[@tanstack/react-start/plugin/rsbuild] tanstackStart() eclipses the client environment's output.assetPrefix, blocking downstream plugins

Which project does this relate to?

Start

Describe the bug

tanstackStart() unconditionally writes assetPrefix to the client rsbuild environment in modifyRsbuildConfig. Because the environment override wins over the root value in rsbuild's merge, Start's write hides whatever the user (or a downstream plugin) had

Complete minimal reproducer

https://github.com/alexandre-marchina/tanstack-start-rsbuild-prefix-repro

Steps to Reproduce the Bug

In the repro above: npm run repro runs three configs (no user config with tanstackStart(), explicit output.assetPrefix with tanstackStart(), and a third without tanstackStart()) and prints the observed env override / effective / root values for each

Expected behavior

• No output.assetPrefix and no server.base - Start writes nothing to the client environment
• output.assetPrefix set - forwarded to the client environment
• server.base set - takes precedence

Screenshots or Videos

No response

Platform

• @tanstack/start-plugin-core: 1.168.x (latest)
• @rsbuild/core: 2.1.5
• @tanstack/react-start: 1.168.x
• Environment: macOS, Node.js >= 20

Start at tanstackStart() and its modifyRsbuildConfig handling for the client environment. Run npm run repro in the linked reproducer to compare the env override, effective, and root values across the three configurations. Done means assetPrefix is omitted when unset, forwarded when set, and overridden by server.base.