[Bug?]: "use server" in async generator is broken

[theturtle32]

Duplicates

• I have searched the existing issues

Latest version

• I have tested the latest version

Current behavior 😯

Adding the "use server" directive to an async generator fails with the error "use server" in generator function not supported.

The error is raised from the plugin tanstack-start-directive-vite-plugin

Error location is @tanstack/directive-functions-plugin/dist/esm/compilers.js:327:10
Version of @tanstack/directive-functions-plugin is 1.115.0

Expected behavior 🤔

"use server" in async generator functions should work. And it did work before adopting @tanstack/server-functions-plugin in #1715

And indeed, if you hoist "use server" to the module level, the functionality DOES work even today, but then you get warnings about module-level "use server" being unsupported when you run pnpm run build, which I think is an otherwise unrelated issue (#1799).

Steps to reproduce 🕹

Steps:

1. pnpm create solid
2. Create a SolidStart project using the basic template.
3. Add the following files:

/* src/server-functions.tsx */

// Say hello in a different language every second, forever.
export async function* sayHello() {
  "use server";
  const greetings = [
    "Hello, World!",      // English
    "¡Hola, Mundo!",      // Spanish
    "Bonjour le monde !", // French
    "Hallo Welt!",        // German
    "Ciao mondo!",        // Italian
    "Olá Mundo!",         // Portuguese
    "こんにちは世界",        // Japanese
    "你好，世界"            // Chinese (Mandarin)
  ];

  for (const greeting of greetings) {
    yield greeting;
    await new Promise(resolve => setTimeout(resolve, 1000)); // Wait for 1 second
  }
}

/* src/routes/generator-server-fn.tsx */

import { Title } from "@solidjs/meta";
import { For, onMount } from "solid-js";
import { createStore } from "solid-js/store";
import { sayHello } from '~/server-functions';

export default function GeneratorServerFn() {
  const [store, setStore] = createStore<Array<string>>([]);

  onMount(async () => {
    const greetings = await sayHello();
    for await (const greeting of greetings) {
      setStore(store.length, greeting);
    }
  });

  return (
    <main>
      <Title>Generator Server Fn</Title>
      <h1>Generator Server Fn</h1>
      <For each={store}>
        {(message) => <div class="message">{message}</div>}
      </For>
    </main>
  );
}

4. pnpm run dev
5. Visit http://localhost:3000/generator-server-fn
6. Observe the error "use server" in generator function not supported occurring.
7. Optional: hoist "use server" to the top of the src/server-functions.ts file and refresh, and then observe that the server function runs and streams a new "hello, world" phrase to be added to the page every second.

Context 🔦

No response

Your environment 🌎

System:
  OS: macOS Sequoia 15.3.2
  Chip: Apple M1 Pro

Node.js v22.14.0

pnpm 10.8.0

@solidjs/start@1.1.3

[theturtle32]

Seroval absolutely does support streaming from an async generator. And this used to work previously.

In the project I'm working on, I'm using a server function (an async generator) to request a response be generated from an LLM, and then streaming the chunks from the LLM to the client via async iteration. Worked perfectly before adopting @tanstack/server-functions-plugin, and works fine in the dev server if I hoist "use server" to the top of the file instead of placing it inside my function.

[theturtle32]

There was a previous bug #1477 reported and resolved around async generators as server functions, but from before @tanstack/server-functions-plugin was adopted.

[theturtle32]

This appears to be because there is explicit code in @tanstack/directive-functions-plugin to block supporting generators as server functions. I don't know why that was done, perhaps that limitation is relevant to TanStack Start, but it is not relevant to SolidStart.

https://github.com/TanStack/router/blob/3abf4c19f7e7a37a3531512bf6dfa166ecedfeb7/packages/directive-functions-plugin/src/compilers.ts#L298-L310

Excerpt:

// Handle class and object methods which are not supported
const isGenerator =
  directiveFn.isFunction() && directiveFn.node.generator

const isClassMethod = directiveFn.isClassMethod()
const isObjectMethod = directiveFn.isObjectMethod()

if (isClassMethod || isObjectMethod || isGenerator) {
  throw codeFrameError(
    opts.code,
    directiveFn.node.loc,
    `"${opts.directive}" in ${isClassMethod ? 'class' : isObjectMethod ? 'object method' : 'generator function'} not supported`,
  )
}

[theturtle32]

Added a failing e2e test to the suite that illustrates the issue and should stop failing once a fix is implemented.
https://github.com/solidjs/solid-start/pull/1887/files

Result of (currently failing) test execution via GitHub Actions: https://github.com/theturtle32/solid-start/actions/runs/14804995916/job/41571655312

[lxsmnsyc]

This used to work before thanks to Vinxi allowing it. Seems like TS directive didn't support this. Upstream issue.

[theturtle32]

Upstream in the sense that the fix most likely needs to happen in the TanStack Router repo, yes. But not upstream in the sense that this regression was inadvertently introduced when @brenelz swapped out @vinxi/server-functions with @tanstack/server-functions-plugin in #1715. So I think this should be tracked here, given that this is advertised functionality of SolidStart's approach to server functions that no longer works.

[lxsmnsyc]

@theturtle32 We planned to move on from Vinxi, so while it's a regression on Start's part, it must be moved to the upstream for support.

[theturtle32]

Oh! I thought Vinxi was supposed to be a new foundation not just for SolidStart, but for other frameworks to also build on top of. I guess the thinking around that has changed?

[theturtle32]

In any case, I think SolidStart should keep an e2e test to ensure the functionality continues to work in the future as things continue to evolve.