1. MCP (Model Context Protocol): Agent -> Tools
When an AI agent needs to interact with the physical or digital world—like querying a database, hitting an API, or reading a local file—it uses the Model Context Protocol (MCP).
The Flow: A user issues a request via an MCP Host (such as Claude Desktop, Cursor, or VS Code). If the host needs external data, it instantiates an MCP Client. This client packages and routes the request using the MCP Protocol to an MCP Server, which processes the request and securely exposes external tools (like PostgreSQL, GitHub, or Google Drive) to return a structured response.
Best For: Connecting an LLM directly to development environments, enterprise data sources, and local file systems.
2. A2A: Agent -> Agent (Registry-Based)
When a single agent doesn't have the specialized skills required to solve a problem, it needs to delegate. The registry-driven Agent-to-Agent (A2A) pattern handles this collaborative routing.
The Flow: Agent A receives a user request but needs assistance. It queries a central Agent Registry which stores "Agent Cards" (manifests of capabilities). Once a capable peer (Agent B) is discovered, the task is delegated to it. Agent B executes the task and returns the results back to Agent A to continue the workflow.
Best For: Complex, multi-agent workflows within a managed ecosystem where agents have distinct, specialized roles (e.g., a researcher agent handing off data to a writer agent).
3. ACP: Agent -> Agent (REST/HTTP-Based)
If you want agents to collaborate but prefer standard web architecture over centralized registries, Agent-to-Agent via REST (ACP) is the way to go.
The Flow: Agent A reads a static Metadata Manifest containing the capabilities of surrounding agents. It discovers Agent B and sends a standard REST/HTTP request directly to it. Depending on the complexity of the task, Agent B can execute and return the data via a Synchronous or Asynchronous Response, passing the results back to Agent A.
Best For: Decoupled, microservice-style AI architectures that rely on traditional web infrastructure and require flexible sync/async processing.
Which one should you build on?
Choose MCP if you are building developer tools or need your LLM to interact seamlessly with local code, databases, and APIs.
Choose A2A if you are designing a dynamic, closed-loop multi-agent system where agents need to actively discover each other’s capabilities on the fly.
Choose ACP if you want a clean, web-standard approach to connecting agents using existing HTTP infrastructure and decoupled microservices.






