A practical comparison of MCP and direct tool APIs: integration reuse, permissions, testing, performance and maintenance for business agents.
Choose the interface per integration rather than adopting a universal rule. MCP can help expose a consistent tool or context interface across compatible applications. A direct API may be simpler for a tightly scoped internal workflow. Either approach still needs business rules, permissions, test cases and reliable handling of external actions.
What MCP provides
The MCP specification defines a protocol for connecting model applications with external tools and context. It uses JSON-RPC and describes hosts, clients, servers and supported capabilities such as tools, resources and prompts. Interoperability depends on the capabilities and versions implemented at both ends; a published server does not guarantee that every client supports every feature.
Where MCP can be useful
Consider MCP when several compatible applications need a shared integration or when partners need a documented extension interface. A maintained server may reduce repeated adapter work. Assess who publishes it, the tools it exposes, required access, version support and how updates are handled. Connection setup, account selection and permission review remain necessary even when the server already exists.
Where a direct API may be simpler
If one application needs a small, stable business operation, a direct typed API can keep the contract explicit. Complex transactions may belong in a backend service regardless of the agent interface. Do not assume a universal MCP latency penalty or that MCP prevents testing or record-level controls: measure the actual path and enforce authorisation in the service behind either interface.
A decision framework per integration
Ask who owns the systems, who will consume the interface and what operation is required. Check version compatibility, authentication, record permissions, tool schemas, error behaviour and maintenance ownership. Measure representative calls, including failures. Compare implementation and support effort, not just the presence of an existing server or the number of lines in a prototype.
A hypothetical mixed architecture
A research assistant could use a reviewed MCP server to search an approved document system while calling a direct internal API to create a draft record. Both paths use the authenticated user's permitted scope. The application requires approval before publishing or sending anything and records the result. This is an illustrative design, not a report of Afiniti Global deployments or a prescribed percentage split.
Security and testing mistakes to avoid
Do not trust tool descriptions or retrieved content as authority to expand permissions. Apply least privilege, validate arguments and enforce tenant and record access server-side. For remote services, follow the applicable authorisation specification; local and remote setups have different requirements. The MCP security guidance addresses risks including token passthrough and confused-deputy attacks. Authentication alone does not establish that an action is appropriate.
What the interface does not decide
Neither MCP nor a custom API supplies the whole agent architecture. Retrieval quality, approval policy, duplicate-action protection and outcome verification remain application concerns. Both can expose useful tools, and both can be poorly designed. Prefer narrowly defined operations with stable results that the execution layer can validate and test.
A practical next step
List each proposed tool and its read or write effect. Prototype the most important integration using the intended account permissions, then test errors, unavailable dependencies and repeated requests. Record the interface version and the maintainer. Keep a working direct integration unless a specific reuse, compatibility or maintenance benefit justifies changing it.