MCP vs A2A: Different Protocols for Tools and Agent Delegation

An A2A client discovers a remote agent through an Agent Card and delegates a task. The receiving agent may then use MCP to discover and invoke a tool. Either agent may make a separate HTTP request to an LLM while doing the work. Those are three transitions, with three authorization decisions and three evidence requirements.

The official A2A comparison describes MCP as the vertical connection to tools and resources and A2A as the horizontal connection between agents. Treating MCP vs A2A as a winner-takes-all contest produces a bad architecture diagram and an even worse audit trail.

TL;DR

• A2A handles remote agent discovery and delegated work through Agent Cards and tasks. Messages carry parts and artifacts.
• MCP gives a host access to tools and resources through a stateless JSON-RPC client-host-server protocol.
• One workflow can use A2A for delegation and MCP for a tool call. It can also use a separate HTTP request for model inference.
• Identity and authorization need explicit handling at every hop. Audit evidence and state need explicit handling too.
• DeepInspect governs a routed authenticated HTTP LLM request, not the A2A task, MCP tools/call, or local stdio process.

A2A owns the remote-agent delegation layer

The A2A project's comparison of A2A and MCP places A2A on the horizontal axis between agents. A client agent discovers a remote agent and delegates work without requiring access to the remote agent's internal implementation.

A2A's key concepts describe remote agents as HTTP endpoints. An Agent Card advertises identity and capabilities for discovery. Work proceeds through stateful tasks. Tasks track status and messages carry communication. Parts hold message content, while artifacts are outputs produced by the task.

That object model fits work that lasts longer than one request and can produce intermediate state. Picture a whiteboard with a claims agent on the left and a specialist coding agent on the right. The arrow between them is an A2A task. The coding agent remains opaque; the requesting agent cares about advertised capability and task state. It also cares about the returned artifact. A2A gives that arrow a protocol vocabulary.

MCP owns the tool and context layer inside an agent

The latest MCP architecture specification defines a stateless JSON-RPC client-host-server protocol focused on context exchange. A host manages one or more clients, and each client connects to a server that exposes tools and resources.

In the delegated claims example, the receiving specialist agent might use MCP to discover a policy lookup tool or fetch a resource. It might also invoke a calculation. The MCP server supplies the interface the agent can use. The A2A task is the unit of delegated work between agents, while the MCP interaction gives the receiving agent access to a capability needed to complete that work.

The MCP security vs API security guide examines the runtime discovery and tool trust issues at that boundary. For this comparison, keep the layers separate. An Agent Card advertises an agent, while MCP capability discovery exposes context and tools to an agent host.

One delegated task crosses three control points

Start with the full sequence: agent A discovers agent B through its A2A Agent Card and creates a task. Agent B receives messages and works toward an artifact. Next, agent B's host uses MCP to find and invoke a tool. Finally, agent B sends a prompt to an LLM through a separate model API request.

Each transition needs its own decision record:

• The A2A record should connect the requesting principal, client agent, remote agent, task identifier, requested capability, state changes, and resulting artifact.
• The MCP record should identify the host, server, capability, invocation arguments or resource reference, authorization result, and returned content.
• The LLM record should connect the authenticated caller context supplied by the application, model route, data classification, policy version, decision, and timestamp.

Combining these into one line such as "agent completed task" destroys the chain an incident responder needs. The multi-agent system security guide covers the broader delegation risks. Here, the design requirement is a stable correlation identifier across distinct records.

Discovery objects answer different questions

An A2A Agent Card tells a client about a remote agent and supports discovery before delegation. MCP discovery happens inside the host-client-server relationship and presents resources or tools that the host may use.

Neither discovery object should become an authorization decision by itself. An advertised capability says that an agent or server can perform an operation. Policy must still determine that this caller may request this operation with this data under the current purpose. Discovery changes over time, so authorization should evaluate the live request against policy rather than copy an old capability list into a permanent allow rule.

I dislike architecture documents that draw discovery and authorization as one box. That shortcut survives the design review and fails during the first privilege investigation. Keep separate evidence for what was advertised and what was requested. Record what policy allowed too. The MCP policy enforcement guide goes deeper on policy at the tool boundary.

State behaves differently across the two protocols

A2A represents delegated work as a stateful task with messages. Parts and artifacts remain attached to that task. The task gives the participants a durable unit for progress and results. MCP's 2026-07-28 architecture describes a stateless JSON-RPC protocol for context exchange. The application around MCP can maintain workflow state, but the core interaction model and the A2A task model solve different problems.

This distinction affects recovery and audit. An A2A client may need to reconnect to a task and inspect its current state. An MCP host needs to know which capability invocation supported that task and how the response influenced the next action. A model request needs its own pass or redact decision even when it occurs halfway through a longer A2A task. A block decision needs its own record too.

Treat the A2A task identifier as a correlation value. Carry the initiating principal and delegated scope explicitly through the application. Correlate that context with each MCP invocation and LLM request where the architecture permits it.

Transport determines which network controls can see the call

A2A models remote agents as HTTP endpoints. MCP supports Streamable HTTP and stdio transports. Those transport choices create different enforcement opportunities.

Remote HTTP traffic can pass through network and application policy points designed for that protocol. Local stdio runs between processes on the host, so a network proxy lacks that traffic. The tool invocation and the later LLM request also remain separate, even when both use HTTP. Assign MCP tools/call authorization to an MCP-aware policy point. Classify the later model prompt at the LLM request boundary.

The practical control map should label each arrow with transport and owner. Local stdio requires host controls. Delegated tasks require A2A-aware authorization, while capabilities require MCP-aware authorization. Apply LLM request policy to the model call. The AI agent security guide provides the surrounding control model.

Protocol selection follows the relationship being modeled

Use A2A when one remote agent needs to discover another agent, delegate a unit of work, track state, exchange messages, and receive artifacts while the remote implementation stays opaque. Use MCP when an agent host needs a consistent protocol for tools and resources. Prompts can also be supplied by an MCP server.

Use both when the remote agent needs tools to complete delegated work. An A2A task can contain work that invokes an MCP capability, while a separate model API call supports execution. Security teams should resist collapsing the diagram after implementation because separate arrows preserve ownership and make failures diagnosable.

The decision also affects procurement. An A2A-capable agent product and an MCP server solve different integration needs. For remote delegation, inspect the Agent Card and task controls. An MCP review should cover transport, server authorization, and capability controls. The LLM request path needs route policy and independent decision records.

DeepInspect

DeepInspect sits between an authenticated user or agent and an LLM on deliberately routed HTTP model traffic. At that boundary, it can evaluate identity context supplied by the application, classify the prompt, enforce route and data policy, and write a per-decision audit record for the model request.

It does not authorize the A2A task, approve the MCP tools/call, inspect local stdio, or govern downstream business actions. Those controls belong at their respective protocol and host boundaries. In a composed A2A and MCP workflow, DeepInspect covers the separate LLM request arrow when that HTTP traffic traverses the proxy.

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