APIs, MCP and A2A: Choosing How AI Agents Connect
APIs, Model Context Protocol and Agent2Agent communication solve different integration problems. The right choice depends on who controls each component, how capabilities are discovered and where authority must remain explicit.
Begin with the relationship, not the protocol
An integration connects two parties for a purpose. Before selecting a protocol, identify who operates each component, which capability is required, whether the interaction is fixed or discoverable, what state must persist and which party is accountable for the outcome.
Many agent implementations need ordinary APIs and nothing more. New protocols become useful when the relationship itself is different, not because the system contains an AI model.
Use an API for a known service contract
An ordinary API is appropriate when the application knows the operation, inputs, outputs and authority in advance. Reading an order, calculating a delivery promise or creating a maintenance request are conventional service calls even when an agent decides when to use them.
APIs are mature, widely supported and straightforward to place behind authentication, validation and transaction controls. An agent framework can expose an API operation as a tool without introducing another network protocol.
Use MCP for discoverable tools and resources
MCP is useful when compatible agent clients need a standard way to discover tools, resources or prompts. A provider can publish one MCP server rather than build a custom connector for every client. A business can place several internal capabilities behind a governed MCP gateway.
MCP changes the connection and discovery layer. It does not remove the need for narrow tool contracts, scoped identity and business validation. The MCP business guide explains server families, gateways and official examples in more detail.
Use A2A for independent agents
Agent2Agent, or A2A, is designed for communication between independent agent systems. An agent can advertise its capabilities, receive a task, report status and return an artefact without exposing its internal tools or memory. An interoperable agent-to-agent protocol is useful when different teams or organisations operate the agents and need an interoperable task relationship.
A2A is not a reason to split one workflow into several agents. Inside one controlled application, direct function calls or framework orchestration are usually simpler.
The protocols can coexist
A co-ordinating agent might receive a task from another organisation through A2A, retrieve information from MCP servers and use an ordinary API for a material transaction. The architecture remains understandable when each interface has one clear role.
Problems arise when the same action is exposed through several routes with different permissions or when an agent can bypass the controlled transaction service by selecting a broader tool.
A logistics example
A distributor's exception agent uses ordinary APIs to read inventory and create a reservation because those are known internal contracts. It uses MCP to discover approved document search and work-management tools shared across several assistants. The distributor considers A2A only when an independently operated carrier agent can accept a disruption task, provide status and return a confirmed recovery option.
If the carrier exposes a stable API instead, A2A may add no value. The choice depends on the operating relationship, not on a preference for the newest protocol.
Compare security at the trust boundary
For every interface, document the calling identity, delegated user, permission scope, data disclosed, action limit, trace, timeout and revocation route. Validate inputs and returned artefacts. Treat descriptions supplied by an external server or agent as untrusted content that cannot override the local policy.
Where a tool can change money, access, inventory or customer state, enforce the rule in the receiving service. Protocol compliance is not transaction assurance.
Plan for version and failure
Record the protocol version, tool or capability version and compatibility policy. Decide how the workflow behaves when discovery changes, an external agent becomes unavailable or a long-running task outlives its credentials. Use stable identifiers so a retry can locate the original task instead of creating another one.
What Marketways delivers
Marketways models the participating systems and owners, selects the least complex interface that supports the relationship, specifies permissions and failure handling, and tests complete task trajectories. The output is an integration decision record and control map that engineering, process and assurance owners can review together.