mcp_a2a_comparison

This document outlines the key functionalities of the Model Context Protocol (MCP) and the Agent-to-Agent (A2A) protocol, derived from their respective JSON schema definitions (schema.json for MCP, dated 2025-03-26, and a2a.json for A2A). It then presents a comparative table to identify common and unique functionalities and analyze interoperability.

Part 1: Model Context Protocol (MCP) Functionality

Derived from schema.json. MCP facilitates interactions between a "client" (e.g., an LLM-hosting environment or agent framework) and a "server" (providing tools, resources, or prompts).

Client-to-Server Request Methods:

initialize: Client initiates connection with the server, exchanging capabilities (ClientCapabilities, ServerCapabilities), protocol versions, and implementation information.
ping: Client sends to server to check liveness and ensure the connection is active.
resources/list: Client requests a list of available data resources from the server.
resources/templates/list: Client requests a list of resource templates (e.g., for URI construction) from the server.
resources/read: Client requests the actual content of a specific data resource (identified by URI) from the server.
resources/subscribe: Client subscribes to receive notifications from the server when a specific resource is updated.
resources/unsubscribe: Client unsubscribes from notifications for a specific resource.
prompts/list: Client requests a list of available prompts or prompt templates from the server.
prompts/get: Client requests a specific prompt from the server, potentially providing arguments for templating.
tools/list: Client requests a list of available tools (callable functions) from the server.
tools/call: Client invokes a named tool on the server, providing specific arguments for that tool.
logging/setLevel: Client requests the server to set or adjust the verbosity level of log messages that the server sends back to the client.
completion/complete: Client requests autocompletion suggestions from the server, typically for arguments of tools or resource names.

Server-to-Client Request Methods:

ping: Server sends to client to check liveness (bidirectional with client's ping).
sampling/createMessage: Server requests the client (which often manages or is an LLM itself) to generate a message (perform LLM sampling) based on provided context, messages, model preferences, etc.
roots/list: Server requests a list of root URIs (e.g., file system paths representing the client's workspace context) from the client.

Notification Methods (Direction Indicated):

notifications/cancelled (Bidirectional): Sent by either client or server to cancel a previously issued, still in-flight request.
notifications/initialized (Client-to-Server): Client informs the server that it has completed its side of the initialization process.
notifications/progress (Bidirectional): Sent by the receiver of a long-running request to the sender to provide an update on the progress of that request.
notifications/roots/list_changed (Client-to-Server): Client informs the server that its list of available root URIs has changed.
notifications/resources/list_changed (Server-to-Client): Server informs the client that its list of available data resources has changed.
notifications/resources/updated (Server-to-Client): Server informs a subscribed client that a specific resource has been updated.
notifications/prompts/list_changed (Server-to-Client): Server informs the client that its list of available prompts has changed.
notifications/tools/list_changed (Server-to-Client): Server informs the client that its list of available tools has changed.
notifications/message (Server-to-Client): Server sends a log message to the client (related to logging/setLevel).

Part 2: Agent-to-Agent (A2A) Protocol Functionality

Derived from a2a.json. A2A focuses on agent-to-agent communication using a task-based paradigm.

A2A Request Methods (from one agent to another):

tasks/send: Initiates a new task or sends a subsequent message within an existing task to a target agent. The message payload (containing role and parts like text, file, or structured data) carries the communicative intent. This is the primary method for instructing another agent or requesting it to use one of its skills.
tasks/get: Retrieves the current status, history of messages, and any generated Artifacts for a specific task from the target agent.
tasks/cancel: Requests the target agent to cancel an ongoing task it is managing.
tasks/sendSubscribe: Similar to tasks/send, but also subscribes the sender to a stream of real-time updates for the task (e.g., TaskStatusUpdateEvent, TaskArtifactUpdateEvent).
tasks/resubscribe: Allows an agent to re-subscribe to an existing task for streaming updates, typically used after a disconnection or to join an ongoing stream.
tasks/pushNotification/set: Configures how an agent wishes to receive push notifications (e.g., via a webhook) for a specific task from the target agent.
tasks/pushNotification/get: Retrieves the current push notification configuration for a specific task from the target agent.

Key A2A Data Structures Defining Functionality (not RPC methods):

AgentCard: A rich descriptor for an agent, detailing its name, endpoint URL, version, capabilities (e.g., streaming, pushNotifications), authentication methods, and a list of AgentSkills. It's fundamental for agent discovery and understanding.
AgentSkill: Defines a specific capability or function an agent offers, including its ID, name, description, and supported input/output modes.
Message: The core structure for communication content within a task, composed of one or more Parts.
Part: Can be TextPart, FilePart, or DataPart (for arbitrary JSON data), enabling flexible and rich message payloads.
Artifact: Represents data or files generated, exchanged, or referenced during a task.
Task: A central object representing the stateful, potentially long-running interaction between agents, including its ID, status, history of messages, and artifacts.
TaskStatusUpdateEvent, TaskArtifactUpdateEvent: These are not incoming RPC notification methods but rather event structures pushed from the serving agent to the subscribed client agent over an established stream (initiated by tasks/sendSubscribe or tasks/resubscribe).

Part 3: Comparative Table of Functionality

This table maps the detailed methods to broader conceptual functions, highlighting MCP and A2A equivalents and noting areas of uniqueness or required dual implementation for an entity participating in both protocols.

Unified Functional Name	MCP Equivalent Method(s) / Concept	A2A Equivalent Method(s) / Concept	Notes / Dual Implementation?
Connection & Session Setup
Initialize Session/Exchange Capabilities	`initialize`	`N/A` (Agent capabilities via `AgentCard`; auth via `AgentAuthentication` within Card or per request)	MCP has explicit session init. A2A relies on pre-discovery of `AgentCard` & its URL. Different paradigms.
Check Liveness	`ping`	`N/A` (No A2A-specific ping method)	MCP has explicit ping.
Discovery & Capability Listing
Discover Agent Identity/Overall Capabilities	`N/A` (Server capabilities via `InitializeResult`)	`AgentCard` (data structure. A2A docs suggest MCP server may list AgentCards as resources for discovery)	A2A `AgentCard` is richer for autonomous agents. Discovery of AgentCards might use MCP resource listing. Dual setup for full discoverability.
List Available Tools/Skills	`tools/list`	Skills listed in `AgentCard` (data structure)	MCP lists tools of a server. A2A skills are part of agent's self-description. If an agent is both, dual mechanism for advertising.
List Available Data Resources	`resources/list`, `resources/templates/list`	`N/A` (Data exchanged as `Message` parts or `Artifacts` within tasks)	MCP has explicit browseable resources. A2A is task-data oriented. Different paradigms.
List Available Prompt Templates	`prompts/list`	`N/A`	MCP specific.
Core Interaction & Tasking
Invoke Named Function/Skill with Arguments	`tools/call`	`tasks/send` or `tasks/sendSubscribe` (intent & args in `Message` payload)	Dual implementation required. Different methods & payload structures for the same underlying agent skill.
Send General Message/Data to Entity	`N/A` (MCP methods are specific to resource/tool/prompt ops)	`tasks/send` or `tasks/sendSubscribe` (using `Message` parts)	A2A is designed for rich message exchange within tasks.
Get Data/Resource Content	`resources/read`	`N/A` (Task artifacts are part of `Task` object via `tasks/get` or streamed events)	MCP for direct resource reads. A2A for task outputs. Dual implementation if same data offered both ways.
Get Prompt Content	`prompts/get`	`N/A`	MCP specific.
Get Status/Results of Operation/Task	`CallToolResult` (direct for `tools/call`), `notifications/progress`	`tasks/get` (for `Task` object); streamed events (`TaskStatusUpdateEvent`, `TaskArtifactUpdateEvent`)	Dual implementation required. Different ways to get results/status.
Cancel In-Progress Operation/Task	`notifications/cancelled`	`tasks/cancel`	Conceptually similar. `notifications/cancelled` is a generic JSON-RPC notification for any MCP request. `tasks/cancel` is a specific A2A request for an A2A task. Dual implementation for an entity processing both.
Asynchronous Operations & Notifications
Subscribe to Data/Resource Updates	`resources/subscribe` (results in `notifications/resources/updated`)	`tasks/sendSubscribe` / `tasks/resubscribe` (for task events/stream)	Dual implementation required. MCP for resource changes, A2A for task stream.
Unsubscribe from Data/Resource Updates	`resources/unsubscribe`	`N/A` (Stream ends with task completion/cancellation, or connection drop)	MCP explicit. A2A implicit or via task cancellation.
Receive Progress Notifications	`notifications/progress` (for MCP requests)	`TaskStatusUpdateEvent` / `TaskArtifactUpdateEvent` (streamed for A2A tasks)	Dual implementation required. Different notification mechanisms.
Receive Entity List Change Notifications	`notifications/resources/list_changed`, `notifications/prompts/list_changed`, `notifications/tools/list_changed`	`N/A` (Agent capabilities in `AgentCard` are not actively pushed as "list changed" by A2A itself; discovery is pull-based)	MCP specific for server offerings.
Configure/Receive Push Notifications	`N/A`	`tasks/pushNotification/set`, `tasks/pushNotification/get`	A2A specific for tasks, enabling out-of-band notifications.
Client-Side Specifics (MCP Context)
Control Server Logging Output	`logging/setLevel` (client tells server what to log to it)	`N/A`	MCP specific.
Request Input Autocompletion	`completion/complete`	`N/A`	MCP specific.
Server-Initiated Requests (MCP Context)
Request Client to Perform LLM Sampling	`sampling/createMessage`	`N/A` (An A2A agent could ask another A2A agent that fronts an LLM to do this via `tasks/send`, but it's a peer request, not MCP's client-as-LLM-service model)	Different paradigms. MCP assumes client has direct LLM access.
Request Client's File System Roots	`roots/list`	`N/A`	MCP specific for client-provided workspace context.

Part 4: Key Implications, Overlaps, and Differences

This section summarizes the practical consequences for an agent or service operating in an ecosystem where both MCP and A2A are present.

Core Action Invocation Requires Dual Implementation:
- To offer a specific capability (e.g., "analyze_image") to both an MCP client (like an LLM calling a tool) and an A2A peer agent, an entity must implement:
  1. An MCP endpoint to handle tools/call with params.name == "analyze_image".
  2. An A2A endpoint to handle tasks/send (or tasks/sendSubscribe) and parse the Message payload to identify the request for the "analyze_image" skill and its arguments.
- Similarly, reporting results, status, and handling cancellations for such an action would require adhering to the distinct mechanisms of each protocol.
Discovery and Capability Advertisement:
- Designed Interoperability Point: A2A documentation suggests A2A AgentCards (which describe A2A agent capabilities and skills) can be listed as resources on an MCP server. This allows MCP-style discovery mechanisms to find A2A agents.
- Dual Presence for Full Visibility: An entity that is both a rich MCP tool/resource provider and a collaborative A2A agent would need:
  1. To implement MCP methods like tools/list, resources/list for its MCP offerings.
  2. To have its AgentCard published (e.g., via an MCP resource listing) for its A2A persona.
Asynchronous Operations and Notifications:
- Both protocols support asynchronous operations, but through different mechanisms.
- MCP uses specific notifications like notifications/progress for requests and notifications/resources/updated for subscribed resource changes.
- A2A uses its task-based model with status updates (TaskStatusUpdateEvent, TaskArtifactUpdateEvent) typically delivered via streaming (from tasks/sendSubscribe) or polling (tasks/get), and has dedicated methods for configuring task-based push notifications.
- An entity handling long operations for both MCP and A2A clients would need to implement both notification/update systems.
Unique Strengths and Focus Areas:
- MCP excels at:
  - Standardizing how LLMs and agent frameworks (clients) connect to and consume discrete tools, data resources, and prompt templates offered by servers.
  - Providing specific mechanisms for client-side LLM sampling (sampling/createMessage) and server access to client-side context (roots/list).
  - Fine-grained listing and discovery of server-offered tools, resources, and prompts.
- A2A excels at:
  - Facilitating complex, stateful, and potentially long-running collaborative tasks between autonomous agents.
  - Rich, flexible message payloads (Message with Parts) for nuanced inter-agent communication.
  - A comprehensive AgentCard for detailed agent self-description, including skills, capabilities, and authentication.
  - Built-in support for streaming task updates and configurable push notifications for tasks.
Paradigm Differences:
- MCP often implies a more direct client-server interaction for specific functionalities (tool call, resource read).
- A2A interactions are framed within a "Task," which provides a stateful context for potentially multi-turn message exchanges and artifact generation.
- The "tool is an agent" idea highlights that an entity might need to bridge these paradigms internally if it offers its capabilities through both protocols.

In summary, while MCP and A2A are designed to be complementary for building broad agentic applications (MCP for agent-to-tool, A2A for agent-to-agent), an individual entity aiming to be a first-class citizen accessible via both protocols for its core capabilities will face significant dual implementation requirements for handling requests, responses, and asynchronous updates.

webr3/mcp_a2a_comparison.md