Shopify's cart attribute limit

Shopify's cart attribute limit is somewhere between 19.9 MB and 20 MB, or roughly:

Max attribute value size ≈ 20,873,216 characters (just under 20 MB).

To test this out use the script below on your Shopify store:

413 (Payload Too Large) error will throw on the larger attribute.

const sizes = [
  20873216, 20900000,21000000
];

(async () => {
  for (const size of sizes) {
    const largeString = 'x'.repeat(size);
    const attributes = { long_data: largeString };

    console.log(`🚀 Trying size: ${size} characters (${(size / 1024 / 1024).toFixed(2)} MB)`);

    try {
      const response = await fetch('/cart/update.js', {
        method: 'POST',
        headers: {
          'Content-Type': 'application/json',
          'Accept': 'application/json'
        },
        body: JSON.stringify({ attributes })
      });

      const data = await response.json();
      const returnedLength = data.attributes?.long_data?.length || 0;

      console.log(`✅ Size ${size}: Saved length = ${returnedLength}`);
    } catch (err) {
      console.error(`❌ Size ${size}: Cart update failed`, err);
    }

    await new Promise((res) => setTimeout(res, 1000)); // throttle-safe
  }
})();