Anthropic does not support assistant-message prefill and returns a 400
error when the conversation ends with an assistant turn. This commonly
happens when heartbeat/system messages accumulate trailing assistant
replies in the session history.
The _merge_consecutive method already handles same-role merging but did
not strip trailing assistant messages. The base provider's
_enforce_role_alternation (used by OpenAI-compat) does strip them, but
AnthropicProvider uses its own _merge_consecutive instead.
Add a trailing-assistant stripping loop to _merge_consecutive, matching
the behavior already present in _enforce_role_alternation.
Includes 7 new tests covering merge + strip behavior.
hussein1362andXubin Ren
2f02342083