Bug Report: reactStartHandler fails with AbortError due to missing duplex option

[mez]

Environment

• Package: @convex-dev/better-auth@0.8.6
• Framework: TanStack Start ^1.132.27
• Node.js: ^20.19.9 (with modern Fetch API)
• Runtime: Node.js server-side handlers

Description

The reactStartHandler function fails with an AbortError when proxying POST requests with a body to Convex. The error occurs
because the implementation is missing the required duplex option when creating a new Request object with a streaming body.

export const reactStartHandler = (request, opts) => {
      const requestUrl = new URL(request.url);
      const convexSiteUrl = opts?.convexSiteUrl ?? process.env.VITE_CONVEX_SITE_URL;
      if (!convexSiteUrl) {
          throw new Error("VITE_CONVEX_SITE_URL is not set");
      }
      const nextUrl = `${convexSiteUrl}${requestUrl.pathname}${requestUrl.search}`;
      request.headers.set("accept-encoding", "application/json"); // ← Issue #1: immutable headers
      return fetch(nextUrl, new Request(request, { redirect: "manual" })); // ← Issue #2: missing duplex
  };

Reproduction

  // apps/web/src/routes/api/auth/$.ts
  import { createFileRoute } from "@tanstack/react-router";
  import { reactStartHandler } from "@convex-dev/better-auth/react-start";

  export const Route = createFileRoute("/api/auth/$")({
    server: {
      handlers: {
        POST: async ({ request }) => {
          const convexSiteUrl = process.env.CONVEX_SITE_URL;
          return reactStartHandler(request, { convexSiteUrl });
        },
      },
    },
  });

Try to sign up via POST to /api/auth/sign-up/email - it will fail with AbortError.

Suggested Fix (** with claude's help **)

  export const reactStartHandler = (request: Request, opts?: { convexSiteUrl?: string; verbose?: boolean }) => {
      const requestUrl = new URL(request.url);
      const convexSiteUrl = opts?.convexSiteUrl ?? process.env.VITE_CONVEX_SITE_URL;
      if (!convexSiteUrl) {
          throw new Error("VITE_CONVEX_SITE_URL is not set");
      }
      const nextUrl = `${convexSiteUrl}${requestUrl.pathname}${requestUrl.search}`;

      // Create mutable headers
      const headers = new Headers(request.headers);
      headers.set("accept-encoding", "application/json");

      // Include duplex option for streaming bodies
      return fetch(nextUrl, {
          method: request.method,
          headers: headers,
          body: request.method !== 'GET' && request.method !== 'HEAD' ? request.body : undefined,
          redirect: "manual",
          // @ts-ignore - duplex is required for streaming request bodies in modern fetch
          duplex: 'half',
      });
  };

My current Workaround

Manually implement the proxy handler.

If you confirm this I can submit a pull request with the update!

better-auth/src/react-start/index.ts

Line 124 in 9607e64

return fetch(nextUrl, new Request(request, { redirect: "manual" }));

[Volko61]

Using OP solution didn't worked but using a discord thread (titled Can't get better-auth working with the Tanstack Start and Convex stack) sent to claude worked :

Here is what was generated

import { createFileRoute } from '@tanstack/react-router'

const convexSiteUrl = process.env.VITE_CONVEX_SITE_URL

async function handler({ request }: { request: Request }) {
  try {
    const requestUrl = new URL(request.url)

    if (!convexSiteUrl) {
      throw new Error('VITE_CONVEX_SITE_URL is not set')
    }
    
    const nextUrl = `${convexSiteUrl}${requestUrl.pathname}${requestUrl.search}`

    // Create a new request with the proper headers
    const headers = new Headers(request.headers)
    headers.set('accept-encoding', 'application/json')

    const response = await fetch(nextUrl, {
      method: request.method,
      headers: headers,
      body: request.body,
      redirect: 'manual',
      // @ts-ignore - duplex is needed for streaming requests in Node
      duplex: 'half'
    })

    const body = response.body
    const status = response.status
    const statusText = response.statusText

    const responseHeaders = new Headers(response.headers)

    // Ensure no caching for auth endpoints
    responseHeaders.set('Cache-Control', 'no-cache, no-store, must-revalidate')
    responseHeaders.set('Pragma', 'no-cache')
    responseHeaders.set('Expires', '0')

    return new Response(body, {
      status,
      statusText,
      headers: responseHeaders,
    })
  } catch (e: any) {
    console.error('Auth handler error:', e)

    let message = 'Internal Server Error'

    if (process.env.NODE_ENV !== 'production') {
      message = e?.message || message
    }

    return new Response(JSON.stringify({ error: message }), { 
      status: 500,
      headers: {
        'Content-Type': 'application/json'
      }
    })
  }
}

export const Route = createFileRoute('/api/auth/$')({
  server: {
    handlers: {
      GET: handler,
      POST: handler,
    },
  },
})

Might help someone

[Volko61]

Okay, previous doesn't seems to be enought.
I let the AI try to fix it and now it seems to be working

I don't know if it can help someone