I think the issue is with how pnpm resolves dependencies in a monorepo with isolated installs.

My use case:

Root node_modules/.pnpm:

• contains rescript
• contains @rescript/runtime

some/app package.json:

"devDependencies": {
  "rescript": "^12.0.0"
},

some/app node_modules:

• contains rescript
• does not contain @rescript/runtime
• Vite import fails because the compiled source file contains
  import * as Stdlib_Dict from "@rescript/runtime/lib/es6/Stdlib_Dict.js"

With pnpm’s isolated installs, a package can only import modules that appear in its own dependencies (or devDependencies), so from pnpm’s point of view some/app directly depends on @rescript/runtime and must declare it.

fix

some/app package.json:

"dependencies": {
  "@rescript/runtime": "^12.0.0"
},
"devDependencies": {
  "rescript": "^12.0.0"
},

not a fix

Adding rescript to dependencies does not solve the issue; you get the same error. Even if rescript itself depends on @rescript/runtime, pnpm will not expose @rescript/runtime as a direct dependency of some/app, so Node/Vite still cannot resolve it from some/app’s node_modules.

Error [ERR_MODULE_NOT_FOUND]: Cannot find package '@rescript/runtime'