MCP vs A2A: Tools, Collaboration, and the Missing Identity Layer
MCP vs A2A is not a winner-take-all choice: MCP connects an agent to tools, resources, prompts, and APIs; A2A lets agents discover one another, negotiate interactions, and manage tasks. The missing layer is persistent identity: a stable .agent record that says which agent, tool server, Agent Card, manifest, and operator belong together.
For HeadlessDomains.com, the practical path is to treat .agent as the identity anchor across both protocol surfaces. MCP can expose callable tools. A2A can coordinate peer agents. A .agent identity can bind the operator, manifests, endpoints, and proof links into one verifiable record.
Quick Comparison
| Layer | MCP | A2A | .agent anchor |
|---|---|---|---|
| Primary job | Connect an AI application to tools, resources, prompts, and context. | Let independent agents discover one another, exchange messages, and manage tasks. | Publish the canonical identity and verification path for both surfaces. |
| Main artifact | MCP server, tool definitions, resources, prompts, authorization metadata. | Agent Card, supported interfaces, skills, security schemes, tasks, artifacts. | agent.json, DNS/TXT records, SKILL.md, endpoint links, proof links. |
| Best question | Which tool can this agent call, with which inputs? | Which peer agent can collaborate on this task? | Which public identity controls these endpoints? |
| Risk to govern | Tool invocation, data exposure, credential scope, untrusted tool descriptions. | Peer-agent trust, task state, auth delegation, Agent Card exposure. | Continuity, endpoint ownership, operator provenance, revocation path. |
What MCP Covers
The current Model Context Protocol specification describes a standard way for AI applications to share context, expose capabilities, and build composable integrations. MCP servers can offer resources, prompts, and tools, while clients and hosts manage the model-facing application flow.
MCP is strongest when an agent has to call a known capability: search a database, invoke an API, read a resource, run a workflow, or fetch context through a server. The MCP architecture also includes transport choices such as stdio and Streamable HTTP, plus optional authorization for HTTP transports.
What A2A Covers
Google introduced Agent2Agent as an open protocol for agent interoperability, designed to complement MCP. The A2A specification describes peer agents that can discover capabilities, negotiate interaction formats, exchange messages, and manage task progress without sharing private internal memory or tools.
The latest A2A specification lists version 1.0.0 and centers discovery on the Agent Card. An Agent Card can describe names, descriptions, supported interfaces, protocol versions, capabilities, skills, security schemes, and optional signatures. The well-known discovery path is /.well-known/agent-card.json.
The Missing Identity Layer
MCP and A2A describe interaction surfaces, but neither protocol alone gives an agent a durable public identity across domains, tool servers, Agent Cards, manifests, directories, and payments. Without an identity anchor, the same operator can appear as separate URLs, server entries, cards, and credentials.
A .agent identity gives those artifacts one canonical inspection point. The record can link to an MCP server, A2A Agent Card, agent.json manifest, SKILL.md workflow, OpenAPI file, payment metadata, policy page, and proof endpoint. Agents can then start with the name, inspect the record, and decide which protocol surface to use.
Implementation Checklist
- Publish an
agent.jsonmanifest that names the operator, agent purpose, endpoints, proof links, auth model, and update policy. - Register or configure a .agent identity that points to the manifest, MCP server, A2A Agent Card, and SKILL.md file.
- Expose MCP tools only with scoped credentials, clear descriptions, and host-side consent for sensitive actions.
- Publish an A2A Agent Card with supported interfaces, protocol versions, capabilities, skills, security schemes, and optional signatures.
- Keep public cards free of plaintext secrets, private endpoints, and sensitive operational metadata.
- Version endpoint URLs and protocol metadata so agents can detect stale integrations.
- Add a revocation and rotation path for compromised tool servers, Agent Cards, manifests, and keys.
Example Identity Bundle
{"identity":"atlas.agent","operator":"Atlas AI Services","manifest":"https://atlas.agent/.well-known/agent.json","mcp":{"server":"https://api.atlas.agent/mcp","auth":"oauth"},"a2a":{"agent_card":"https://atlas.agent/.well-known/agent-card.json","protocol_version":"1.0"},"proof":"https://atlas.agent/proof.txt","skill":"https://atlas.agent/SKILL.md"}Where HeadlessDomains.com Fits
HeadlessDomains.com gives agent teams a persistent identity layer for the agentic web. A .agent record can bind machine-readable manifests, tool endpoints, collaboration cards, workflow playbooks, and verification metadata under one name that humans and agents can inspect.
Use The Agent Identity Stack as the hub for this publishing model. Then use .agent as the identity anchor across MCP tools and A2A collaboration so protocol metadata resolves back to one verifiable record.
Internal Reading Path
- The Agent Identity Stack
- llms.txt vs SKILL.md vs agent.json
- How to Make Your Website AI-Agent Readable
- agent.json Examples for Public AI Agent Identity
- Register a .agent identity
Sources
- Model Context Protocol specification
- Model Context Protocol architecture
- Model Context Protocol authorization
- A2A Protocol specification
- A2A and MCP comparison
- Google Developers Blog announcement for A2A
FAQ
Is MCP replacing A2A?
No. MCP handles tool and context access. A2A handles peer-agent communication and task coordination. Product teams can use both when an agent must call tools and also collaborate with other agents.
Is A2A replacing MCP?
No. A2A does not remove the tool layer. A remote agent can expose an A2A interface for collaboration while using MCP servers behind the scenes for tools, resources, prompts, and API access.
Where should identity live?
Identity should live in a stable public record that agents can resolve before choosing a protocol surface. A .agent identity can point to agent.json, MCP metadata, an A2A Agent Card, SKILL.md, proof links, and policy pages.
Does an Agent Card prove ownership?
An Agent Card can describe an agent and may include signatures, but ownership still depends on the serving domain, authentication model, key management, and proof chain. A .agent record helps connect those signals.
What should teams publish first?
Publish the identity record first, then attach the manifest, tool server, Agent Card, and workflow playbook. That sequence gives agents a stable place to inspect before invoking tools or delegating tasks.