APIs, MCP and A2A: Choosing How AI Agents Connect
APIs, Model Context Protocol and Agent2Agent serve different relationships. An API gives software a defined way to request a service. MCP helps an AI application discover and use approved tools, resources and prompts. A2A allows independently operated agents to exchange tasks, status and results. A business should select each interface according to the relationship it must govern, rather than the novelty of the protocol.
The integration decision begins with ownership and authority
An AI agent becomes operational when it can obtain information or request an action from another system. The design of that connection determines which data the agent can see, which changes it can request and which organisation remains accountable when the request fails. The protocol decision is therefore part of the operating design, not merely a technical preference.
Management should first identify the parties on each side of the connection. The same organisation may control both systems, or an external supplier may operate one of them. Management must then define the capability required, the authority delegated and the result that the receiving system must confirm. These conditions reveal whether the business needs a stable service call, discoverable AI capabilities or a task relationship between independent agents.
An API supports a known service contract
An API is usually the simplest choice when the calling application already knows the operation it needs. For example, an agent may read an order, obtain current inventory or submit a maintenance request through existing APIs. The application knows the endpoint, required inputs, expected response and permitted action before the request begins.
This certainty makes an API suitable for material transactions. The receiving service can authenticate the caller, validate the request, enforce business rules and return a transaction identifier. An AI framework may present the operation to a model as a tool, but the underlying API should still enforce the same permissions and validation that apply to any other caller.
MCP standardises capability discovery for AI applications
An organisation may want several AI applications to use the same approved tools and information sources. Building a separate connector for every application duplicates integration effort and allows descriptions or permissions to diverge. MCP addresses that problem by giving compatible hosts a common way to discover and invoke tools, read resources and obtain prompts exposed by an MCP server.
MCP standardises the connection between an AI application and those capabilities. It does not decide whether a user may approve a payment, whether an order change is valid or whether returned content is trustworthy. The host and the receiving service must still authenticate the relevant identity, restrict permissions, validate inputs and outputs, and record consequential actions. A business should therefore use MCP to simplify governed access, while retaining its business controls behind the protocol.
A2A carries tasks between independently operated agents
A different problem arises when one agent needs another agent to complete a task without revealing its internal tools or reasoning process. The A2A protocol allows an agent to advertise capabilities through an Agent Card, receive a message, manage a task and return status updates or artefacts. This arrangement can be useful when different teams, vendors or organisations operate the participating agents.
However, A2A does not make a multi-agent design necessary. A single application can usually coordinate its own internal components through direct function calls or framework orchestration. A2A becomes relevant when the independence of the participating agents creates a real interoperability requirement, especially when a task may continue over time or cross an organisational boundary.
One business process may use all three interfaces
The three approaches are complementary because they govern different relationships. An agent may use MCP to discover an approved capability, call an ordinary API to complete a controlled transaction and use A2A to delegate a separate task to an independently operated agent. The architecture remains clear only when each interface has a distinct responsibility.
Duplicated routes weaken that clarity. If the same inventory reservation can be created through a narrow transaction API, a broad MCP tool and an external agent, the organisation may apply different permissions or validation to the same business action. Management must select one authoritative route for each consequential action and require every higher-level protocol to use that route.
A logistics exception shows where each boundary belongs
Consider a distributor that wants an AI agent to respond when a shipment will miss a customer commitment. The agent first needs inventory, order and transport data. Existing APIs are appropriate for these stable internal records because the distributor controls the systems and knows the required operations.
The distributor also operates several assistants that need approved document search and case-management tools. An MCP server can publish those capabilities once so that compatible assistants discover the same descriptions and schemas. The server reduces connector duplication, while the underlying services continue to enforce access and transaction rules.
The distributor should consider A2A only if a carrier operates an independent agent that can accept a disruption task, report progress and return a confirmed recovery option. A stable carrier API may still be the better choice when the distributor requires only a fixed status or booking operation. The deciding issue is whether the carrier provides a service endpoint or assumes responsibility for a continuing task.
Security follows the trust boundary and the business consequence
Every connection requires a verified caller, a defined permission scope and a record of the request and result. MCP and A2A add discovered descriptions, returned content and longer interactions that an agent may use in later decisions. The receiving application must treat those materials as untrusted until it validates their source, structure and permitted effect.
The strongest control belongs in the system that changes the business record. If an agent requests a payment, reservation, access change or customer communication, the receiving service must enforce the approval, value limit and transaction rule. A protocol can carry the request and its credentials, but protocol compliance cannot prove that the business action is authorised or correct.
Failure and change must be designed before deployment
Connections change over time. An MCP server may revise a tool definition, an A2A Agent Card may advertise a new capability and an API may release a new version. The organisation must decide which changes require review, how clients negotiate supported versions and how it will revoke a capability that no longer meets policy.
The agentic workflow must also preserve state when a connection fails. A retry should locate the original transaction or task instead of creating a duplicate. Long-running tasks need stable identifiers, time limits and an owner who can resolve an uncertain outcome. These controls allow the business to recover the process without guessing which action the agent completed.
How Marketways selects the integration approach
Marketways begins with the business process, participating organisations and decisions that the connected systems must support. We identify the authoritative system for each action, the information that may cross each boundary, the identity on whose behalf the request occurs and the evidence required to confirm completion.
We then select the least complex interface that supports those requirements. The resulting design may retain existing APIs, introduce MCP for shared AI capabilities or use A2A for a justified relationship between independent agents. Marketways also specifies permissions, failure handling, evaluation scenarios and management controls so that engineering teams can implement the connection without transferring undefined authority to the agent.