What is ACP and how does it position relative to MCP and A2A in the agent protocol landscape?
Describe ACP (Agent Communication Protocol) and explain how it relates to MCP and A2A in the agent protocol ecosystem. How is each protocol governed?
MCP connects agents to tools. A2A delegates tasks across orgs. IBM's REST-based ACP merged into A2A in 2025. MCP and A2A now share one home, the Linux Foundation's Agentic AI Foundation.
Think of a busy office. MCP is the standard wall socket every gadget plugs into to get power and data. It connects one worker to all their tools. A2A is the courier service that hands a sealed work order to another company's office and waits for the finished result. ACP was an open inbox where any worker could drop a rich message, attach files, and have a back and forth conversation, even if the two sides never agreed on a format in advance. The inbox company later merged into the courier service. Sockets and couriers do different jobs, so neither replaces the other. They also now share one building manager, a neutral foundation, though each keeps its own rulebook.
Concept explanation~2 min read
Everything you need to truly understand this topic: intuition, mechanics, step by step explanation, code, formulas, and worked example. Click to expand.
Concept explanation~2 min read
Everything you need to truly understand this topic: intuition, mechanics, step by step explanation, code, formulas, and worked example. Click to expand.
The trap is to treat MCP, A2A, and ACP as three competitors, where one must eventually win. That framing misses the point. Each protocol was built for a boundary. The useful skill is placing each one at the boundary it solves rather than ranking them against each other.
The cleaner mental model is that an agent system has multiple boundaries. There is the boundary between an agent and its tools or data. There is the boundary between one agent and another agent, especially across organizations. And there is the looser boundary of general agent to agent conversation that is richer than a single task handoff. Each protocol was built to serve one of these. MCP and A2A coexist because their boundaries differ. ACP's boundary overlapped A2A's, and in August 2025 it merged into A2A.
The tool layer is settled with MCP, and the agent-messaging layer has consolidated around A2A.
MCP: the settled tool-connectivity layer
MCP is the Model Context Protocol, released by Anthropic in November 2024. It standardizes the boundary between an LLM host application and external capability providers. A server exposes three primitives: tools (callable functions), resources (read-only data referenced by URI), and prompts (reusable templates). The host discovers these over JSON-RPC and surfaces them to the model, typically by registering each tool through the model vendor's function-calling format.
The problem it solved was the M×N integration explosion. Before MCP, every agent app hand-wrote a custom integration for every tool, so M apps times N tools meant M×N pieces of glue. MCP collapses that to M+N: write one server per tool, one client per host, and any host talks to any server. That reframing is the most important point, because it explains why the protocol exists.
Be precise about what MCP is not. It is not a way for one agent to delegate work to another agent, and it is not a model-side calling convention. It sits below the model's function-calling format and above the concrete tool, acting as the integration plane.
MCP is effectively the default at the tool layer. It is an open spec, so although Anthropic created it, any provider can implement it. That openness plus an early ecosystem is why it spread beyond a single vendor and why competitors did not displace it.
Situations where this technique stops working.
2–4 min · Everything important, quickly.
| Dimension | MCP | A2A | ACP |
|---|---|---|---|
| Boundary | Agent to tools and data | Agent to agent, cross-org delegation | Agent to agent, general messaging |
| Origin | Anthropic, Nov 2024 | Google, April 2025 | IBM Research, BeeAI |
| Core unit | Tools, resources, prompts | Capability card, task lifecycle | Rich multi-turn messages |
| Transport style | JSON-RPC over stdio or HTTP | HTTP-based task exchange | REST-based messaging |
| Governance | Linux Foundation's Agentic AI Foundation (since 2025) | Linux Foundation; Agentic AI Foundation since 2026 | Merged into A2A (2025) |
| Status | Dominant at tool layer | Leading cross-org delegation | Merged into A2A |
Real products, models, and research that use this idea.
- MCP is an open spec with reference servers, hosted by the Linux Foundation's Agentic AI Foundation since December 2025. Cursor and Zed adopted it so one server runs across hosts.
- Google introduced A2A in April 2025 with launch partners, then donated it to the Linux Foundation for neutral governance.
What an interviewer would ask next. Try answering before peeking at the approach.
QWhy did MCP win the tool-connectivity layer when other tool standards existed?
Point at the M×N integration pain it targeted, early reference servers and SDKs, fast host adoption by Cursor and Zed, and the resulting network effects.
Red flags & common mistakes
The phrases that signal junior thinking. Click to expand.
Red flags & common mistakes
The phrases that signal junior thinking. Click to expand.
Treating ACP as a live rival to A2A. IBM merged ACP into A2A in August 2025, and MCP and A2A sit at different boundaries: tool connectivity and cross-org delegation.
60 second bullets to scan on the way to the call.
The boundary each protocol owns: tools, delegation, general messaging
Origin org and rough date for MCP, A2A, and ACP
Primary sources. Browse if you want the original framing.
Same topic, related formats. Practice these next.