The Anthropic client steered JSON replies by appending an assistant turn
containing an opening brace and gluing the brace back onto the reply.
Current Claude models (the 4.6 / 4.7 / 4.8 family, Opus 5, Sonnet 5,
Fable 5 and 5.1) reject an assistant prefill with HTTP 400, so every
JSON surface on the provider (briefing, status cards, document and lab
extraction) failed for a self-hoster on a current model while plain chat
kept working.
Drop the prefill on every model. The request now carries a plain
instruction on the last user turn to reply with a single JSON object and
nothing else, and the reply is narrowed to the outermost object body via
a shared extractJsonObject helper, so a fenced or prefixed answer still
parses. The insights stripJsonFences helper delegates to the same
function rather than keeping a second copy. The tool-use path is
unchanged.
Reported independently by @sweidinger.
