the Google-initiated open protocol (2025, later a Linux Foundation project) for agents discovering and working with EACH OTHER - agent cards describing capabilities at well-known URLs, tasks with lifecycle, messages and artifacts between peers.
The discovery door: the card at the well-known path tells strangers where the evidence lives.
Hand-authored SVG for this glossary, 2026. Surfaces per main.rs routes; the calibration word "candidate" per the paper's own claim discipline.
Etymology and backstory
if MCP wires agents to tools, A2A wires agents to agents - the complementary layer for delegation across organizational boundaries. The agent card (a JSON self-description at /.well-known/agent-card.json) is its discovery primitive, deliberately shaped like other well-known-URI conventions of the web.
Ecosystem
cross-vendor agent interop efforts, enterprise agent registries; usually described alongside MCP as the two-protocol stack of the agentic web - which is exactly how the paper's four-surface inventory treats them.
In codexproof
the gateway serves an agent card at the well-known path (main.rs line 85), advertising the evidence endpoints so a non-MCP agent framework can discover and integrate by plain HTTP - discovery for strangers, one GET away (Q15's fourth surface).