windmill-labs/windmill · error

Cannot preview step of type '${moduleValue?.type ?? "unknown

Error message

Cannot preview step of type '${moduleValue?.type ?? "unknown"}'. Supported types: rawscript, script, flow.

What it means

Step preview only supports module types rawscript, script, and flow; when the resolved module has another type (e.g. approval, wait, loop, brancheval, or an unknown type due to a malformed module), the command refuses to run it as a job and throws.

Source

Thrown at cli/src/commands/flow/flow.ts:846

        // Anchor to the script's own deployed path so its relative imports
        // resolve against the workspace tree (or temp_script_refs).
        path: moduleValue.path,
        flow_path: flowWmPath,
        args,
        tag,
        temp_script_refs: tempScriptRefs,
      },
    });
  } else if (moduleValue?.type === "flow") {
    log.info(colors.yellow(`Previewing step '${stepId}' (flow ${moduleValue.path})...`));
    jobId = await wmill.runFlowByPath({
      workspace: workspace.workspaceId,
      path: moduleValue.path,
      tag,
      requestBody: args,
    });
  } else {
    throw new Error(
      `Cannot preview step of type '${moduleValue?.type ?? "unknown"}'. Supported types: rawscript, script, flow.`
    );
  }

  const { result, success } = await pollForJobResult(workspace.workspaceId, jobId);

  if (!success) {
    if (silent) {
      console.log(JSON.stringify(result));
    } else {
      log.info(colors.red.bold(`Step '${stepId}' failed:`));
      log.info(JSON.stringify(result, null, 2));
    }
    process.exitCode = 1;
    return;
  }

  if (silent) {

View on GitHub (pinned to e474e8803c)

Solutions

1. Preview an executable step (rawscript/script/flow) instead of a control-flow module
2. If the type is 'unknown', fix the module entry in flow.yaml so it has a valid `type` field
3. Test control-flow behavior by previewing the whole flow rather than the step

Example fix

// before
wmill flow preview .flow/__flow --step myForLoop
// after
wmill flow preview .flow/__flow --step innerScript  # a rawscript/script/flow module

const supported = ['rawscript', 'script', 'flow'];
const mod = findStepInFlowValue(flow.value, stepId);
if (!mod || !supported.includes(mod.value?.type)) throw new Error(`step ${stepId} is not previewable`);

function isPreviewableModule(m: any): m is { value: { type: 'rawscript' | 'script' | 'flow' } } {
  return ['rawscript', 'script', 'flow'].includes(m?.value?.type);
}

When it happens

Trigger: `wmill flow preview --step <id>` where the step is a control-flow module (forloop, branch, approval, sleep, etc.) or the module object is malformed so its type is undefined (printed as 'unknown').

Common situations: Trying to preview a forloop/branchone step; a hand-edited flow.yaml missing the `type` field so the module reads as unknown; attempting to preview trigger/router modules.

Related errors

• App ${appPath} not found
• Dependency generation failed: ${queueResponse.status} ${queu
• Failed to poll dependencies job ${jobId}: ${e?.message ?? e}
• No response body for SSE stream
• Datatable '${dtName}' already exists in this workspace

AI-assisted analysis of windmill-labs/windmill@e474e8803c (2026-09-03). Data as JSON: /api/errors/097994ebfc2fc763. Report an issue: GitHub.