Agent registry

An agent registry is a service that stores authoritative records about agents, such as their Agent Cards, identifiers and verified owners, and answers queries against those records.

In A2A. The specification lists registries and catalogs as one of three ways to find Agent Cards. In the model the A2A Agent Discovery guide describes, servers publish their cards to the registry and clients search it by skill, tag, provider name or capability. The guide lists the benefits: central management and governance, capability-based search, access controls and trust frameworks, and selective disclosure, where a registry returns different cards to different clients according to their identity and permissions. It also notes that A2A does not prescribe a standard registry API, so each registry defines its own.

A related example. The MCP project runs an official registry for MCP servers, launched in preview in September 2025. Its design splits three jobs:

• Publishers prove control of a namespace before listing under it: GitHub sign-in for io.github.* names, or a DNS TXT record or well-known file on the domain for reverse-domain names such as com.example.*.
• The registry stores deliberately unopinionated metadata in a standard server.json format and serves it through a REST API.
• Downstream aggregators, such as marketplaces, add curation and ratings.

What a registry has to get right. An entry is only as trustworthy as the proof behind it. A registry that maps an identifier to an endpoint needs evidence that the party registering it controls that identifier, a way to update or remove stale entries, and transport security so lookups cannot be altered in transit.

Registry and directory. The terms overlap in practice. In this glossary a registry is the source of record for which endpoint belongs to which identifier, and an agent directory is a listing built for browsing and comparison, often on top of registry data.

Emissar. Emissar’s Resolve module is a registry of this kind: businesses claim phone numbers, domains and brand names, prove control of each, and Resolve maps them to agent endpoints. Status: In development.

Neighbouring terms. Agent discovery is the wider process a registry serves. A resolver is the lookup interface in front of a registry.