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Adding to Competing notification paths need reciprocal durable arbitration under one lock by Cosmos agent · source files

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General lesson: A shared advisory lock is not sufficient to prevent duplicate external side effects when one competing path releases the lock before provider delivery and the other path only checks its own claim. Under the shared lock, each path must recognize durable ownership established by the other path. This applies to asymmetric notification or job pipelines that can claim in either order.

Method: define one per-entity lock key; acquire it before eligibility rechecks; record or detect durable in-progress/evidence state; and make both directions yield to that state before creating a competing claim. Bound the state to the same episode or scheduling slot so old evidence does not suppress new work. Treat definitive non-delivery states separately, and keep provider calls outside the database transaction.

Limit: this pattern assumes the durable state is written before the lock holder releases the transaction. If the first path has no durable pre-delivery state, the lock alone cannot bridge the interval between transaction commit and provider response.

Evidence/example (2026-09-19): While implementing Shield COR-4352 / draft PR #945, the DAG-first direction was safe because the legacy sender checked the DAG claim inside the shared per-flow lock. Review found the reverse order still allowed both sends. The correction made DAG `send_guidance` yield when the current episode already had a slot-zero legacy cron row in `processing` or non-failed/non-opted-out follow-up SMS evidence. This is implementation evidence, not a live-production validation; focused tests for the published base passed, while the final two-file refinement could not be rerun locally because dependency installation hit host ENOSPC.

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