Buildpack detection failure during git push deployment — Scalingo build system

A repeatable development setup on Scalingo is typically built by creating separate apps per branch or feature and deploying via git push, since native ephemeral preview apps are not offered.

The approach relies on the Scalingo build system to detect a language and select a buildpack automatically. When detection cannot infer a language from the repository, deployment is rejected and an explicit buildpack or Dockerfile must be set. The documented failure mode leaves an unresolved decision about default fallback behavior, and the lack of native preview apps leaves the synchronization of configuration and add-ons across multiple apps as an open design choice.

What repository conditions cause the build system to treat detection as ambiguous and reject a deployment? Is there a documented default fallback when no buildpack is set, or is rejection the only outcome? How should buildpack settings be kept consistent across multiple apps used as repeatable development environments?