feat(sveltekit): add Lambda response streaming support

examples/aws-svelte-kit/src/routes/streaming/+page.server.ts

@@ -0,0 +1,32 @@
+/** @type {import('./$types').PageServerLoad} */
+export async function load() {
+	// This data is available immediately
+	const instant = { message: "This rendered instantly!", timestamp: Date.now() };
+
+	// These promises are NOT awaited — SvelteKit streams them to the client
+	const slow = new Promise((resolve) => {
+		setTimeout(() => {
+			resolve({
+				message: "This was streamed after 2 seconds!",
+				timestamp: Date.now(),
+			});
+		}, 2000);
+	});
+
+	const slower = new Promise((resolve) => {
+		setTimeout(() => {
+			resolve({
+				message: "This was streamed after 4 seconds!",
+				timestamp: Date.now(),
+			});
+		}, 4000);
+	});
+
+	return {
+		instant,
+		streamed: {
+			slow,
+			slower,
+		},
+	};
+}

examples/aws-svelte-kit/src/routes/streaming/+page.svelte

@@ -0,0 +1,71 @@
+<script>
+  /** @type {import('./$types').PageData} */
+  export let data;
+</script>
+
+<style>
+  section {
+    max-width: 600px;
+    margin: 2rem auto;
+    font-family: system-ui, sans-serif;
+  }
+  .card {
+    border: 1px solid #ddd;
+    border-radius: 8px;
+    padding: 1rem;
+    margin-bottom: 1rem;
+  }
+  .card h3 {
+    margin: 0 0 0.5rem;
+  }
+  .loading {
+    color: #888;
+    animation: pulse 1.5s infinite;
+  }
+  @keyframes pulse {
+    0%, 100% { opacity: 1; }
+    50% { opacity: 0.4; }
+  }
+  .done {
+    color: #16a34a;
+  }
+  code {
+    background: #f3f4f6;
+    padding: 0.15rem 0.4rem;
+    border-radius: 4px;
+    font-size: 0.85em;
+  }
+</style>
+
+<section>
+  <h1>SvelteKit Streaming Demo</h1>
+  <p>This page demonstrates Lambda response streaming. The shell and instant data arrive first, then streamed promises resolve progressively.</p>
+
+  <div class="card">
+    <h3>Instant Data</h3>
+    <p class="done">{data.instant.message}</p>
+    <p><code>timestamp: {data.instant.timestamp}</code></p>
+  </div>
+
+  <div class="card">
+    <h3>Streamed — 2s delay</h3>
+    {#await data.streamed.slow}
+      <p class="loading">Waiting for data...</p>
+    {:then result}
+      <p class="done">{result.message}</p>
+      <p><code>timestamp: {result.timestamp}</code></p>
+    {/await}
+  </div>
+
+  <div class="card">
+    <h3>Streamed — 4s delay</h3>
+    {#await data.streamed.slower}
+      <p class="loading">Waiting for data...</p>
+    {:then result}
+      <p class="done">{result.message}</p>
+      <p><code>timestamp: {result.timestamp}</code></p>
+    {/await}
+  </div>
+
+  <p><a href="/">← Back to home</a></p>
+</section>

platform/functions/sveltekit-server/server.mjs

@@ -0,0 +1,112 @@
+// Streaming Lambda handler for SvelteKit.
+// Copied by SST at deploy time — replaces the buffered handler from svelte-kit-sst.
+
+import fs from "node:fs";
+import path from "node:path";
+import { installPolyfills } from "@sveltejs/kit/node/polyfills";
+import { Server } from "./index.js";
+import { manifest } from "./manifest.js";
+import prerenderedFiles from "./lambda-handler/prerendered-file-list.js";
+
+installPolyfills();
+
+const app = new Server(manifest);
+await app.init({ env: process.env });
+
+export function convertLambdaRequestToNode(event) {
+  if (event.headers["x-forwarded-host"]) {
+    event.headers.host = event.headers["x-forwarded-host"];
+  }
+
+  const search = event.rawQueryString.length ? `?${event.rawQueryString}` : "";
+  const url = new URL(event.rawPath + search, `https://${event.headers.host}`);
+
+  const headers = new Headers();
+  for (let [header, value] of Object.entries(event.headers)) {
+    if (value) {
+      headers.append(header, value);
+    }
+  }
+
+  return new Request(url.href, {
+    method: event.requestContext.http.method,
+    headers,
+    body:
+      event.body && event.isBase64Encoded
+        ? Buffer.from(event.body, "base64")
+        : event.body,
+  });
+}
+
+export function isPrerenderedFile(uri, prerenderedFiles) {
+  uri = uri.replace(/^\/|\/$/g, "");
+
+  if (uri === "") {
+    return prerenderedFiles.includes("index.html") ? "index.html" : undefined;
+  }
+
+  if (prerenderedFiles.includes(uri)) {
+    return uri;
+  }
+  if (prerenderedFiles.includes(uri + "/index.html")) {
+    return uri + "/index.html";
+  }
+  if (prerenderedFiles.includes(uri + ".html")) {
+    return uri + ".html";
+  }
+}
+
+export const handler = awslambda.streamifyResponse(
+  async (event, responseStream, context) => {
+    context.callbackWaitsForEmptyEventLoop = false;
+
+    // Check for prerendered files on GET requests
+    if (event.requestContext?.http?.method === "GET") {
+      const filePath = isPrerenderedFile(event.rawPath, prerenderedFiles);
+      if (filePath) {
+        const body = fs.readFileSync(
+          path.join("prerendered", filePath),
+          "utf8",
+        );
+        const writer = awslambda.HttpResponseStream.from(responseStream, {
+          statusCode: 200,
+          headers: {
+            "content-type": "text/html",
+            "cache-control":
+              "public, max-age=0, s-maxage=31536000, must-revalidate",
+          },
+        });
+        writer.write(body);
+        writer.end();
+        return;
+      }
+    }
+
+    // Handle dynamic requests through SvelteKit
+    const request = convertLambdaRequestToNode(event);
+    const response = await app.respond(request, {
+      getClientAddress: () => event.requestContext.http.sourceIp,
+    });
+
+    const writer = awslambda.HttpResponseStream.from(responseStream, {
+      statusCode: response.status,
+      headers: {
+        ...Object.fromEntries(response.headers.entries()),
+        "Transfer-Encoding": "chunked",
+      },
+      cookies: response.headers.getSetCookie(),
+    });
+
+    if (response.body) {
+      const reader = response.body.getReader();
+      let readResult = await reader.read();
+      while (!readResult.done) {
+        writer.write(readResult.value);
+        readResult = await reader.read();
+      }
+    } else {
+      writer.write(" ");
+    }
+    writer.end();
+  },
+);

platform/src/components/aws/svelte-kit.ts

@@ -437,14 +437,22 @@ export class SvelteKit extends SsrSite {
         }
       } catch (e) {}
 
+      // Copy streaming handler into the server output directory
+      fs.copyFileSync(
+        path.join(
+          $cli.paths.platform,
+          "functions",
+          "sveltekit-server",
+          "server.mjs",
+        ),
+        path.join(serverOutputPath, "server.mjs"),
+      );
+
       return {
         base: basepath,
         server: {
-          handler: path.join(
-            serverOutputPath,
-            "lambda-handler",
-            "index.handler",
-          ),
+          handler: path.join(serverOutputPath, "server.handler"),
+          streaming: true,
           nodejs: {
             esbuild: {
               minify: process.env.SST_DEBUG ? false : true,