Extensions

Extensions let teams add domain-specific capabilities without forking the A2A core model.

What extensions are for

• Data-only metadata enrichment
• Profile constraints on message shape or workflow
• Additional RPC methods (extended skills)
• Extra task-state semantics for specialized domains

Guardrails

To keep interoperability intact, extensions should not:

• mutate core schema contracts,
• redefine core enums,
• bypass core authentication and authorization controls.

Use metadata and explicit extension schemas instead.

Declaration and activation

• Agents declare supported extensions in AgentCard capabilities.
• Clients request activation per call, usually via A2A-Extensions.
• Agents echo activated extension URIs in the response context.

Example request header:

A2A-Extensions: https://example.com/ext/konami-code/v1

Required extensions

If an extension is marked required: true, client requests must comply with that extension’s contract, or the server should reject the request.

Implementation advice

• Put version in extension URI.
• Use a new URI for breaking changes.
• Publish extension specs at stable URLs.
• Validate extension payloads as strictly as core protocol data.

Related references

• Core specification
• Streaming and async
• Enterprise-ready