diff --git a/apps/server/src/serve-node.test.ts b/apps/server/src/serve-node.test.ts
index 250d0b75e..3cafa78e5 100644
--- a/apps/server/src/serve-node.test.ts
+++ b/apps/server/src/serve-node.test.ts
@@ -120,4 +120,117 @@ describe("serve", () => {
       await server.stop();
     }
   });
+
+  // A long-lived response has no other way to learn its reader is gone. Without these two
+  // signals an SSE producer keeps streaming into a dead socket for the life of the process,
+  // holding its upstream engine subscription open with it.
+  test("aborts request.signal when the client hangs up mid-stream", async () => {
+    const encoder = new TextEncoder();
+    let aborted = false;
+    let sawRequest = (): void => {};
+    const requestSeen = new Promise<void>((resolve) => {
+      sawRequest = resolve;
+    });
+
+    const server = await serve({
+      hostname: "127.0.0.1",
+      port: 0,
+      fetch: (request) =>
+        new Response(
+          new ReadableStream<Uint8Array>({
+            start(controller) {
+              controller.enqueue(encoder.encode("open\n"));
+              sawRequest();
+              request.signal.addEventListener("abort", () => {
+                aborted = true;
+                try {
+                  controller.close();
+                } catch {
+                  // Already closed by the transport.
+                }
+              }, { once: true });
+            },
+          }),
+          { headers: { "content-type": "text/event-stream" } },
+        ),
+    });
+
+    try {
+      const controller = new AbortController();
+      const response = await fetch(`http://127.0.0.1:${server.port}/stream`, { signal: controller.signal });
+      await response.body!.getReader().read();
+      await requestSeen;
+
+      controller.abort();
+      for (let attempt = 0; attempt < 50 && !aborted; attempt += 1) await delay(10);
+
+      expect(aborted).toBe(true);
+    } finally {
+      await server.stop();
+    }
+  });
+
+  test("cancels the response stream when the client hangs up mid-stream", async () => {
+    const encoder = new TextEncoder();
+    let cancelled = false;
+    let sawRequest = (): void => {};
+    const requestSeen = new Promise<void>((resolve) => {
+      sawRequest = resolve;
+    });
+
+    const server = await serve({
+      hostname: "127.0.0.1",
+      port: 0,
+      fetch: () =>
+        new Response(
+          new ReadableStream<Uint8Array>({
+            start(controller) {
+              controller.enqueue(encoder.encode("open\n"));
+              sawRequest();
+            },
+            cancel() {
+              cancelled = true;
+            },
+          }),
+          { headers: { "content-type": "text/event-stream" } },
+        ),
+    });
+
+    try {
+      const controller = new AbortController();
+      const response = await fetch(`http://127.0.0.1:${server.port}/stream`, { signal: controller.signal });
+      await response.body!.getReader().read();
+      await requestSeen;
+
+      controller.abort();
+      for (let attempt = 0; attempt < 50 && !cancelled; attempt += 1) await delay(10);
+
+      expect(cancelled).toBe(true);
+    } finally {
+      await server.stop();
+    }
+  });
+
+  test("a normal response still completes and does not report a disconnect", async () => {
+    let aborted = false;
+    const server = await serve({
+      hostname: "127.0.0.1",
+      port: 0,
+      fetch: (request) => {
+        request.signal.addEventListener("abort", () => {
+          aborted = true;
+        }, { once: true });
+        return Response.json({ ok: true });
+      },
+    });
+
+    try {
+      const response = await fetch(`http://127.0.0.1:${server.port}/health`);
+      expect(await response.json()).toEqual({ ok: true });
+      await delay(50);
+      expect(aborted).toBe(false);
+    } finally {
+      await server.stop();
+    }
+  });
 });
diff --git a/apps/server/src/serve-node.ts b/apps/server/src/serve-node.ts
index 2b1440f9e..8ca1a6264 100644
--- a/apps/server/src/serve-node.ts
+++ b/apps/server/src/serve-node.ts
@@ -79,7 +79,31 @@ async function waitForDrainOrClose(nodeRes: ServerResponse): Promise<void> {
 /**
  * Convert a Node.js IncomingMessage into a Web API Request.
  */
-function toWebRequest(nodeReq: IncomingMessage, hostname: string, port: number): Request {
+/**
+ * Signals that the client went away.
+ *
+ * `close` also fires on a perfectly normal response, so the finished flag is what
+ * separates "the exchange completed" from "the socket died under us". Without this a
+ * long-lived response — an SSE stream — has no way to learn its reader is gone, and its
+ * producer keeps running for the life of the process.
+ */
+function createDisconnectSignal(nodeReq: IncomingMessage, nodeRes: ServerResponse): AbortSignal {
+  const controller = new AbortController();
+  const abort = (): void => {
+    if (nodeRes.writableFinished) return;
+    if (!controller.signal.aborted) controller.abort();
+  };
+  nodeRes.once("close", abort);
+  nodeReq.once("aborted", abort);
+  return controller.signal;
+}
+
+function toWebRequest(
+  nodeReq: IncomingMessage,
+  hostname: string,
+  port: number,
+  signal?: AbortSignal,
+): Request {
   const url = `http://${hostname}:${port}${nodeReq.url ?? "/"}`;
   const method = nodeReq.method ?? "GET";
   const headers = new Headers();
@@ -106,6 +130,7 @@ function toWebRequest(nodeReq: IncomingMessage, hostname: string, port: number):
     method,
     headers,
     body,
+    ...(signal ? { signal } : {}),
     // @ts-expect-error duplex is required for streaming request bodies in Node
     duplex: hasBody ? "half" : undefined,
   });
@@ -114,7 +139,11 @@ function toWebRequest(nodeReq: IncomingMessage, hostname: string, port: number):
 /**
  * Write a Web API Response to a Node.js ServerResponse.
  */
-async function writeWebResponse(webRes: Response, nodeRes: ServerResponse): Promise<void> {
+async function writeWebResponse(
+  webRes: Response,
+  nodeRes: ServerResponse,
+  disconnected?: AbortSignal,
+): Promise<void> {
   const headersObj: Record<string, string | string[]> = {};
   webRes.headers.forEach((value, key) => {
     const existing = headersObj[key];
@@ -135,17 +164,36 @@ async function writeWebResponse(webRes: Response, nodeRes: ServerResponse): Prom
   }
 
   const reader = webRes.body.getReader();
+  let drained = false;
+
+  // An idle stream parks in `reader.read()` with nothing to wake it, so noticing the
+  // disconnect between chunks is not enough — the read itself has to be cut short.
+  // Cancelling does both: it resolves the pending read and reaches the stream's
+  // `cancel()` callback, which is how the producer learns to release its upstream.
+  const cancelOnDisconnect = (): void => {
+    void reader.cancel().catch(() => undefined);
+  };
+  disconnected?.addEventListener("abort", cancelOnDisconnect, { once: true });
+
   try {
     while (true) {
       const { done, value } = await reader.read();
-      if (done) break;
+      if (done) {
+        drained = true;
+        break;
+      }
       if (!isResponseWritable(nodeRes)) break;
       if (!nodeRes.write(value)) {
         await waitForDrainOrClose(nodeRes);
       }
     }
   } finally {
-    reader.releaseLock();
+    disconnected?.removeEventListener("abort", cancelOnDisconnect);
+    if (!drained) {
+      await reader.cancel().catch(() => undefined);
+    } else {
+      reader.releaseLock();
+    }
     endResponse(nodeRes);
   }
 }
@@ -168,9 +216,10 @@ export function serve(options: ServeOptions): Promise<ServeResult> {
     });
 
     try {
-      const webReq = toWebRequest(nodeReq, hostname, boundPort);
+      const disconnected = createDisconnectSignal(nodeReq, nodeRes);
+      const webReq = toWebRequest(nodeReq, hostname, boundPort, disconnected);
       const webRes = await fetchHandler(webReq);
-      await writeWebResponse(webRes, nodeRes);
+      await writeWebResponse(webRes, nodeRes, disconnected);
     } catch (error) {
       if (isExpectedConnectionAbort(error)) {
         if (isResponseWritable(nodeRes) && !nodeRes.headersSent) {