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🤖 ## Summary When a Buzz agent falls behind on incoming messages, its connection can make the backlog worse while trying to recover. The connection buffers messages from the relay server until the agent is ready to process them; if that buffer overflows, recovery previously requested history for **every subscribed channel** and paused socket reads while sending those requests. That adds traffic to an already overloaded connection. This change requests history only for affected subscriptions, once the code consuming those messages has room, with at least five seconds between attempts. The recovery path now: - Combines repeated losses into one pending recovery per affected subscription, keeping the oldest dropped timestamp so replay starts early enough. - Waits until at least half the consumer queue is free and the relay's existing rate-limit delay has expired. The queue wakes recovery when space becomes available; recovery does not periodically sample capacity or hold queue space away from live messages. - Attempts one subscription at a time, choosing the least recently attempted so a busy channel cannot crowd out other channels or membership notifications. The five-second delay starts when an attempt finishes, including a failed write; failed writes leave recovery pending. Recovery is paced by available capacity, not by how often messages are lost. This is not a larger buffer or a cutoff that abandons recovery. Subscription identifiers, message filters, replay timestamp overlap and duplicate filtering are unchanged; no downstream agent changes are required. This targets a reproducible overload **amplifier**, not every cause of overload or every catch-up limitation. The initial live overload's cause has not been established. Recovery remains best effort: a successful request write is not proof of delivery, and existing history/retention limits, bounded duplicate tracking and replay limitations still apply. There is no exactly-once or complete catch-up guarantee. A stalled write can still pause socket reads for the existing ten-second timeout; the pacing bound does not cover initial subscriptions, reconnects or other retry paths. ### Related issue Closest related: #5014 (channel re-subscription); also #6661 (membership reconciliation) and #6090 (relay backpressure gap signaling). This addresses local overflow recovery scheduling, not those separate mechanisms. ### Testing Recorded offline comparisons against the previous behavior, with the final implementation at `8000636f3073167c5a5107bb179c7d91160f1729`: | Same fixture: 18 subscriptions, three overload rounds | Before | After | | --- | --- | --- | | Recovery history requests | 108 | 3 | | Ping-response delay | About 4.6 seconds | Below the measurement's 1 ms resolution | A separate bounded-history fixture delivered all 320 events plus subsequent live traffic in **both** versions. Regression coverage exercises the real socket-handling task, including intermittent consumer capacity, fairness, failed writes and cancellation of capacity waits before live delivery. These are synthetic results, not production throughput measurements or evidence of a deployed cure. The full local `RUST_TEST_THREADS=4 just ci` run passed on September 4, 2026. Earlier unsuccessful local runs remain part of the validation history. The [recorded validation evidence and separate desktop follow-up](https://github.com/block/buzz/pull/7325#issuecomment-5540592398) preserve the original desktop mock-history scroll failure, its passing rerun and the remaining investigation. That desktop path does not run the agent connection code; neither this repair nor the passing rerun fixes the observed scroll problem. --------- Signed-off-by: Logan Johnson <loganj@squareup.com>