naptha/tesseract.js · critical · Error

Failed to load TesseractCore

Error message

Failed to load TesseractCore

What it means

In the browser worker, getCore.js resolves a core JS file (defaulting to the jsdelivr CDN, picking SIMD/relaxed-SIMD/LSTM variants) and calls importScripts() on it. After the import it expects global.TesseractCore to be defined (with a fallback to global.TesseractCoreWASM for core <= 4.0.3). If neither exists, it throws 'Failed to load TesseractCore'.

Source

Thrown at src/worker-script/browser/getCore.js:54

          corePathImportFile = `${corePathImport.replace(/\/$/, '')}/tesseract-core-simd.wasm.js`;
        }
      } else if (lstmOnly) {
        corePathImportFile = `${corePathImport.replace(/\/$/, '')}/tesseract-core-lstm.wasm.js`;
      } else {
        corePathImportFile = `${corePathImport.replace(/\/$/, '')}/tesseract-core.wasm.js`;
      }
    }

    // Create a module named `global.TesseractCore`
    global.importScripts(corePathImportFile);

    // Tesseract.js-core versions through 4.0.3 create a module named `global.TesseractCoreWASM`,
    // so we account for that here to preserve backwards compatibility.
    // This part can be removed when Tesseract.js-core v4.0.3 becomes incompatible for other reasons
    if (typeof global.TesseractCore === 'undefined' && typeof global.TesseractCoreWASM !== 'undefined' && typeof WebAssembly === 'object') {
      global.TesseractCore = global.TesseractCoreWASM;
    } else if (typeof global.TesseractCore === 'undefined') {
      throw Error('Failed to load TesseractCore');
    }
    res.progress({ status: statusText, progress: 1 });
  }
  return global.TesseractCore;
};

View on GitHub (pinned to a1ca80d9e3)

Solutions

1. Self-host the matching tesseract.js-core version and set corePath to that directory or to the specific .wasm.js file.
2. Add the core origin (CDN or self-hosted) to the script-src directive of your Content-Security-Policy.
3. Verify network access to the core URL from the browser (open it directly / check devtools).
4. Ensure corePath ends in a directory (lib loads tesseract-core*.wasm.js) or a full .wasm.js filename — never the raw .wasm binary.

Example fix

// before
const worker = await createWorker('eng', 1, {
  corePath: 'https://cdn.jsdelivr.net/npm/tesseract.js-core', // blocked by CSP
});

// after
const worker = await createWorker('eng', 1, {
  corePath: '/vendor/tesseract-core/', // self-hosted, allowed in CSP script-src
});

// Validate that corePath is reachable and is the .wasm.js loader, not the binary.
async function assertCoreReachable(corePath) {
  const url = corePath.endsWith('.js')
    ? corePath
    : `${corePath.replace(/\/$/, '')}/tesseract-core.wasm.js`;
  const resp = await fetch(url, { method: 'HEAD' });
  if (!resp.ok) throw new Error(`Core not reachable at ${url} (HTTP ${resp.status})`);
  const ct = resp.headers.get('content-type') || '';
  if (!/javascript|text|octet-stream/.test(ct) && ct !== '') {
    throw new Error(`Core URL did not return a JS file (${ct})`);
  }
}
await assertCoreReachable(options.corePath || '/vendor/tesseract-core/');

const isValidCorePath = (p) =>
  typeof p === 'string' &&
  (p.endsWith('.js') || !/\.wasm$/.test(p)) &&
  /^https?:\/\//.test(p) || p.startsWith('/');

try {
  worker = await createWorker('eng', 1, { corePath });
} catch (e) {
  if (/Failed to load TesseractCore/i.test(e.message)) {
    // fall back to the pinned CDN or a self-hosted copy
    worker = await createWorker('eng', 1, { corePath: '/vendor/tesseract-core/' });
  } else { throw e; }
}

When it happens

Trigger: corePath pointing to a missing file or wrong directory; CSP script-src blocking the CDN or self-hosted origin; offline with no local core; importScripts silently failing (network/CSP); pointing corePath at a .wasm binary instead of the .wasm.js loader; core version mismatch with the tesseract.js runtime.

Common situations: Production deployment behind a strict Content-Security-Policy; air-gapped intranet; self-hosting the core but with a path typo; CDN blocked in the user's region; pinning an incompatible tesseract.js-core version.

Related errors

• Network error while fetching ${fetchUrl}. Response code: ${r
• Legacy model requested but code missing.
• `worker.detect` requires Legacy model, which was not loaded.
• initialization failed
• File could not be read! Code=${code}

AI-assisted analysis of naptha/tesseract.js@a1ca80d9e3 (2026-08-13). Data as JSON: /api/errors/2a43cedb1a4e913e. Report an issue: GitHub.