Bun.serve HTML routes rewrite import.meta.env.X to hmr.importMeta.env.X, but the runtime never defines env — TypeError in the browser #48817

What version of Bun is running?

1.4.0 (macOS arm64). Also reproduced on 1.4.4.

What platform is your computer?

macOS arm64

What steps can reproduce the bug?

index.html:

<!doctype html><html><body><script type="module" src="./main.ts"></script></body></html>

main.ts:

console.log("MY_VAR", (import.meta as any).env.MY_VAR);

serve.ts:

import index from "./index.html";
const s = Bun.serve({ port: 0, routes: { "/": index }, development: true });
const html = await (await fetch(s.url)).text();
const src = html.match(/src="([^"]+\.js)"/)?.[1]!;
const js = await (await fetch(new URL(src, s.url))).text();
console.log(js.split("\n").filter((l) => l.includes("MY_VAR")).join("\n"));
s.stop(true);

bun serve.ts

What is the expected behavior?

import.meta.env.X in browser code served through Bun.serve HTML routes resolves to a defined object — e.g. populated from BUN_PUBLIC_* variables / [serve.static] env config — or is statically replaced like DEV/PROD/MODE already are. The page must not crash.

What do you see instead?

The dev bundle rewrites the read to hmr.importMeta.env:

console.log("MY_VAR", hmr.importMeta.env.MY_VAR);

but the client runtime's importMeta getter only defines url, main, require and hot — there is no env property:

get importMeta(){let Z={url:`${location.origin}/${this.id}`,main:!1,require:this.require.bind(this),get hot(){throw Error("import.meta.hot cannot be used indirectly.")}}; ...

So any import.meta.env.X read in browser code throws at runtime:

TypeError: Cannot read properties of undefined (reading 'MY_VAR')

which crashes the whole module on page load.

Notes:

• Reproduced on 1.4.0 and 1.4.4, with and without --hot.
• Adding env = "BUN_PUBLIC_*" under [serve.static] in bunfig.toml and exporting BUN_PUBLIC_* variables does not populate importMeta.env either.
• DEV, PROD and MODE are statically replaced (true/false/"development" in dev); every other key is rewritten to hmr.importMeta.env.* and crashes.
• With development: false the read is left verbatim as import.meta.env.MY_VAR, which is equally undefined in a real browser.
• Related: Bun.build / bun build leave import.meta.env.DEV, PROD and MODE in a browser bundle, while Bun.serve HTML routes replace them #47170 reports the same rewrite asymmetry for DEV/PROD/MODE between Bun.serve HTML routes and Bun.build. This issue is specifically about the runtime env object never being defined, which turns the rewrite into a guaranteed TypeError for any non-special-cased key.

Workaround we shipped downstream

Guard the read (typeof import.meta.env === "undefined", which keeps member expressions intact for Vite define replacement) and fetch env values from a JSON route the dev server provides.