystem deletion, model calls, plugin state, settings/state persistence, secret handling.
 - Imported dependency areas include: `abc`, `asyncio`, `collections`, `collections.abc`, `enum`, `helpers`, `json`, `langchain_core.messages`, `math`, `plugins._model_config.helpers.model_config`, `typing`, `uuid`.
 

tests/test_history.py

@@ -0,0 +1,14 @@
+from helpers.history import output_langchain
+from langchain_core.messages import AIMessage, HumanMessage
+
+
+def test_output_langchain_omits_leading_assistant_messages():
+    messages = output_langchain(
+        [
+            {"ai": True, "content": "Welcome"},
+            {"ai": False, "content": "Hello"},
+            {"ai": True, "content": "Hi"},
+        ]
+    )
+
+    assert messages == [HumanMessage("Hello"), AIMessage("Hi")]