wrangler >=4.127.0 added `workflows[].concurrency.limit`. The limit is the maximum number of Workflow instances that can run concurrently. Older versions don't have it.

const { spawnSync } = require("child_process");
const fs = require("fs");
const path = require("path");
const dir = "/w/proj";
fs.rmSync(dir, { recursive: true, force: true });
fs.mkdirSync(dir, { recursive: true });
const home = fs.mkdtempSync("/tmp/home-");
function wrangler(args, extraEnv, dropEnv) {
  const env = Object.assign({}, process.env, { CI: "1", WRANGLER_SEND_METRICS: "false", NO_COLOR: "1", FORCE_COLOR: "0", HOME: home }, extraEnv || {});
  for (const k of dropEnv || []) delete env[k];
  const r = spawnSync(process.execPath, ["--no-warnings", "/w/node_modules/wrangler/wrangler-dist/cli.js", ...args], { cwd: dir, encoding: "utf8", timeout: 40000, env });
  const out = (String(r.stdout || "") + String(r.stderr || "")).replace(/\x1b\[[0-9;]*m/g, "");
  return { status: r.status, out };
}
function symptom(msg) { console.error("FAIL: " + msg); process.exit(1); }
function broken(msg, out) {
  console.error("ERROR: " + msg);
  if (out) console.error(out.slice(-2000));
  process.exit(2);
}
function write(name, content) {
  fs.writeFileSync(path.join(dir, name), typeof content === "string" ? content : JSON.stringify(content, null, 2));
}
write("index.js", 'export class MyWorkflow {}\nexport default { fetch() { return new Response("ok"); } };\n');
const cfg = (limit) => ({ name: "wf-concurrency", main: "index.js", compatibility_date: "2026-01-01", workflows: [{ binding: "WF", name: "my-workflow", class_name: "MyWorkflow", concurrency: { limit } }] });
write("wrangler.json", cfg(10));
let r = wrangler(["deploy", "--dry-run", "--outdir", "out"]);
if (/Unexpected fields found in workflows\[0\] field: "concurrency"/.test(r.out)) symptom("workflows[].concurrency is an unexpected field");
if (r.status !== 0 || !r.out.includes("--dry-run: exiting now.")) broken("dry run with concurrency.limit 10 failed", r.out);
write("wrangler.json", cfg(0));
r = wrangler(["deploy", "--dry-run", "--outdir", "out2"]);
if (r.status === 0 || !r.out.includes('"concurrency.limit" field must be a positive integer')) broken("concurrency.limit 0 was not rejected", r.out);
console.log("OK: workflows[].concurrency.limit is validated");

const { spawnSync } = require("child_process");
const fs = require("fs");
const path = require("path");
const dir = "/w/proj";
fs.rmSync(dir, { recursive: true, force: true });
fs.mkdirSync(dir, { recursive: true });
const home = fs.mkdtempSync("/tmp/home-");
function wrangler(args, extraEnv, dropEnv) {
  const env = Object.assign({}, process.env, { CI: "1", WRANGLER_SEND_METRICS: "false", NO_COLOR: "1", FORCE_COLOR: "0", HOME: home }, extraEnv || {});
  for (const k of dropEnv || []) delete env[k];
  const r = spawnSync(process.execPath, ["--no-warnings", "/w/node_modules/wrangler/wrangler-dist/cli.js", ...args], { cwd: dir, encoding: "utf8", timeout: 40000, env });
  const out = (String(r.stdout || "") + String(r.stderr || "")).replace(/\x1b\[[0-9;]*m/g, "");
  return { status: r.status, out };
}
function symptom(msg) { console.error("FAIL: " + msg); process.exit(1); }
function broken(msg, out) {
  console.error("ERROR: " + msg);
  if (out) console.error(out.slice(-2000));
  process.exit(2);
}
function write(name, content) {
  fs.writeFileSync(path.join(dir, name), typeof content === "string" ? content : JSON.stringify(content, null, 2));
}
write("index.js", 'export class MyWorkflow {}\nexport default { fetch() { return new Response("ok"); } };\n');
const cfg = (limit) => ({ name: "wf-concurrency", main: "index.js", compatibility_date: "2026-01-01", workflows: [{ binding: "WF", name: "my-workflow", class_name: "MyWorkflow", concurrency: { limit } }] });
write("wrangler.json", cfg(10));
let r = wrangler(["deploy", "--dry-run", "--outdir", "out"]);
if (/Unexpected fields found in workflows\[0\] field: "concurrency"/.test(r.out)) symptom("workflows[].concurrency is an unexpected field");
if (r.status !== 0 || !r.out.includes("--dry-run: exiting now.")) broken("dry run with concurrency.limit 10 failed", r.out);
write("wrangler.json", cfg(0));
r = wrangler(["deploy", "--dry-run", "--outdir", "out2"]);
if (r.status === 0 || !r.out.includes('"concurrency.limit" field must be a positive integer')) broken("concurrency.limit 0 was not rejected", r.out);
console.log("OK: workflows[].concurrency.limit is validated");