Build fails when I use a server function middleware with a server function

The middleware reads the user's data from a session cookie.

From what I understand, the error is about me trying to run server-only code on the client side (like reading the encrypted session data), but I created my middleware using the

server()

server()

server()

server() method, so I'm confused.

Reproduction
I forked the basic example and added a commit to it to reproduce the issue.

Commit: https://github.com/fulopkovacs/tanstack-start-middleware-issue/commit/60d7283c05ab71353d6014f062242f72fd8be12e#diff-6aa551c33ca5fabc618706416b028e5ce253ca1e09a72d61fe0163b09e2bbcae
Repo: https://github.com/fulopkovacs/tanstack-start-middleware-issue

This is the error:

✗ Build failed in 825ms

[12:16:35 PM]  ERROR  node_modules/.pnpm/@tanstack+start-server-core@1.120.10/node_modules/@tanstack/start-server-core/dist/esm/transformStreamWithRouter.js (2:9): "Readable" is not exported by "__vite-browser-external", imported by "node_modules/.pnpm/@tanstack+start-server-core@1.120.10/node_modules/@tanstack/start-server-core/dist/esm/transformStreamWithRouter.js".
file: /Users/fulopkovacs/dev-learning/2025/tanstack-start-weirdness/start-basic/node_modules/.pnpm/@tanstack+start-server-core@1.120.10/node_modules/@tanstack/start-server-core/dist/esm/transformStreamWithRouter.js:2:9

1: import { ReadableStream } from "node:stream/web";
2: import { Readable } from "node:stream";
           ^
3: import { createControlledPromise } from "@tanstack/router-core";
4: function transformReadableStreamWithRouter(router, routerStream) {

[see the full error in the attachment]

✗ Build failed in 825ms

[12:16:35 PM]  ERROR  node_modules/.pnpm/@tanstack+start-server-core@1.120.10/node_modules/@tanstack/start-server-core/dist/esm/transformStreamWithRouter.js (2:9): "Readable" is not exported by "__vite-browser-external", imported by "node_modules/.pnpm/@tanstack+start-server-core@1.120.10/node_modules/@tanstack/start-server-core/dist/esm/transformStreamWithRouter.js".
file: /Users/fulopkovacs/dev-learning/2025/tanstack-start-weirdness/start-basic/node_modules/.pnpm/@tanstack+start-server-core@1.120.10/node_modules/@tanstack/start-server-core/dist/esm/transformStreamWithRouter.js:2:9

1: import { ReadableStream } from "node:stream/web";
2: import { Readable } from "node:stream";
           ^
3: import { createControlledPromise } from "@tanstack/router-core";
4: function transformReadableStreamWithRouter(router, routerStream) {

[see the full error in the attachment]

✗ Build failed in 825ms

[12:16:35 PM]  ERROR  node_modules/.pnpm/@tanstack+start-server-core@1.120.10/node_modules/@tanstack/start-server-core/dist/esm/transformStreamWithRouter.js (2:9): "Readable" is not exported by "__vite-browser-external", imported by "node_modules/.pnpm/@tanstack+start-server-core@1.120.10/node_modules/@tanstack/start-server-core/dist/esm/transformStreamWithRouter.js".
file: /Users/fulopkovacs/dev-learning/2025/tanstack-start-weirdness/start-basic/node_modules/.pnpm/@tanstack+start-server-core@1.120.10/node_modules/@tanstack/start-server-core/dist/esm/transformStreamWithRouter.js:2:9

1: import { ReadableStream } from "node:stream/web";
2: import { Readable } from "node:stream";
           ^
3: import { createControlledPromise } from "@tanstack/router-core";
4: function transformReadableStreamWithRouter(router, routerStream) {

[see the full error in the attachment]

✗ Build failed in 825ms

[12:16:35 PM]  ERROR  node_modules/.pnpm/@tanstack+start-server-core@1.120.10/node_modules/@tanstack/start-server-core/dist/esm/transformStreamWithRouter.js (2:9): "Readable" is not exported by "__vite-browser-external", imported by "node_modules/.pnpm/@tanstack+start-server-core@1.120.10/node_modules/@tanstack/start-server-core/dist/esm/transformStreamWithRouter.js".
file: /Users/fulopkovacs/dev-learning/2025/tanstack-start-weirdness/start-basic/node_modules/.pnpm/@tanstack+start-server-core@1.120.10/node_modules/@tanstack/start-server-core/dist/esm/transformStreamWithRouter.js:2:9

1: import { ReadableStream } from "node:stream/web";
2: import { Readable } from "node:stream";
           ^
3: import { createControlledPromise } from "@tanstack/router-core";
4: function transformReadableStreamWithRouter(router, routerStream) {

[see the full error in the attachment]

I started using TanStack Start a week ago and am still a newbie.

Would be really grateful for some help with this!

GitHub

GitHub - fulopkovacs/tanstack-start-middleware-issue: Repro of an i...

Repro of an issue with server function middlewares. - fulopkovacs/tanstack-start-middleware-issue

GitHub

Build error · fulopkovacs/tanstack-start-middleware-issue@60d7283

Build fails when I use a server function middleware with a server function

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