Agent protocols: A2A, MCP and the two ACPs

At a glance

A2A (Agent2Agent)

A2A is an open protocol that lets agents built on different frameworks communicate. Agents interact "without needing to share internal memory, tools, or proprietary logic", and publish Agent Cards so others can discover them. Google announced A2A in April 2025; the Linux Foundation launched the A2A project on 23 June 2025 with more than 100 supporting companies and founding members Google Cloud, AWS, Cisco, Salesforce, SAP, Microsoft and ServiceNow. The technical steering committee includes AWS, Cisco, Google, IBM Research, Microsoft, Salesforce, SAP and ServiceNow. Version 1.0.0, released 12 March 2026, was a major release with breaking changes, including OAuth 2.0 modernization and a specification refactor. Version 1.0.1 followed on 28 May 2026 and prefers the application/a2a+json media type. Version 0.3.0 (30 July 2025) added mTLS and signed Agent Cards; several products still target v0.3.

Buyer note: ask vendors which A2A version they support. Microsoft Foundry supports v1.0 (GA) and v0.3 (preview); Gemini Enterprise registration requires a v0.3-compatible agent, with v1.0.0+ via compatibility packages.

MCP (Model Context Protocol)

MCP is an open standard for connecting AI applications to external systems, described by its documentation as "like a USB-C port for AI applications". It uses JSON-RPC 2.0 between hosts, clients and servers. Servers expose resources, prompts and tools; clients can support elicitation. Extensions include Tasks for long-running work, Skills over MCP and MCP Apps for inline interfaces. The latest specification revision is dated 28 July 2026. MCP is a founding project of the Linux Foundation's Agentic AI Foundation (AAIF), alongside goose and AGENTS.md; AAIF also hosts agentgateway and lists A2A among its projects.

Buyer note: MCP is about tools, not agent collaboration. An agent can be exposed as an MCP tool, but that makes it a function another agent calls, not a peer.

The two ACPs

Agent Communication Protocol (IBM / BeeAI). Launched by IBM in March 2025 and contributed to the Linux Foundation's LF AI & Data. On 29 August 2025, LF AI & Data announced that ACP was joining forces with A2A: the ACP team stopped active development, IBM's Kate Blair joined the A2A technical steering committee, and BeeAI moved to A2A through adapters. If an RFP response cites this ACP, ask about the A2A migration path.

Agent Client Protocol. A different standard that connects code editors and IDEs to coding agents, often described as an "LSP for agents". It runs JSON-RPC over stdio for local agents, reuses MCP JSON types, and has remote transport over HTTP or WebSocket in progress. It matters for developer tooling (for example, running coding agents inside an editor) rather than for enterprise agent-to-agent traffic.

How to tell them apart: if the context is enterprise agents talking to each other, "ACP" almost always means the IBM protocol, now folded into A2A. If the context is editors and coding agents, it means the Agent Client Protocol.

Where protocols stop

Protocols define how a call is made. They do not define:

• which of many agents should handle a piece of work;
• how to stop two agents replying to each other indefinitely;
• whether each recipient processed a message, and exactly once;
• how an agent that crashed catches up;
• where the shared history and memory of a task lives;
• where a person reviews, approves or overrides.

These are coordination functions. BAND supports MCP and A2A and provides them in its collaboration layer: @mention routing, loop prevention with mandatory mentions and per-room limits, a per-agent, per-message lifecycle with attempt history, exactly-once processing, two-phase sync for catch-up, Memories, and human inspect, approve and override.

Vendor protocol support (September 2026)

Source: a2a-protocol.org/latest , github.com/a2aproject/A2A/releases , linuxfoundation.org/press/linux-foundation-launches-the-agent2agent-protocol-project-to-enable-secure-intelligent-communication-between-ai-agents , modelcontextprotocol.io/specification/latest , modelcontextprotocol.io/docs/getting-started/intro , aaif.io/projects , lfaidata.foundation/communityblog/2025/08/29/acp-joins-forces-with-a2a-under-the-linux-foundations-lf-ai-data , agentclientprotocol.com · Reviewed Sep 2026

Frequently asked questions