Preview failed: ${response.status} - ${response.statusText}

Error message

Preview failed: ${response.status} - ${response.statusText} - ${await response.text()}

What it means

The preview command POSTs a form to the instance's preview/app endpoint. If the HTTP response is not ok, the CLI throws 'Preview failed' embedding the status code, status text, and response body via `detectAuthGatewayChallenge` having already checked for auth-gateway interception. This surfaces server-side validation or auth failures from the raw HTTP layer.

Source

Thrown at cli/src/commands/script/script.ts:2053

      workspace.remote +
      "api/w/" +
      workspace.workspaceId +
      "/jobs/run/preview_bundle";

    const extraHeaders = getHeaders();
    const response = await fetch(url, {
      method: "POST",
      headers: {
        Authorization: `Bearer ${workspace.token}`,
        ...extraHeaders,
      },
      body: form,
    });

    await detectAuthGatewayChallenge(response, url);

    if (!response.ok) {
      throw new Error(
        `Preview failed: ${response.status} - ${response.statusText} - ${await response.text()}`
      );
    }

    const jobId = await response.text();
    if (!opts.silent) {
      await track_job(workspace.workspaceId, jobId);
    }

    // Wait for the job to complete and get the result
    while (true) {
      try {
        const completedJob = await wmill.getCompletedJob({
          workspace: workspace.workspaceId,
          id: jobId,
        });

        const result = completedJob.result ?? {};

View on GitHub (pinned to e474e8803c)

Solutions

Read the status and body in the message: 401/403 → re-login with `wmill login` or refresh the token.
400 → fix the script error reported in the response body.
Verify the instance URL and workspace with `wmill workspace show`.
If behind a gateway/proxy, check its logs; 502/503 means the instance or upstream is down.

Example fix

// before
wmill script preview f/my_script.py   # 401 from expired token
// after
wmill login
wmill script preview f/my_script.py

Defensive patterns

Strategy: try-catch

Validate before calling

const cfg = JSON.parse(readFileSync(configPath, 'utf8'));
const res = await fetch(cfg.baseUrl + '/api/version');
if (!res.ok) console.error('Instance unreachable or auth issue before preview');

Try / catch

try {
  await wmill.script.preview(file, opts);
} catch (e) {
  const m = String(e.message).match(/Preview failed: (\d+)/);
  if (m) {
    const status = Number(m[1]);
    if (status === 401 || status === 403) console.error('Re-authenticate: wmill login');
    else if (status >= 500) console.error('Server-side issue; check instance/proxy health.');
  }
  throw e;
}

Prevention

Refresh credentials (`wmill login`) before long CI sessions.
Validate script syntax locally (lint/format) before previewing.
Confirm `--base-url`/workspace points at the intended instance, especially behind proxies.

When it happens

Trigger: Previewing a script whose code fails server-side validation (400), hitting an endpoint that requires auth while the token is expired (401/403), the path/endpoint not existing on the target instance (404), or a reverse proxy returning 502/503.

Common situations: Expired or mis-scoped API token; pointing the CLI at the wrong instance URL (`--base-url`/workspace config) behind a proxy; previewing code with syntax errors the server rejects; corporate SSO/gateway challenging the request.

Understand the failure class

Background: 'Something went wrong' / 'Request failed (500)' / 'HTTP error! status: 404' — what failed HTTP requests actually mean and how to find the real cause — this error's family across 28 libraries.

Related errors

AI-assisted analysis of windmill-labs/windmill@e474e8803c (2026-09-03). Data as JSON: /api/errors/efb1aabfd01734ff. Report an issue: GitHub.