[Bug]: Persistent cache fully invalidated when switching between `build` and `serve` with the same config

[Armine-i]

System Info

System:
OS: macOS (Darwin 25.2.0)
CPU: ARM64 (Apple Silicon)

Binaries:
Node: v24.12.0
npm: 11.10.0

NPM packages:
@rspack/cli: ^1.7.6
@rspack/core: ^1.7.6

Details

When using experiments.cache with type: 'persistent', switching between rspack build and rspack serve using the same config file causes the entire persistent cache to be invalidated and rewritten. No module-level build artifacts are reused, even though the config file, loaders, and source files are identical.

Partial cache validation is supported in Webpack 5's filesystem cache and we currently benefit a great deal from it. This is a blocker for a workflow we rely on in our large codebase: CI runs rspack build to produce a warm persistent cache, and that cache is shipped to other environments such as remote developer environments so developers benefit from it on their first rspack serve cold start.

Reproduce link

https://github.com/Armine-i/rspack-repro/tree/mina-repro

Reproduce Steps

1. Run rspack build -c rspack/rspack.config.ts: cache is created under a single hash folder
2. Run rspack serve -c rspack/rspack.config.ts: the same hash folder's contents are fully invalidated and rewritten
3. Run rspack build -c rspack/rspack.config.ts again: fully invalidated and rewritten again

Each switch between build and serve produces a cache miss despite no source files or config changing.

[jerrykingxyz]

We can answer these questions in two parts:

1. Why is there only one hash folder?

rspack generates a hash folder based on config.name, config.mode, and persistent caching options. In your case, both build and development modes are set to "development," so rspack will use the same cache.

2. Why is the cache invalid?

In development mode, rspack defaults to asynchronous writing, while in production mode, it defaults to synchronous writing. Therefore, during a build with development mode, the program may exit before the cache is fully written, causing subsequent startups to be unable to use the cache.

In summary, using the build command in development mode causes this problem. I would consider whether it's possible to ensure the cache is fully written when the program stops, similar to webpack, to avoid this issue. Returning to your project, I don't understand why you're using the build command in development mode. Could you provide more context?