wrangler >=4.126.0 changed what `.cloudflare/output/v0/config.json` does. The mode a build was produced in is now written to .cloudflare/output/v0/config.json as a mode field, alongside the account and compliance settings. Silent: code written for older versions still runs, without an error.

const fs = require("fs");
const path = require("path");
const { spawnSync } = require("child_process");
function symptom(msg) { console.error("FAIL: " + msg); process.exit(1); }
function broken(msg) { console.error("ERROR: " + msg); process.exit(2); }
const dir = fs.mkdtempSync("/tmp/proj-");
fs.mkdirSync(path.join(dir, "src"));
fs.writeFileSync(path.join(dir, "src/index.js"), "export default { async fetch() { return new Response('hello'); } };\n");
fs.writeFileSync(path.join(dir, "cloudflare.config.ts"), "export default { type: 'worker', name: 'mode-test', compatibilityDate: '2026-05-18', entrypoint: './src/index.js' };\nexport const settings = { type: 'settings', accountId: 'acc-123', complianceRegion: 'public' };\n");
const r = spawnSync(process.execPath, ["/w/node_modules/wrangler/bin/wrangler.js", "build", "--experimental-new-config", "--experimental-cf-build-output", "--env", "staging"], {
  cwd: dir,
  encoding: "utf8",
  timeout: 80000,
  env: { ...process.env, CI: "1", WRANGLER_SEND_METRICS: "false", CLOUDFLARE_ENV: "" }
});
if (r.error) broken("wrangler build did not run: " + r.error.message);
if (r.status !== 0) broken("wrangler build exited " + r.status + ": " + (r.stderr || r.stdout).slice(-800));
const cfgPath = path.join(dir, ".cloudflare/output/v0/config.json");
let got;
try { got = JSON.parse(fs.readFileSync(cfgPath, "utf8")); } catch (e) { broken("top-level config.json not readable: " + e.message); }
if (got.accountId !== "acc-123") broken("settings not written: " + JSON.stringify(got));
if (got.mode !== "staging") symptom("build with --env staging wrote no mode field to .cloudflare/output/v0/config.json");
console.log("OK: " + JSON.stringify(got));

const fs = require("fs");
const path = require("path");
const { spawnSync } = require("child_process");
function symptom(msg) { console.error("FAIL: " + msg); process.exit(1); }
function broken(msg) { console.error("ERROR: " + msg); process.exit(2); }
const dir = fs.mkdtempSync("/tmp/proj-");
fs.mkdirSync(path.join(dir, "src"));
fs.writeFileSync(path.join(dir, "src/index.js"), "export default { async fetch() { return new Response('hello'); } };\n");
fs.writeFileSync(path.join(dir, "cloudflare.config.ts"), "export default { type: 'worker', name: 'mode-test', compatibilityDate: '2026-05-18', entrypoint: './src/index.js' };\nexport const settings = { type: 'settings', accountId: 'acc-123', complianceRegion: 'public' };\n");
const r = spawnSync(process.execPath, ["/w/node_modules/wrangler/bin/wrangler.js", "build", "--experimental-new-config", "--experimental-cf-build-output", "--env", "staging"], {
  cwd: dir,
  encoding: "utf8",
  timeout: 80000,
  env: { ...process.env, CI: "1", WRANGLER_SEND_METRICS: "false", CLOUDFLARE_ENV: "" }
});
if (r.error) broken("wrangler build did not run: " + r.error.message);
if (r.status !== 0) broken("wrangler build exited " + r.status + ": " + (r.stderr || r.stdout).slice(-800));
const cfgPath = path.join(dir, ".cloudflare/output/v0/config.json");
let got;
try { got = JSON.parse(fs.readFileSync(cfgPath, "utf8")); } catch (e) { broken("top-level config.json not readable: " + e.message); }
if (got.accountId !== "acc-123") broken("settings not written: " + JSON.stringify(got));
if (got.mode !== "staging") symptom("build with --env staging wrote no mode field to .cloudflare/output/v0/config.json");
console.log("OK: " + JSON.stringify(got));