fix: ensure single system message and merge historical system prompts

backend/app/api/websocket.py

@@ -96,6 +96,35 @@ async def get_chat_history(
     return out
 
 
+def merge_system_prompt(system_prompt: str, last_system: str) -> str:
+    if not last_system:
+        return system_prompt
+
+    # find the longest common prefix
+    min_len = min(len(system_prompt), len(last_system))
+    i = 0
+    while i < min_len and system_prompt[i] == last_system[i]:
+        i += 1
+
+    # prevent meaningless matching (like only matching to "You are ")
+    if i < 20:
+        return system_prompt + "\n\n" + last_system
+
+    common_prefix = system_prompt[:i].strip()
+    rest_a = system_prompt[i:].strip()
+    rest_b = last_system[i:].strip()
+
+    parts = []
+    if common_prefix:
+        parts.append(common_prefix)
+    if rest_a:
+        parts.append(rest_a)
+    if rest_b:
+        parts.append(rest_b)
+
+    return "\n\n".join(parts)
+
+
 async def call_llm(
     model: LLMModel,
     messages: list[dict],
@@ -155,8 +184,16 @@ async def call_llm(
     tools_for_llm = await get_agent_tools_for_llm(agent_id) if agent_id else AGENT_TOOLS
 
     # Convert messages to LLMMessage format
-    api_messages = [LLMMessage(role="system", content=system_prompt)]
+    last_system = None
+    filtered_messages = []
     for msg in messages:
+        if msg.get("role") == "system":
+            last_system = msg.get("content")
+        else:
+            filtered_messages.append(msg)
+    final_system = merge_system_prompt(system_prompt, last_system)
+    api_messages = [LLMMessage(role="system", content=final_system)]
+    for msg in filtered_messages:
         api_messages.append(LLMMessage(
             role=msg.get("role", "user"),
             content=msg.get("content"),