What are agent registries and how do Google Cloud, AWS, and Microsoft index AI agent capabilities?

Agent registries are the production-grade directories where autonomous AI agents discover business capabilities, the machine-economy equivalent of Google My Business, rebuilt for AI crawlers instead of human eyes. Three major registries are live in production: (1) Google Cloud Agent Registry, shipped via the gcloud alpha agent-registry CLI integrated with Google's Agent Development Kit (ADK), indexes agent-card.json manifests directly into the search framework powering Google Assistant and Gemini's native checkout. (2) AWS Bedrock AgentCore Runtime includes a native AWS Agent Registry for enterprise B2B capability matchmaking, where corporate orchestrators programmatically search for verified vendor capabilities using IAM-managed access tokens. (3) Microsoft Entra Agent ID treats autonomous agents as first-class identity citizens inside corporate networks, verifying security boundaries and cryptographic signatures via a core directory. For the open web, a2aregistry.org provides a community-driven global clearinghouse, while Cloudflare Bot Management distributes Signature Agent Registries that parse .well-known intent cards and build verified whitelists. Slickrock.dev builds the endpoints that get listed in these registries, wrapping legacy infrastructure into ADK-compliant servers that output signed agent-card.json manifests.