Why agent protocols

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An agent on its own can only use what it was built with. To work with a tool, another agent or a user interface, it needs a common language. Without one, every pair of parties needs its own custom integration, which is the problem Anthropic described for data sources when it introduced MCP: every new source needed its own implementation, so connected systems did not scale. A protocol is that agreed language.

What the language covers

Four jobs, and a protocol takes on some or all of them.

  • Discover each other: learn that another party exists and where it lives.
  • Describe capabilities: what it can do and what input it takes.
  • Communicate: a message format both sides can parse.
  • Exchange context: pass along the data and state the task needs.

Together these give interoperability: parts built by different teams, on different frameworks, work together without custom glue.

Four names, three directions

ProtocolConnectsIn one line
MCPAgent and tool or dataStandardizes how a model gets context and calls tools.
A2AAgent and agentLets independent agents discover each other, delegate tasks and share results.
AG-UIAgent and user interfaceStreams agent state and events to a user-facing app.
ACPAgent and agentIBM Research’s agent protocol, since folded into A2A.
What each protocol connects

The two worth reading in detail are MCPMCPModel Context Protocol: one standard way for an agent to reach tools and data.Opens in a new tab and A2A, ACP and AG-UIA2A, ACP and AG-UIThe other three protocols: agent to agent, the one that merged, and agent to user.Opens in a new tab. The multi-agent patterns (see choosing a patternChoosing a patternRouter, handoff, group chat or swarm: what each one trades away.Opens in a new tab) describe how agents split work inside one system. Protocols matter where the parties are built separately.

Still settling

These are emerging standards, and the set is not settled. ACP launched in March 2025, and by August 2025 its team had stopped development to join A2A. MCP moved to the Linux Foundation’s Agentic AI Foundation in December 2025, and its own handshake and transports changed between spec revisions (see the MCP lifecycleMCP lifecycleDiscovery, order, delivery: how one tool call moves between client and server.Opens in a new tab). Names and versions here are a snapshot. Read the current spec before building on one.

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