Concurrent lcm_expand_query calls from the same origin session could both pass the existing recursion guard, spawn delegated sub-agents, and deadlock on the shared session lane. This change adds a per-origin-session in-flight gate so overlapping expansions fail fast instead of wedging the gateway pipeline.

• What changed

  • Added a concurrency guard in the delegated expansion path for lcm_expand_query.
  • The first request claims the origin session slot before any delegated sub-agent work starts.
  • A second overlapping request from the same origin session now returns immediately with:
    • errorCode: "EXPANSION_CONCURRENCY_BLOCKED"
    • reason: "origin_session_in_flight"

• Guard behavior

  • The new guard is scoped to the origin session, not the delegated child session.
  • It is independent of the existing recursion/depth guard:
    • recursion guard still blocks delegated re-entry
    • concurrency guard blocks parallel top-level expansion from the same origin
  • The slot is always released after completion/failure via cleanup, so later retries can proceed normally.

• Error surface

  • Returned errors include explicit recovery guidance to fall back to lighter tools such as lcm_grep or lcm_describe when another expansion is already running.
  • Telemetry continues to emit a block event for these fast-fail cases.

• Release note

  • Added a patch changeset for this user-visible behavior change.

const concurrencyCheck = acquireExpansionConcurrencySlot({
  originSessionKey,
  requestId,
});

if (concurrencyCheck.blocked) {
  return jsonResult({
    errorCode: concurrencyCheck.code,
    error: concurrencyCheck.message,
    requestId: concurrencyCheck.requestId,
    originSessionKey: concurrencyCheck.originSessionKey,
    reason: concurrencyCheck.reason,
  });
}