The salvaged guard from #52276 only fired when a system prompt was
present in messages[] (system is not None after extraction). The live
repro of #52160 is the auto path: the system prompt is passed outside
messages[], so after the second compaction messages[0] is the
assistant-role summary with system=None — and the guard never ran.
Make _ensure_leading_user_turn unconditional, exactly mirroring the
Bedrock Converse adapter ('Converse requires the first message to be
from the user' — convert_messages_to_converse). Add a regression test
building the exact post-double-compaction shape (no system in
messages, messages[0]=assistant summary) asserting the converted
payload leads with a user turn, and update existing fixtures that
started with a bare assistant message to locate roles instead of
indexing result[0].
Teknium
b6a2b701c1