At the moment, Librarian uses pipeline-state.json and pipeline-config.json to record the state of the pipeline and repo-specific configuration. To implement this, we currently have:

• protos are defined in https://github.com/googleapis/librarian/tree/main/proto
• a custom Bash script https://github.com/googleapis/librarian/blob/main/generate-proto.sh is used to generate code
• generated Go code lives under https://github.com/googleapis/librarian/tree/main/internal/statepb, which brings in an additional proto dependency,

  librarian/go.mod

  Line 13 in ecd0ebc

  google.golang.org/protobuf v1.36.6

By comparison, in the https://github.com/googleapis/google-cloud-rust, the sidekick CLI tool uses TOML to define its specifications. This approach:

• Uses a single struct: https://github.com/googleapis/google-cloud-rust/blob/a82945194a96ba1ef320db8efa04ae9029f5e4c3/generator/internal/config/config.go#L40
• Supports serialization/deserialization via one function: https://github.com/googleapis/google-cloud-rust/blob/a82945194a96ba1ef320db8efa04ae9029f5e4c3/generator/internal/config/config.go#L248
• Requires no additional external dependencies

As part of our goal to unify our infrastructure, we should consider standardizing on one specification (proposing TOML because it is already being used in google-cloud-rust, but could be something else if that makes more sense and @coryan agrees), unless there is a strong reason to diverge and keep the existing protobuf-based approach.

This would allow us to:

• simplify the librarian codebase
• avoid the need to install protoc for development
• aligns with existing patterns already used in sidekick

/cc @jskeet @ldetmer @codyoss