V8 15.6: new Function("") now allowed during requests, which breaks code-generation probes (zod v4) in deployed Workers #7740

Summary

Since the V8 15.6.75 update (3f8a3dd), new Function('') is allowed inside a request. Real code generation, like new Function('return 1'), is still blocked. Libraries probe whether code generation works by trying new Function(''). That probe now passes, so they switch to code-generating paths, and those throw EvalError: Code generation from strings disallowed for this context on the first real use.

This broke Workers that were deployed weeks or months ago, with no redeploy. It started around 2026-10-13 20:00 UTC, which we assume is when 15.6 rolled out to production.

Most visible case: zod v4

zod v4 (and the zod/v4 build in zod 3.25.x) decides once whether to JIT-compile object parsers:

// zod 3.25.27 (dist/esm/v4/core/util.js)
export const allowsEval = cached(() => {
  try {
    const F = Function;
    new F("");
    return true;
  } catch (_) {
    return false;
  }
});

Before 15.6, the probe threw, so zod used its interpreter and everything worked. On 15.6 the probe passes, zod compiles parsers with new Function(code), and every parse fails with EvalError.

Later zod versions (3.25.76 and 4.x) skip the probe when navigator.userAgent includes Cloudflare. navigator only exists from compat date 2022-03-21, though, so Workers with an older compat date or none (2021-11-01) are still affected on current zod. Workers bundling zod 3.25.27 to 3.25.75 are affected on any compat date.

Repro

export default {
  fetch() {
    let probe;
    try { new Function(""); probe = "allowed"; } catch { probe = "blocked"; }
    let real;
    try { new Function("return 1")(); real = "allowed"; } catch (e) { real = String(e); }
    return new Response(JSON.stringify({ probe, real }));
  },
};

Expected from the setup-test change in 3f8a3dd:

• V8 15.5: {"probe":"blocked","real":"EvalError: ..."}
• V8 15.6: {"probe":"allowed","real":"EvalError: ..."}

A minimal zod case: a schema built at module scope with zod 3.25.27, import { z } from "zod/v4", then schema.parse({...}) inside fetch.

Impact

• Deployed Workers that use zod v4 started returning errors with no change on the customer's side. For us, that was MCP servers returning 400 "Invalid JSON-RPC message", plus some auth policies.
• Others hit the same thing the same day: fix(worker): disable zod v4 JIT so /verify-siwf stops throwing EvalError farcasterxyz/quick-auth#35.
• The 15.6 commit message says the update "needs no compatibility flag". This behavior change is observable, though, and it breaks already-deployed code.

Ask

One of:

1. Keep new Function('') blocked wherever real code generation is blocked, so feature probes keep giving the right answer; or
2. Put the new allowance behind a compatibility date/flag, so existing Workers keep the 15.5 behavior.

We've worked around it on our side by forcing zod's jitless mode on workerd. That only reaches Workers once they're rebuilt.

Related: #6344 (empty-function allowance), #7676.