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Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →An A2A Agent Card is a JSON metadata document that an A2A server publishes so a client can learn who the agent is, what it can do, where to send requests, and what authentication it requires before it starts a task. Of the five formats compared here, it is the only one whose contents are described in enough detail to explain properly, which makes it the natural anchor for the rest of the map.
The other four, Agent Manifest, agents.json, NANDA AgentFacts and MCP metadata, sit close to it in purpose. They answer different questions, and the sources available for this comparison do not show that their schemas can be treated as interchangeable. The most useful test is functional. A discovery card tells a client about an agent and how to reach it. A governance manifest declares operating or policy context. An API description tells an agent how to use an API. An AgentFacts record is registry metadata. MCP is a client-server protocol with its own tool and server description layer.
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What an A2A Agent Card contains
The A2A specification defines the card as a JSON document published by an A2A server. It describes the server’s identity, capabilities, skills, endpoint and authentication requirements. A client reads it to decide whether an agent fits a task and to set up the interaction.
“A2A Servers MUST make an Agent Card available.” (A2A Protocol Specification, Section 8.1)
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The same section says that “The Agent Card describes the server’s identity, capabilities, skills, and interaction requirements.” That requirement is what makes the card the entry point of A2A discovery: it is the standard place a client looks for an agent’s details.
Identity, capabilities and skills
Identity tells a client which agent it is talking to. Capabilities and skills describe what the agent can do. The card also carries protocol-version information, so a client can tell which A2A version the agent declares. Skill entries have a second use in some registries. Google Cloud Agent Registry uses skill fields for keyword search indexing, so the wording of a skill affects how the agent surfaces in keyword search, not only how a person reads it.
Endpoint and supported interfaces
The card names the endpoint a client should use. Its supportedInterfaces field declares the protocol combinations the agent supports, in order of preference. A client reading that list learns more than whether the agent exists. It learns which interface the agent prefers to be called through.
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How a client finds a card
The specification names three discovery routes:
- A well-known URI on the agent’s host.
- Curated registries or catalogs, where cards are indexed and can be searched.
- Direct configuration, where the client is set up with the agent’s details instead of finding them.
The well-known path depends on the protocol version
The well-known path differs between protocol versions. An implementation should tie its path to the version it supports rather than assume one URI works for every deployment.
| Specification version (as surfaced, early October 2026) | Well-known path |
|---|---|
| Current A2A specification | /.well-known/agent-card.json |
| v0.3 specification | /.well-known/agent.json |
To check whether an agent serves its card at the current path, request it directly:
curl -i https://agent.example.com/.well-known/agent-card.json- 200 OK with a JSON body: the card is served at the current path. Check that the protocol version it declares matches what your client supports.
- 404 Not Found: the agent may implement v0.3 instead. Retry at
/.well-known/agent.json. If that also fails, use the agent’s own documentation or a registry entry to locate its card.
Registries and catalogs
A registry or catalog lets a client search for agents without knowing each host in advance. Registration can also be a validation point. The vendor examples later in this article check cards against the A2A specification when they are registered.
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Security and publication rules
What a public card may contain
A public card should not expose sensitive credentials or internal implementation details. It may advertise the authentication the agent requires, which is part of what a card is for, but it is not a place to publish a reusable API key. Treat anything in a public card as readable by anyone who can reach it.
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A server can offer a fuller version of its card through an authenticated endpoint. Under A2A guidance, these extended cards may hold more information than the public card and should be access-controlled. Use that route for details that only authorized callers should see.
HTTPS and JWS signatures
A2A guidance recommends HTTPS for serving cards. A card may also be signed with JSON Web Signatures (JWS), and clients should verify the signature whenever one is present. A successful verification gives tamper detection: it shows the card was not altered after it was signed. It does not show that the agent is suitable for your task or that you are authorized to use it. Those decisions remain with the consumer’s own trust and policy rules.
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Two axes for comparing the formats
Two questions sort the five formats. The practical axis asks what a format does: whether it describes an agent, advertises a service endpoint, defines API usage, provides governance declarations, or specifies runtime communication. The trust axis asks who publishes it, whether its contents are public or access-controlled, how clients validate it, and how credentials are obtained.
A2A answers most of these questions in the sources reviewed. For the other four formats, the trust answers are largely not established, so a full security comparison across all five would claim more than the evidence supports.
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| Format or layer | Role, as the sources characterize it | Advertises a service endpoint |
|---|---|---|
| A2A Agent Card | Discovery document: identity, capabilities, skills, endpoint and authentication requirements | Yes |
| Agent Manifest | Declarative governance manifest: declared operating or policy context | Not automatically |
| agents.json | Description of how an agent uses an API | Not stated |
| NANDA AgentFacts | Registry-approach metadata, cited alongside A2A Agent Cards and MCP’s mcp.json | Not stated |
| MCP metadata | Tool and server description layer of a client-server protocol; mcp.json is the registry approach cited for it | Not stated |
Agent Manifest: governance context, not a locator
Read an Agent Manifest as a statement of declared operating or policy context. It is not automatically a live endpoint locator, so do not expect it to tell a client where to send a request.
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agents.json: API instructions, not agent-to-agent tasks
The comparison describes agents.json as a description of API use: it tells an agent how to call an API. That is a different job from an A2A card, which sets up interaction with another agent. The two are easy to confuse because both are JSON documents, so check the job a given file performs before you wire it into a client.
NANDA AgentFacts: registry metadata
A survey of AI agent registry solutions lists NANDA AgentFacts as one registry approach, alongside A2A Agent Cards and MCP’s mcp.json. Treat an AgentFacts record as registry metadata, and do not assume it does the same job as an A2A card.
MCP metadata: a tool layer, not an Agent Card
MCP is a client-server protocol. Its description layer covers the servers and tools a client connects to, and the survey cites mcp.json as its registry and metadata approach. Keep MCP separate from A2A. A2A covers discovery and task exchange between peer agents, while MCP covers how a client reaches tools. Do not present MCP metadata as an Agent Card schema, even though both appear in the same registry comparison.
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Vendor validation in practice
Two cloud products document validation of A2A cards. Where the sources do not give a detail, the table says so.
| Service | Validation documented | Card versions named | Other documented detail |
|---|---|---|---|
| Google Cloud Agent Registry | Registration validates A2A cards against the external specification | 0.3 and 1.0 | Skill fields drive keyword search indexing. Registered cards have a 10 KB maximum file size; this is a limit of the Agent Registry product, not of the A2A specification. |
| AWS Bedrock AgentCore | Agent cards are validated against the A2A card specification | Not stated | Not stated |
These examples show that managed platforms can check a card’s structure at registration. They do not show how many cards are in circulation, or whether any individual card is accurate or secure. A passed validation confirms that the card matches the format. It does not vouch for the agent behind it.
Quick Recap
What the evidence does not establish
- Normative schemas for four formats. No primary schema is reproduced here for Agent Manifest, agents.json, NANDA AgentFacts or MCP server metadata, so the rows for those formats describe role only. The publisher and version of agents.json also need confirmation from its primary source.
- Field-level equivalence. No field mapping between any two formats is established. Do not translate one format’s fields into another’s.
- Adoption. Vendor validation and a survey listing are not a count of deployed cards or registries.
- Signing key trust. This article does not establish how a client should decide which signing keys to trust.
- Currency. Well-known paths, version support and registry features were checked in early October 2026 and can change. Confirm them against the A2A specification and each vendor’s documentation before you build against them.
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