Jeff R.

The deadlock happens when returning results. (Wasm's single thread blocks indefinitely waiting for results that will come over the network, and so that thread can't receive those results from the extension. With send, the thread is yielded periodically, so the deadlock is broken.) ATTACH is a bit special; I'm unsure if the deadlock condition can occur in that case. I'd go through send for everything that talks to MotherDuck to be safe, but you could run some tests and see if it hangs - it's usually pretty easy to trigger

Using the extension directly is certainly valid. The client package exists mainly to help with that. Note that there are some pitfalls to using the extension directly; the package guides you around these. For example, using the default query method with a remote MotherDuck query can result in deadlock. (The client package uses the send method, which, being more async, doesn't have this problem.)

Note that the latest versions of the MotherDuck Wasm client don't require cross-origin isolation. Version 1.5.2-r.1 and later (of @motherduck/wasm-client) use more recent MotherDuck Wasm extensions, which as you mention no longer requires cross-origin isolation.

Wasm extensions other than MD come from the usual place at extensions.duckdb.org, for example https://extensions.duckdb.org/v1.5.3/wasm_eh/icu.duckdb_extension.wasm

The package does bake in the version of duckdb-wasm, and fetches those assets as well, for example:

https://app.motherduck.com/duckdb-wasm-assets/1.33.1-dev55.0/duckdb-eh.wasm
https://app.motherduck.com/duckdb-wasm-assets/1.33.1-dev55.0/duckdb-browser-eh.worker.js

(The MD native extension checks this automatically; for the MD Wasm extension, the wasm-client package does this.)

The MD wasm extension is also fetched from ext.motherduck.com. Example: https://ext.motherduck.com/v1.5.3-2026-06-118/v1.5.3/wasm_eh/motherduck.duckdb_extension.wasm

Fabian I looked into this and found a bug with how @motherduck/wasm-client recovers from failed initialization (as you describe). I've (just) published a fix in version 1.5.3-r.2. Please give this a try and let us know if it helps.

See my comments in https://github.com/duckdb/duckdb-ui/issues/153.

No timelines. Being able to better collaborate on SQL (of which import/export is a part) is on our near-term roadmap, however. The DuckDB UI and the MotherDuck (web) UI share code, so in most cases a feature will land in both at the same time. Import/export of files is necessarily a little different in the browser vs local, but we aim to for the two experiences to be as similar as possible.