## The problem

Using stytch-node inside Vercel serverless functions crashed with Unhandled Promise Rejection: TypeError: First parameter has member 'readable' that is not a ReadableStream. The same code worked locally and in a normal Node VM. The cause was the isomorphic-unfetch polyfill, whose fetch implementation was incompatible with the serverless runtime's streams.

## What works

Fixed upstream in PR #255, which dropped isomorphic-unfetch entirely and switched the SDK to the built-in fetch API (also fixing Cloudflare Workers and Vercel Edge runtimes). Upgrade stytch-node to a release from October 2023 or later containing that change; the maintainer verified it across Cloudflare Workers, Vercel serverless, and Edge runtimes. There is no reliable workaround on the broken versions since the polyfill itself is the problem.
Source: https://github.com/stytchauth/stytch-node/issues/248