# Agent Card

Category: [Agents & Protocols](https://seo-geo-glossary.com/en/glossary/category/agents-protocols/) › [Agent protocols](https://seo-geo-glossary.com/en/glossary/category/agents-protocols/agent-protocols/) · Also known as: AgentCard, A2A Agent Card

An **Agent Card** is a JSON document in which an AI agent describes itself under the Agent2Agent (A2A) protocol: its name, what it can do, where to reach it and how to authenticate. Other agents read the card before they send it a task.

## What does an Agent Card contain?

The A2A specification (v1.0.0) calls the Agent Card a self-describing manifest for an agent. Required fields are `name`, `description`, `version`, `supportedInterfaces` (the endpoints and protocol bindings, preferred one first), `capabilities`, `defaultInputModes`, `defaultOutputModes` and `skills`. Optional fields include `provider`, `documentationUrl`, `securitySchemes`, `securityRequirements`, `signatures` and `iconUrl`.

Input and output modes are media types such as `text/plain` or `application/json`. Each skill lists an ID, a name, a description and tags, so a client agent can decide whether the agent fits a job.

## How do other agents find it?

The specification names three discovery routes: a well-known URI at `https://{domain}/.well-known/agent-card.json`, registries or catalogs of agents, and direct configuration. A card can carry JSON Web Signatures so clients can check that it has not been altered. An agent can also offer an extended card with more skills or details that is only returned after authentication; since v1.0 this is flagged in `capabilities.extendedAgentCard`.

Platforms use the format too. Google Cloud Marketplace asks partners to provide an Agent Card for each listed AI agent.

## Example

```json
{
  "name": "Store Locator Agent",
  "description": "Finds the nearest store and its opening hours.",
  "version": "1.2.0",
  "supportedInterfaces": [
    { "url": "https://agent.example.com/a2a", "protocolBinding": "JSONRPC", "protocolVersion": "1.0" }
  ],
  "capabilities": { "streaming": false },
  "defaultInputModes": ["text/plain"],
  "defaultOutputModes": ["application/json"],
  "skills": [
    { "id": "find-store", "name": "Find store", "description": "Returns stores near a postal code.", "tags": ["retail", "locations"] }
  ]
}
```

## Common mistakes

- Publishing the card at the old path `/.well-known/agent.json` that early guides used.
- Using a top-level `url` field instead of `supportedInterfaces`, which v1.0 requires.
- Keeping the extended-card flag at the top level. `supportsExtendedAgentCard` moved to `capabilities.extendedAgentCard`.

## FAQ

**What is an A2A Agent Card?**
The discovery document of the A2A protocol. It tells a client agent what a remote agent offers and how to call it.

**Is an Agent Card the same as llms.txt?**
No. An llms.txt file helps language models read a website. An Agent Card describes an agent as a service that other agents can call.

**Does every website need one?**
Only sites that run their own agent over A2A. A site without an agent gains nothing from it.

## At a glance

- **Type:** JSON document (file) in the Agent2Agent protocol
- **Used in:** A2A; Google Cloud Marketplace for AI agents
- **Default path:** /.well-known/agent-card.json
- **Visible?:** public; extended card only after authentication
- **Docs:** [A2A Protocol Specification](https://a2a-protocol.org/latest/specification/)

Last checked: October 8, 2026

*Diagram: Which protocol covers what?.* Map of agent protocols as of 2026-10-08: which connection each protocol covers, who publishes it and what its status is. In commerce, ACP, UCP and AP2 overlap, and several protocols build on MCP or A2A. Versions change quickly.

## Related terms

- [Agent2Agent protocol (A2A)](https://seo-geo-glossary.com/en/glossary/agent2agent-protocol/index.md)
- [AI agent](https://seo-geo-glossary.com/en/glossary/ai-agent/index.md)
- [MCP (Model Context Protocol)](https://seo-geo-glossary.com/en/glossary/mcp/index.md)
- [Agent readiness](https://seo-geo-glossary.com/en/glossary/agent-readiness/index.md)
- [llms.txt](https://seo-geo-glossary.com/en/glossary/llms-txt/index.md)

## Sources

- [Agent2Agent (A2A) Protocol Specification v1.0.0, sections 4.4.1 AgentCard, 8.2 Discovery Mechanisms, A.2.2](https://a2a-protocol.org/latest/specification/)
- [Add an Agent Card for your AI agent – Google Cloud Marketplace](https://docs.cloud.google.com/marketplace/docs/partners/ai-agents/agent-card)
- [The /llms.txt file – llmstxt.org](https://llmstxt.org/)

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Last updated: October 8, 2026
URL: https://seo-geo-glossary.com/en/glossary/agent-card/