Glossary · Network and discovery
Resolver (agent discovery)
A service that turns an identifier an agent starts with, such as a domain, phone number or DID, into the endpoint and metadata needed to reach an agent.
In agent discovery, a resolver is a component that takes an identifier a client already has and returns the endpoint and metadata needed to reach the agent behind it.
The pattern. The name comes from existing systems. In DNS, resolvers are the programs that answer a client’s query by asking name servers, following referrals when needed (RFC 1034). In the W3C DID standard, a DID resolver takes a DID and returns its DID document, which lists keys and service endpoints. An agent resolver applies the same idea: an identifier goes in, and a verified Agent Card or endpoint comes out.
What A2A covers. Given a domain, an A2A client resolves an agent itself by fetching https://{domain}/.well-known/agent-card.json, and the card’s supportedInterfaces gives the endpoint URL and protocol binding. A2A defines no resolver for other identifiers and no standard registry API.
Identifiers that need a resolver. Illustrative:
| Client starts with | What resolution needs |
|---|---|
example.com |
A well-known URI fetch; no extra service |
+1 800 555 0199 |
A mapping from the phone number to the business’s card, and proof the business controls that number |
| “Example Telecom” | A mapping from a brand name to the right organization, with a way to tell similar names apart |
did:web:example.com |
DID resolution, then a service entry for the agent, if the DID document lists one |
Trust. A resolver’s answer is only as good as its proof of control. A resolver that maps a phone number to an endpoint without checking who controls the number would let an attacker redirect a business’s agent traffic. Clients should also get answers over an authenticated channel, and still validate the card they fetch: its serving domain and any signatures.
Emissar. Emissar’s Resolve module is a resolver and registry of this kind. Businesses claim their phone numbers, domains and brand names and prove control of each; calling agents query Resolve before dialing and receive the verified endpoint, or learn that none exists. Status: In development.
Neighbouring terms. An agent registry is the store of records behind a resolver. An agent endpoint is what a successful resolution returns.