microsoft/TypeScript · error · Error

LKG cannot be created when --bundle=false

Error message

LKG cannot be created when --bundle=false

What it means

The `LKG` (Last Known Good) hereby task copies freshly built JS into lib/ so the compiler can self-host. It is gated on the `--bundle` CLI option because the LKG must contain the bundled entrypoints (_tsc.js, _tsserver.js, etc.); without bundling the LKG would be incomplete and break the self-host build.

Source

Thrown at Herebyfile.mjs:903

//                 path here seems like a bad idea.
export const updateSublime = task({
    name: "update-sublime",
    description: "Updates the sublime plugin's tsserver",
    dependencies: [tsserver],
    run: async () => {
        for (const file of ["built/local/tsserver.js", "built/local/tsserver.js.map"]) {
            await fs.promises.copyFile(file, path.resolve("../TypeScript-Sublime-Plugin/tsserver/", path.basename(file)));
        }
    },
});

export const produceLKG = task({
    name: "LKG",
    description: "Makes a new LKG out of the built js files",
    dependencies: [local],
    run: async () => {
        if (!cmdLineOptions.bundle) {
            throw new Error("LKG cannot be created when --bundle=false");
        }

        const expectedFiles = [
            "built/local/tsc.js",
            "built/local/_tsc.js",
            "built/local/tsserver.js",
            "built/local/_tsserver.js",
            "built/local/tsserverlibrary.js",
            "built/local/tsserverlibrary.d.ts",
            "built/local/typescript.js",
            "built/local/typescript.d.ts",
            "built/local/typingsInstaller.js",
            "built/local/_typingsInstaller.js",
            "built/local/watchGuard.js",
        ].concat(libs().map(lib => lib.target));
        const missingFiles = expectedFiles
            .concat(localizationTargets)
            .filter(f => !fs.existsSync(f));

View on GitHub (pinned to b465fdbfe1)

Solutions

1. Run `hereby LKG --bundle` (pass --bundle explicitly).
2. Ensure `hereby local` completed successfully first, then re-run with --bundle.
3. If you intentionally never bundle, do not run the LKG task — it is not meaningful without bundling.

Example fix

// before
$ hereby LKG

// after
$ hereby LKG --bundle

// Before running LKG, confirm the bundle flag is set.
if (!cmdLineOptions.bundle) {
  throw new Error("Refusing to run produceLKG without --bundle; pass --bundle on the hereby CLI.");
}

When it happens

Trigger: Running `hereby LKG` (or the produce-LKG task) without passing `--bundle` on the hereby command line, so `cmdLineOptions.bundle` is false.

Common situations: Skipping the bundle flag for a faster local cycle; CI that invokes the LKG task directly without forwarding --bundle.

Related errors

• Cannot replace the LKG unless all built targets are present
• Expected ${inputDir} to be a directory
• Expected ${inputFile} to be a file
• tsserverlibrary requires CommonJS; use typescript.js instead
• Diagnostic code ${code} appears more than once.

AI-assisted analysis of microsoft/TypeScript@b465fdbfe1 (2026-08-12). Data as JSON: /api/errors/bedf420675e346f8. Report an issue: GitHub.