~anthropic/claude-sonnet-latest → B (6:4)

Side B fixes a real off-by-one/index-base bug (1-indexed vs 0-indexed thread_post_index) across server rpc, HTML rendering, and types, and backs it with new integration test assertions verifying correctness, giving lasting correctness value. Side A also fixes real bugs (regex split, request body vs input stream, missing try/catch, null-state handling) improving test infra robustness, which is valuable but more localized to test harness code rather than production logic.