WebAssembly exceptions proposal is now implemented · wasmtime

Stream: wasmtime

Topic: WebAssembly exceptions proposal is now implemented

Alex Crichton (Aug 21 2025 at 22:02):

Figured this was worth a shout-out here -- due to huge amount of effort @Chris Fallin has put in the WebAssembly exceptions proposal is now supported in Wasmtime as of https://github.com/bytecodealliance/wasmtime/pull/11326. Support is only in the main branch and will be released as off-by-default in Wasmtime 37 for the first time. If anyone's been waiting to test out various use cases or see if things work, now's the time to test!

If anyone runs into anything please feel free to open an issue as well

WebAssembly exception-handling support. by cfallin · Pull Request #11326 · bytecodealliance/wasmtime

This PR introduces support for the Wasm exception-handling proposal, which introduces a conventional try/catch mechanism to WebAssembly. The PR supports modules that use try_table to register handl...

Alex Crichton (Aug 21 2025 at 22:03):

Also worth noting: this only includes the "standard" exceptions proposal for WebAssembly. The legacy instructions implemented by browsers are not implement and there is no plan to implement them in Wasmtime.

Pat Hickey (Aug 21 2025 at 22:50):

@Chris Fallin up for writing a BA blog post to brag about this?

fitzgen (he/him) (Aug 21 2025 at 22:51):

Alex Crichton said:

Also worth noting: this only includes the "standard" exceptions proposal for WebAssembly. The legacy instructions implemented by browsers are not implement and there is no plan to implement them in Wasmtime.

and you can translate legacy exceptions into standard exceptions via binaryen (I think via wasm-opt, but I haven't actually tested it myself)

Chris Fallin (Aug 21 2025 at 22:52):

Pat Hickey said:

Chris Fallin up for writing a BA blog post to brag about this?

Yep, that's the plan! Still sketching out how it might look (there will definitely be a deep dive through the twisty design question adventures)

Zalim Bashorov (Kotlin_, JetBrains) (Aug 26 2025 at 19:26):

\o/ Yay!
It works fine with a simple Kotlin/Wasm project. More tests coming...

Zalim Bashorov (Kotlin_, JetBrains) (Oct 08 2025 at 13:32):

JFTR: I've recently run a big chunk of our compiler tests (~8K) on wasmtime and all passed. Great job, folks!

fitzgen (he/him) (Oct 08 2025 at 15:55):

@Zalim Bashorov (Kotlin_, JetBrains) that's fantastic to hear!

Chris Fallin (Oct 08 2025 at 15:59):

Indeed, thank you @Zalim Bashorov (Kotlin_, JetBrains) for that confirmation!

Last updated: Dec 06 2025 at 06:05 UTC