From d26ff54def3fa95bc4c670989b5a4e46210b12db Mon Sep 17 00:00:00 2001
From: Nic Polumeyv <162764842+Nic-Polumeyv@users.noreply.github.com>
Date: Sun, 2 Aug 2026 22:28:45 -0400
Subject: [PATCH] fix: stream promised read results lazily

---
 .changeset/wise-moons-read.md            |  5 ++++
 packages/kit/src/runtime/server/index.js | 34 +++++++-----------------
 2 files changed, 15 insertions(+), 24 deletions(-)
 create mode 100644 .changeset/wise-moons-read.md

diff --git a/.changeset/wise-moons-read.md b/.changeset/wise-moons-read.md
new file mode 100644
index 000000000000..839ccf11669d
--- /dev/null
+++ b/.changeset/wise-moons-read.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: stream promised `read` results lazily instead of eagerly buffering them
diff --git a/packages/kit/src/runtime/server/index.js b/packages/kit/src/runtime/server/index.js
index 0b1522391ef0..98dcc85465ca 100644
--- a/packages/kit/src/runtime/server/index.js
+++ b/packages/kit/src/runtime/server/index.js
@@ -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`
+				return /** @type {ReadableStream} */ (
+					/** @type {any} */ (ReadableStream).from(
+						(async function* () {
+							const stream = await result;
+							if (stream) yield* stream;
+						})()
+					)
+				);
 			};
 
 			set_read_implementation(wrapped_read);