The compaction summary's role was selected against the LITERAL
neighbouring messages (compressed[-1] / tail_messages[0]). Mistral-family
chat templates (Devstral, Mistral Small 3.x, Magistral) enforce
user/assistant alternation but exempt the tool flow (tool results and
assistant messages carrying tool_calls) from the check, so a protected
head ending [user, assistant(tool_calls), tool] pinned the summary to
role="user" while the last role the template counts is "user": the
backend rejects the whole request with a Jinja alternation error
(HTTP 500). The summary persists in the stored conversation, every
retry replays the identical poisoned history, and the session is
permanently unrecoverable. Fires on EVERY compaction against a
Mistral-strict backend, captured byte-exact via a tee-proxy in front of
a llama.cpp/llama-swap Devstral deployment.
Fix: compute both neighbour roles through _template_visible_role(),
which skips template-exempt messages. The #52160 (Anthropic user-first)
and #58753 (zero-user-turn) forced-user guards are preserved; their
forced shapes (summary-user followed only by exempt messages) are
alternation-safe. When the visible head ends "assistant" and the
visible tail opens "user", no standalone role can alternate and the
existing merge-into-tail fallback now correctly fires (the literal
logic emitted a standalone user summary there: a second poisoning
shape).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LGN45sMMbwM8cW9T9ga4ou
Matt Ezell
7965462d6c