MCP is an integration protocol for agent hosts and tools. It is not a reason to discard a well-designed API.
The boundary matters. APIs remain the durable contracts between systems: they define ownership, authentication, rate limits, compatibility, observability, and the operational expectations that outlive a particular model or agent framework. MCP makes a useful subset of those capabilities discoverable and callable by an agent.
Treat MCP as an adapter layer
The strongest MCP tools are usually backed by an existing system boundary. An OpenAPI specification, for example, already describes operation names, inputs, and responses. An MCP layer can expose a deliberate subset of that surface while the API remains responsible for the actual business operation.
That is the pattern behind openapi-go-mcp: generate the tool interface from a maintained contract, then keep policy and transport concerns where they belong. Agents gain a reliable tool surface; services do not acquire a second, competing API.