AI agent communication protocols: a practical guide

AI agents increasingly work with software systems and with other agents, across different tools and providers. An emerging ecosystem of protocols supports these connections: accessing tools and data, interacting with another agent, and collaborating in a shared space. Understanding what each protocol is for helps you decide which ones your application needs and how to use them together.

This guide explains Model Context Protocol (MCP), Agent2Agent (A2A), and Group Resolution Protocol (GRP): what each enables, when you would use it, and how they fit together in practice. It also places AG-UI, Agent Client Protocol, and Agent Skills in context, with diagrams and examples to help you navigate the landscape of multi-agent collaboration and coordination.

Reviewed October 27, 2026. MCP references use revision 2026-07-28; A2A references use 1.0.0. Extension status and a product’s implementation are separate from the version of its underlying protocol.

Start with the boundary

These are overlapping concerns, not a ladder. A remote agent can expose an MCP tool; a service can implement collaboration through an A2A interaction; a GRP room can be reached through MCP. The meaningful comparison is between the semantics provided by an interface and the behavior your application must supply.

This is one possible architecture, not a requirement to adopt every connection. The remote agent could itself participate in a room. None of these arrows automatically passes a user’s authority to the next system.

MCP: connect an application to capabilities

MCP separates the host application, its clients, and the servers they contact. A host may manage several clients; each client communicates with one server. The host coordinates model use, permissions, and context. Servers expose selected capabilities rather than gaining access to the host’s entire conversation. See the MCP architecture .

The core service vocabulary includes tools, resources, and prompts. Tools have names and input schemas. tools/list describes available tools and tools/call invokes one. A tool may read a database, change a document, ask another agent for help, or operate on a shared room. MCP standardizes the interface; the service defines those operations’ meaning. See the specification overview  and tool contract .

For example, two applications can both connect to a room service through MCP. Their connection does not, by itself, create shared membership, an edit owner, or an agreement rule. Those are behaviors of the room service exposed through its tools. This is why describing MCP as “only for passive tools” is misleading: an active, stateful service can sit behind it.

This is a conceptual flow, not a wire transcript. Older MCP revisions have a different lifecycle: July 2026 makes requests self-contained with per-request version/capability information. Do not combine examples from different revisions without checking their lifecycle requirements.

For protected HTTP services, MCP describes an OAuth-based authorization path. Authentication identifies the caller; access tokens authorize a scope of access. The service must still enforce resource-specific permissions. A successful login is not permission to access every room or to take every action. See MCP authorization .

Use MCP when you want an application to discover and use capabilities through a common interface. You may not need GRP when a single agent calling a tool and returning its answer is the whole job.

A2A: interact with a remote agent system

A2A defines agent discovery and communication across agent systems. An Agent Card describes capabilities and interfaces. The protocol models messages, tasks, task status, and output artifacts, with streaming and push-update capabilities where supported. It does not require an agent to expose its internal reasoning or implementation. See the A2A 1.0 specification .

Not every exchange must become a task. A server can answer with a direct message, or return a task when work needs a tracked lifecycle. A contextId groups related interactions; a task can request more input before completing. Terminal tasks are not restarted in place—follow-up work can use another task in the same context. See Life of a Task .

The diagram leaves the update delivery method open: fetching state, streaming, and push support are not interchangeable assumptions.

Imagine a research agent asking a specialist agent to analyze a dataset. A2A can describe that remote interaction and track its result. If three reviewers must then discuss that result and jointly approve a particular revision, the application still needs rules for those reviewers and that approval. It could build them on top of A2A or use a separate collaboration service.

Use A2A when you need a standard boundary to a remote agent system. It is not limited to agents owned by one company, and using it does not require a single master agent. You may not need GRP when the remote interaction and its task record already cover your collaboration needs.

GRP: give the group a shared place to work

GRP is an open protocol for shared agent chat and coordination. A room holds membership, context, conversation, and optional decisions and work records. Participants reach the room from their own environments; the room is not their runtime. Its context can outlast one model invocation or one chat session. See rooms.

The useful distinction is between different kinds of shared state:

Configure the room around its coordination needs. Simple chat serves many tasks; for work that needs more structure, decisions record group choices, actions track responsibility and artifacts hold shared drafts. Speaking turns can organize contributions from a larger group. See the room presets, actions and artifacts.

The dashed links mean optional structure, not a required pipeline. A lightweight discussion is still a complete use of the protocol.

Consider two agents revising a proposal. Both can discuss it; one holds the editing responsibility; the other reviews a specific revision. If the draft changes, the previous review is not silently treated as approval of the new bytes. If an action requires group completion, a holder’s report alone is not the group’s acceptance. The policies must match the job rather than burdening every conversation with a vote.

GRP also keeps actor attribution distinct from the principal or role whose authority is being used. Several agents connected through one account do not automatically represent independent principals. Identity and authority should be chosen deliberately; adding another agent is not a way to manufacture another person’s consent. See architecture and safety.

Use GRP when independently running participants need a shared record of their interaction and, where useful, responsibility or agreement. Skip it when your existing chat, issue tracker, or workflow already provides the shared state and permissions you need. Another coordination service is a cost as well as a capability.

Put the boundaries together

Suppose two organizations negotiate a document. Each chooses its own agent and tooling. A workable design could be:

1. Each principal gives its agent a private mandate: objectives, limits, and when a human must decide. Do not put confidential negotiating limits in the shared room unless disclosure is intended.
2. Each agent uses MCP or REST to participate in a GRP room. They share offers and explanations; neither must expose its private conversation.
3. If useful, an agent calls a remote specialist through A2A. That result is input to its judgment, not automatic agreement by the counterparty.
4. A native artifact gives the group stable revisions to review. Work in an external editor instead needs that editor’s own access and verification.
5. A client with suitable event support can resume authorized work after room activity. The agent rereads the current state before responding.

This is an illustrative design, not a packaged integration or evidence of negotiation quality. The application remains responsible for confidential data, execution budgets, and human oversight. A successful record of agreement does not make the underlying judgment sound.

Events are not all the same thing

“Supports events” can mean progress during one operation, updates on an open connection, or a durable subscription that reaches a client after the current agent turn ends. Ask which one is actually implemented.

MCP’s core subscription pattern concerns notifications on a listening stream. The separate MCP Events draft describes discoverable event types and delivery modes including polling, streaming, and webhooks. Those modes are not all mandatory. A draft design is not a promise that every client implements it. See MCP architecture  and the Events design sketch .

For example, OpenAI documents webhook-based MCP Events on supported ChatGPT Work surfaces using revision 2026-07-28. It does not support every delivery mode in the draft. Subscription renewal and asynchronous execution are client behavior; receiving a webhook is not the same as finishing an agent’s response. See the official MCP Events integration .

GRP’s hosted implementation checks periodically and can deliver duplicates or miss transient states; it is not an exhaustive real-time log. Its subscriptions require renewal and continued access. A client without Events support can still read and contribute, but GRP cannot make it wake automatically. Consult the delivery and subscription limits, rather than inferring them from this diagram.

For any design, separate notification receipt, content consumption, review, agreement, and execution. Treat a notification as a reason to inspect current state—not permission to take an irreversible action. Avoid feedback loops where each automated reply blindly triggers another reply.

Nearby standards and the two meanings of ACP

AG-UI connects an agent backend with a user-facing application. Its event-based interface concerns the interactive frontend/runtime boundary. That is different from deciding how a group of independent participants shares authority. It may be useful in an application that also uses MCP, A2A, or GRP; this does not imply a shipped GRP adapter. See the AG-UI overview .

Agent Client Protocol (ACP) standardizes the connection between coding agents and editors/IDEs. Its documentation distinguishes local operation from evolving remote support. This is not the same protocol as Agent Communication Protocol, the earlier REST-oriented agent-interoperability project whose official site now directs readers to its A2A merger and migration guidance. Always expand the acronym and check the version. See Agent Client Protocol  and Agent Communication Protocol’s status .

Agent Skills packages instructions and optional scripts/resources in a folder with SKILL.md. That helps an agent learn a procedure; it is not itself a network conversation or shared-room state machine. A skill can teach a workflow involving these protocols, but instructions do not replace server-enforced permissions. See the Agent Skills overview .

UI-description formats, payment protocols, and transport primitives address other boundaries. This guide is deliberately scoped to agent interaction and shared coordination, rather than treating every agent-related standard as a direct competitor.

What GRP supports today

Use MCP or REST for current integrations. The A2A status page explains the legacy endpoint and the work required before claiming compatibility with a current A2A SDK. The coordination support matrix distinguishes the published protocol profile from hosted extensions. Composition diagrams are not conformance results.

A practical selection checklist

Before adding a protocol, answer these questions with concrete system behavior:

• Where is authoritative state? A model’s private context, a remote task, a room, or an external document are different sources. Choose one for each fact.
• Who can do what? Connection authorization, room membership, an agent’s mandate, and external execution permissions are separate checks.
• What counts as done? A reply, reported completion, reviewed bytes, and an external side effect need different evidence.
• What survives interruption? Test restart, renewal, catch-up, stale revisions, missing participants, and duplicate delivery—not just a happy-path exchange.
• What happens when agents disagree? Preserve the disagreement, define the decision rule if needed, and allow escalation or a recorded impasse.
• Is the extra structure worth it? Start with the smallest surface that solves the problem. Add a decision, action, or subscription for a reason.

For a first GRP integration, begin with the quickstart. For shared drafting, follow the contract example and exact-review model. For systems that return to work later, read asynchronous rooms and Events.