Port the planning turn-admission invariant (FNXC:PlanningTurnAdmission,
2026-07-22) into the Compound Engineering orchestrator: at most one turn
(opening/answer/resume-rehydration) is admitted per CE session, reserved
synchronously and held until the turn settles — a re-entered mobile view
re-submitting a turn now gets CeTurnInProgressError (HTTP 409) instead of
displacing the in-flight turn's live agent, which surfaced as "Failed to
parse agent response: AI returned no valid JSON". cancel()/discard()
force-clear the reservation; releases are token-scoped so a stale release
can't drop a newer turn's slot.
In the engine interactive-ai-session seam: bump the reformat retry from
one to two attempts (non-Anthropic default models comply less reliably
with the JSON-only protocol), and log every failed parse with a bounded
raw-response snippet plus resolved provider/model — including a distinct
empty-assistant-message marker — so support can diagnose these reports
without a repro.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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