fix: initialize user_assistant_msgs to prevent UnboundLocalError in compress_messages

[alvinttang]

Summary

• BaseLLM.compress_messages() raises UnboundLocalError when all messages are system messages or the message list is empty (with compression enabled).

Root Cause

• In compress_messages(), the variable user_assistant_msgs is only assigned inside the else branch of a for-loop (line 369). If all messages have role == "system", the else branch never executes, and user_assistant_msgs remains undefined. The subsequent code on lines 378 and 396 references this variable unconditionally, causing an UnboundLocalError.

Fix

• Initialize user_assistant_msgs = [] before the loop so the variable always has a valid default value. This makes the compression logic safely handle edge cases (all-system messages, empty lists) by producing an empty compressed result instead of crashing.

Test

from metagpt.provider.base_llm import BaseLLM

# Before fix: raises UnboundLocalError
# After fix: returns the system messages without crashing
messages = [{"role": "system", "content": "You are helpful."}]
# compress_messages with any compress_type != NO_COMPRESS