When injects is empty, this returns "export {};" regardless of whether the user has an instrumentation-client file. The PR description says this is intentional ("the alias already handles the user file or empty-module"), which is correct — the resolve.alias at index.ts:1261 maps private-next-instrumentation-client to the user file or empty-module and takes effect when this plugin returns null from resolveId.

However, this means there's an asymmetry in the hook composition path: when injects are present, the composed onRouterTransitionStart fans out to all modules including the user file. When injects are empty, the user's onRouterTransitionStart flows directly through the alias. Both paths work, but it's worth documenting this explicitly in the function's JSDoc (the current doc hints at it but doesn't state the two resolution paths clearly).

The module string is only computed when instrumentationClientInject.length > 0, but clientInjectModule is never reset to null on subsequent config re-evaluations. In Vite's multi-environment build pipeline, config() can fire multiple times. If nextConfig is already set (the if (!nextConfig) guard at line 1042), this code path is still reached because instrumentationPath/instrumentationClientPath/middlewarePath are intentionally re-evaluated outside that guard.

This is fine today because nextConfig is only resolved once (the guard), so instrumentationClientInject won't change across calls. But if the guard is ever removed (e.g., to support config HMR), clientInjectModule would need an explicit reset:

This makes the assignment unconditional and self-clearing, which is safer against future refactors.

Minor: this validates that instrumentationClientInject is an array, but doesn't validate that each element is a string. Next.js uses Zod (z.array(z.string())) in config-schema.ts to reject non-string elements. Consider filtering:

This is consistent with how optimizePackageImports is handled a few lines above (line 1048).

Nit: the generated function signature has no type annotations. This is fine since it's a virtual module consumed by Vite (not user-facing code), and Vite will treat it as JS regardless. But for consistency with ClientInstrumentationHooks, you could annotate the parameters in the generated code:

export function onRouterTransitionStart(url: string, type: string) {

Not blocking — just a polish suggestion.

Good test coverage for the pure code-generation function. One edge case worth adding: what happens when an inject specifier contains characters that could break the generated import (e.g., a specifier with quotes or backslashes)? JSON.stringify handles this correctly, but a test would document the contract:

it("escapes special characters in specifier paths", () => {
  const code = generateInstrumentationClientInjectModule(
    ['./path/with"quote.js'],
    null,
  );
  expect(code).toContain('from "./path/with\\"quote.js"');
});

Also, there's no integration-level test that verifies the Vite plugin wiring end-to-end (i.e., that the virtual module is actually served when instrumentationClientInject is configured). The unit tests cover generateInstrumentationClientInjectModule in isolation, which is great, but a fixture-based test would catch issues like the resolve.alias vs resolveId ordering concern. This could be a follow-up.

Next.js resolves inject specifiers against the project root before emitting require() calls (via this.resolve(rootContext, spec) in the webpack loader). Here, bare specifiers and relative paths are left for Vite's resolver to handle at import resolution time, which should work correctly since the virtual module's importer context is the project root. Just flagging the difference — if a user reports that relative paths in instrumentationClientInject resolve against the wrong directory, this is the place to look.

Nit from the previous review that's still applicable — the generated function has TypeScript type annotations (url: string, type: string) but this is loaded as a virtual module via the load hook, which Vite treats as JavaScript by default (no id.endsWith('.ts') to trigger the TS transform). This works today because Vite 8 / Rolldown processes all modules through OXC regardless, but if you wanted to be explicit you could either:

1. Drop the TS annotations from the generated code (make it plain JS), or
2. Return { code, map: null } with an explicit lang: 'ts' or append .ts to the resolved ID

Not blocking — the current behavior is correct under Vite 8.