The local mesh serves search and files too
The apps that run against the local mesh — the desktop apps and the packaged web app — ask it for searches, for a folder's contents and for uploads over the same web endpoints a hosted portal serves. The local mesh only published one of those. The rest quietly fell through to the app's own start page, which answered successfully with a web page, so the app reported a parsing error rather than a missing feature.
The local mesh now publishes the same set as a portal does, and a check on both sides fails the build if one of them ever stops serving something the apps call.