# ADL vs A2A (Agent2Agent)

**Short answer:** ADL and A2A solve different problems and are designed to work together. **A2A** (Agent2Agent) is a protocol for how agents *communicate*, advertising their capabilities through an **Agent Card** that a client can retrieve to learn how to interact. **ADL** (Agent Definition Language) describes *what an agent is* — its identity, permissions, security, lifecycle, and compliance — in one auditable document. ADL is a superset of the identity information an Agent Card carries, and an ADL document can [generate an A2A Agent Card](/spec/.md#151-a2a-agent-card-generation). You do not choose one over the other; you use ADL to govern the agent and A2A to connect it.

## What is A2A?[​](#what-is-a2a "Direct link to What is A2A?")

A2A (Agent2Agent), introduced by Google and now stewarded under the Linux Foundation, is an open protocol for **agent-to-agent communication**. An agent publishes an **Agent Card** — a JSON document advertising its name, description, skills, and endpoints — typically served at a well-known URL. A client that already knows the agent's address, or finds it through a separate registry or catalog, retrieves the card to learn the agent's capabilities and how to invoke them. A2A standardizes this *capability* discovery and the interaction that follows; it does **not** automatically discover unknown agents across a network — that requires a registry or directory layered on top.

## What is ADL?[​](#what-is-adl "Direct link to What is ADL?")

ADL is an open standard for **defining an AI agent** in a single machine-readable document: identity (including cryptographic identity), capabilities and tools, permission boundaries, security requirements, data classification, lifecycle, and compliance posture. It is the agent's "passport" — the authoritative, auditable record of who the agent is and what it is authorized to do.

## Key differences[​](#key-differences "Direct link to Key differences")

|                             | A2A (Agent Cards)                                 | ADL                                                    |
| --------------------------- | ------------------------------------------------- | ------------------------------------------------------ |
| **Primary purpose**         | How agents communicate and advertise capabilities | What an agent is and may do                            |
| **Scope**                   | Capabilities and endpoints for interaction        | Identity, permissions, security, lifecycle, compliance |
| **Discovery**               | A known agent's Agent Card (per agent)            | Per-domain directory at `/.well-known/adl-agents`      |
| **Identity**                | Partial (name, endpoint)                          | Cryptographic identity (DID) + attestation             |
| **Permissions**             | Not a permissions model                           | Deny-by-default, auditable boundaries                  |
| **Governance / compliance** | Out of scope                                      | NIST 800-53, SOC 2, ISO 27001, EU AI Act mappings      |
| **Lifecycle**               | Partial                                           | Status, versioning, sunset dates, successors           |
| **Relationship**            | Consumes identity                                 | **Generates** A2A Agent Cards                          |

## Discovery[​](#discovery "Direct link to Discovery")

Both standards use well-known URLs, but at different granularities. An A2A **Agent Card** describes a *single* agent, so a client needs that agent's address — or a separate registry — to find it. ADL additionally defines a **per-domain discovery document** at `https://{domain}/.well-known/adl-agents` ([§6.4](/spec/.md#64-discovery)): a directory of the agents a domain publishes, each with a short `description` and `keywords` so a client can triage which ones to retrieve in full. It still requires knowing the *domain* — it is not global, network-wide discovery of unknown agents — and what a discovering agent may then engage is bounded by its `permissions.delegation` envelope and admitted through the Trust Protocol, so discovery widens reach without widening authority.

## How ADL and A2A work together[​](#how-adl-and-a2a-work-together "Direct link to How ADL and A2A work together")

ADL sits *above* A2A. You author one ADL document as the source of truth for the agent, then generate an A2A Agent Card from it for capability discovery and communication. Per the [ADL specification §15.1](/spec/.md#151-a2a-agent-card-generation), an implementation maps ADL fields to an Agent Card: `name`, `description`, `version`, `tools` → skills, `cryptographic_identity.did` → id, and `security.authentication` → authentication.

The result: the Agent Card an agent advertises over A2A is provably derived from the same governed, auditable ADL passport that defines its permissions and compliance — so communication and governance stay in sync.

## When to use which[​](#when-to-use-which "Direct link to When to use which")

* Use **A2A** when you need agents to **advertise their capabilities and call each other**.
* Use **ADL** when you need a **portable, auditable definition** of an agent's identity, permissions, and compliance — and to generate the Agent Card from it.
* Use **both together** for interoperable agents that are also governed and audit-ready.

## FAQ[​](#faq "Direct link to FAQ")

### Is ADL a replacement for A2A?[​](#is-adl-a-replacement-for-a2a "Direct link to Is ADL a replacement for A2A?")

No. They operate at different layers. A2A handles agent-to-agent communication; ADL handles agent identity and governance. ADL complements A2A and can generate A2A Agent Cards.

### Can I use ADL and A2A together?[​](#can-i-use-adl-and-a2a-together "Direct link to Can I use ADL and A2A together?")

Yes — that's the intended pattern. Define the agent in ADL, then generate its A2A Agent Card from the ADL document so capability discovery and governance share one source of truth.

### Does ADL replace the Agent Card?[​](#does-adl-replace-the-agent-card "Direct link to Does ADL replace the Agent Card?")

No. The Agent Card remains the A2A discovery artifact; ADL is the broader passport it can be generated from.

## Related[​](#related "Direct link to Related")

* [ADL vs MCP](/comparisons/adl-vs-mcp.md)
* [All comparisons](/comparisons.md)
* [ADL specification](/spec/.md) · [Interoperability (§15)](/spec/.md#15-interoperability)