Backend systems solved this problem long ago with runtime configuration. You build once and configure the application when it starts.

Static webapps never got a clean equivalent.

Most solutions today are ad-hoc: - env.sh scripts that rewrite bundles - CI pipelines rebuilding per environment - templating HTML files - framework-specific hacks

These approaches are fragile and tied to specific build tools.

I’ve been working on a small protocol called REP (Runtime Environment Protocol) that tries to solve this in a portable way.

The idea is simple: 1. Infrastructure provides configuration as environment variables. 2. A gateway injects them into the HTML response as a JSON payload. 3. The frontend reads them at runtime through a small SDK. 4. The payload is integrity-verified to prevent tampering.

example injected payload: <script id="__rep__" type="application/json"> { "public": { "API_URL": "https://api.example.com" }, "_meta": { "integrity": "sha256-..." } } </script>

The frontend can then read configuration at runtime instead of at build time.

This enables: • build once, deploy anywhere • environment configuration via infrastructure • compatibility with static hosting and CDNs • no rebuilds when configuration changes

The spec focuses on three things: • a standard payload format • secure integrity verification • predictable injection semantics

I’m sharing it mostly to get feedback from people who deal with deploying frontend apps at scale.

Spec: https://github.com/RuachTech/rep/blob/main/spec/REP-RFC-0001.md

URL: https://rep-protocol.dev

Curious if others have run into the same pain or solved this differently.