gchq/CyberChef · error · OperationError

This operation only works in a browser

Error message

This operation only works in a browser

What it means

Thrown by OpticalCharacterRecognition.run when isWorkerEnvironment() is false. OCR uses tesseract.js which spins up a Web Worker and references browser-only globals (self.docURL, self.sendStatusMessage), so it cannot run in the Node.js API / Node package build of CyberChef.

Source

Thrown at src/core/operations/OpticalCharacterRecognition.mjs:59

            },
            {
                name: "OCR Engine Mode",
                type: "option",
                value: OEM_MODES,
                defaultIndex: 1
            }
        ];
    }

    /**
     * @param {ArrayBuffer} input
     * @param {Object[]} args
     * @returns {string}
     */
    async run(input, args) {
        const [showConfidence, oemChoice] = args;

        if (!isWorkerEnvironment()) throw new OperationError("This operation only works in a browser");

        const type = isImage(input);
        if (!type) {
            throw new OperationError("Unsupported file type (supported: jpg,png,pbm,bmp) or no file provided");
        }

        const assetDir = `${self.docURL}/assets/`;
        const oem = OEM_MODES.indexOf(oemChoice);

        try {
            self.sendStatusMessage("Spinning up Tesseract worker...");
            const image = `data:${type};base64,${toBase64(input)}`;
            const worker = await createWorker("eng", oem, {
                workerPath: `${assetDir}tesseract/worker.min.js`,
                langPath: `${assetDir}tesseract/lang-data`,
                corePath: `${assetDir}tesseract/tesseract-core.wasm.js`,
                logger: progress => {
                    if (isWorkerEnvironment()) {

View on GitHub (pinned to 4290ea7539)

Solutions

1. Run this operation in the browser UI or a worker context only.
2. For server-side OCR use tesseract.js directly (or another OCR library) outside CyberChef.
3. Remove the OCR step from Node-side recipes and perform OCR in a browser step.

import { isWorkerEnvironment } from "core/Utils.mjs";
if (!isWorkerEnvironment()) {
  // do not include OCR in Node-side recipes; run it in the browser
}

const canRunOCR = () => typeof self !== "undefined" && isWorkerEnvironment();

When it happens

Trigger: run(input, args) invoked outside a Web Worker — e.g. via the Node REPL (npm run repl), the CyberChef Node library, or a server-side recipe. The check fires before any image validation.

Common situations: Using CyberChef as an npm dependency in a Node service; running recipes via the Node build that include OCR; scripting batch processing server-side; CI pipeline executing recipes headlessly.

Related errors

• Unsupported file type (supported: jpg,png,pbm,bmp) or no fil
• Error performing OCR on image. (${err})
• Sorry, the ${name} operation is not available in the Node.js

AI-assisted analysis of gchq/CyberChef@4290ea7539 (2026-08-13). Data as JSON: /api/errors/502aa27ca0ddc4b1. Report an issue: GitHub.