`bevy run web --unstable multi-threading` panics

[RobertoMaurizzi]

CLI or linter version

bevy_cli both latest release v0.1.0-alpha.2 and built from main at 154bf17

Relevant system information

Debian testing
cargo 1.92.0-nightly (801d9b498 2025-10-04)
rustc 1.92.0-nightly (f6aa851db 2025-10-07)

default .cargo/config.toml (meaning, not there, I didn't rename the file)

What you did

• created a new bevy_new_2d project
• added rust-toolchain.toml to use nightly
• added:

[package.metadata.bevy_cli.unstable]
web-multi-threading = true

• ran bevy run web --unstable multi-threading

What went wrong

I got:

warning: unstable feature specified for `-Ctarget-feature`: `atomics`
  |
  = note: this feature is not stably supported; its behavior can change in the future

warning: `bevy_new_2d_017` (bin "bevy_new_2d_017") generated 1 warning
    Finished `web` profile [optimized + debuginfo] target(s) in 2m 16s
info: bundling JavaScript bindings...

thread 'main' panicked at /home/robm/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wasm-bindgen-cli-support-0.2.104/src/interpreter/mod.rs:218:21:
wasm_bindgen::__rt::wbg_cast::breaks_if_inlined::h1bb6e5dce38e1af3: unknown instruction Block(Block { seq: Id { idx: 1 } })
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
warning: failed to run wasm-bindgen, trying to find automatic fix...
error: command `wasm-bindgen --no-typescript --out-name bevy_new_2d_017 --out-dir /home/robm/bevy_new_2d_017/target/wasm32-unknown-unknown/web --target web /home/robm/bevy_new_2d_017/target/wasm32-unknown-unknown/web/bevy_new_2d_017.wasm` exited with status code exit status: 101

Additional information

It might very well that I've missed an obvious step or there's a missing one in the documentation.
I first tried to enable multithreading on a demo application I have that would really benefit from it but still didn't try to do anything wasm multithreading specific in it.
I then tried on bevy_new_2d and it was the same (both have no wasm multithread specific crates, I think).
It might be I need to add a crate and, that missing, wasm-bindgen misbehaves?

[DaAlbrecht]

Could be an issue with newer wasm-bindgen versions (if you check for example: https://github.com/janhohenheim/foxtrot it should work). Thanks for reporting, we have to look into this!

[RobertoMaurizzi]

Thanks, I checked Foxtrot and it uses "wasm-bindgen = { version = "=0.2.101" }" while I had 0.2.104.
I tried to add an explicit dependency on it under [target.wasm32-unknown-unknown.dependencies]: bevy_cli asked me to install the corresponding version of wasm-bindgen-cli and after that it was able to compile and serve the code (again, unmodified bevy_2d_new except for this change and the others mentioned above) showing it's using the right response headers: {"cross-origin-opener-policy": "same-origin", "cross-origin-embedder-policy": "require-corp"}

Opening the URL however I get:

Uncaught TypeError: [object Int32Array] is not a shared typed array.
    at Atomics.waitAsync (<anonymous>)
    at imports.wbg.__wbg_waitAsync_c3398694eaa5aeab (bevy_new_2d_017.js:2115:29)
    at bevy_new_2d_017-cb5c703e35e0d313.wasm.__wbg_waitAsync_c3398694eaa5aeab externref shim (bevy_new_2d_017_bg.wasm:0x3ea9ee9)
    at bevy_new_2d_017-cb5c703e35e0d313.wasm.wasm_bindgen_futures::task::multithread::Task::run::h494feace3000dc46 (bevy_new_2d_017_bg.wasm:0x1a626fa)
    at bevy_new_2d_017-cb5c703e35e0d313.wasm.wasm_bindgen_futures::queue::QueueState::run_all::h2c4106e058185bd6 (bevy_new_2d_017_bg.wasm:0x3342005)
    at bevy_new_2d_017-cb5c703e35e0d313.wasm.wasm_bindgen_futures::queue::Queue::new::{{closure}}::hc7a6bed311d68e2e (bevy_new_2d_017_bg.wasm:0x3ab4f63)
    at bevy_new_2d_017-cb5c703e35e0d313.wasm.wasm_bindgen::convert::closures::_::invoke::h3537b7d5d7fc49ba (bevy_new_2d_017_bg.wasm:0x3ea716a)
    at bevy_new_2d_017-cb5c703e35e0d313.wasm.closure168317 externref shim (bevy_new_2d_017_bg.wasm:0x3f5f035)
    at __wbg_adapter_61 (bevy_new_2d_017.js:276:10)
    at real (bevy_new_2d_017.js:176:20)

[DaAlbrecht]

Hm what browser are you using? For me, this seems to be working using firefox 143.0.4

[RobertoMaurizzi]

I just tried and I confirm that in Firefox I do get to the start menu. If I press play however I get 2 "Uncaught (in promise) DOMException: WebAssembly.Memory object could not be cloned." with a bit of wasm assembly tracing, then it's stuck.
Firefox 140.3.1esr (64-bit) on Debian testing.

[RobertoMaurizzi]

Partial correction: the 2 ".Memory object could not be cloned." come out in the console before the menu is displayed.
The menu does work, but pressing Play shows the state transition message and nothing else (I waited for a couple minutes).

[RobertoMaurizzi]

What version of wasm-bindgen are you using? (sorry for the multiple messages, it's been quite a day chasing bugs in half rotten C web application servers and my brain isn't exactly in top shape 😅 )

[BD103]

I'm able to reproduce the original error on MacOS using bevy_new_2d, nightly-2025-10-08, and wasm-bindgen 0.2.104:

$ rustc --version
rustc 1.92.0-nightly (f6aa851db 2025-10-07)

$ wasm-bindgen --version
wasm-bindgen 0.2.104

$ bevy run web --unstable multi-threading
...
info: bundling JavaScript bindings...

thread 'main' (39632213) panicked at ~/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wasm-bindgen-cli-support-0.2.104/src/interpreter/mod.rs:218:21:
wasm_bindgen::__rt::wbg_cast::breaks_if_inlined::h054aa67f2d9a88d9: unknown instruction Block(Block { seq: Id { idx: 1 } })
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: wasm_bindgen_cli_support::interpreter::Interpreter::call
   3: wasm_bindgen_cli_support::interpreter::Interpreter::interpret_descriptor
   4: wasm_bindgen_cli_support::descriptors::execute
   5: wasm_bindgen_cli_support::Bindgen::generate_output
   6: wasm_bindgen_cli_support::Bindgen::generate
   7: wasm_bindgen::main

I was also able to reproduce it on the latest nightly, nightly-2025-10-17. I think we're hitting wasm-bindgen/wasm-bindgen#4705, but the fix hasn't been released yet.