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fix: stream promised read results lazily #16622

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fix: stream promised read results lazily #16622

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5 changes: 5 additions & 0 deletions .changeset/wise-moons-read.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
---
'@sveltejs/kit': patch
---

fix: stream promised `read` results lazily instead of eagerly buffering them
34 changes: 10 additions & 24 deletions packages/kit/src/runtime/server/index.js
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Original file line number Diff line number Diff line change
Expand Up @@ -67,31 +67,17 @@ export class Server {
const result = read(file);
if (result instanceof ReadableStream) {
return result;
} else {
return new ReadableStream({
async start(controller) {
try {
const stream = await Promise.resolve(result);
if (!stream) {
controller.close();
return;
}

const reader = stream.getReader();

while (true) {
const { done, value } = await reader.read();
if (done) break;
controller.enqueue(value);
}

controller.close();
} catch (error) {
controller.error(error);
}
}
});
}

// TODO remove the cast once TypeScript's lib includes `ReadableStream.from`

@vercel vercel Bot Aug 3, 2026

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wrapped_read uses ReadableStream.from() and async iteration of a ReadableStream (yield* stream), which are not supported on Cloudflare's workerd runtime, throwing at runtime when server-side asset reads occur on the Cloudflare adapter.

Fix on Vercel

return /** @type {ReadableStream} */ (
/** @type {any} */ (ReadableStream).from(
(async function* () {
const stream = await result;
if (stream) yield* stream;
})()
)
);
};

set_read_implementation(wrapped_read);
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