diff --git a/changelog.d/fixes/14904-deadline-signal-proxied-request.md b/changelog.d/fixes/14904-deadline-signal-proxied-request.md
new file mode 100644
index 000000000000..09aea88e932b
--- /dev/null
+++ b/changelog.d/fixes/14904-deadline-signal-proxied-request.md
@@ -0,0 +1 @@
+- **fix(sse):** `/v1/chat/completions`, `/v1/messages` and `/v1/responses` no longer fail with HTTP 500 `Cannot read private member #state` ([#14904](https://github.com/diegosouzapw/OmniRoute/pull/14904)). Next.js hands route handlers without a `dynamic` export a Proxy around the `NextRequest`, and `withDeadlineSignal()` rebuilt it with `new Request(request, init)`, which reads the input's private fields. The wrapped request is now rebuilt from its public accessors (url, method, headers, redirect, body stream). — thanks @TrippyEngineer
diff --git a/open-sse/utils/earlyStreamKeepalive.ts b/open-sse/utils/earlyStreamKeepalive.ts
index d2d600ebf7ad..59b1f1f0f3b5 100644
--- a/open-sse/utils/earlyStreamKeepalive.ts
+++ b/open-sse/utils/earlyStreamKeepalive.ts
@@ -312,9 +312,12 @@ export function __getDeadlineTokenRegistrySizeForTests(): number {
  *
  * The wrapped request MUST be the one the route hands downstream (admission,
  * body parse, `handleChat`): the handler snapshots `request.signal` after
- * admission, so wrapping after that point would not propagate. Rebuilt via
- * `new Request(request, { signal, headers })`, which preserves method, url and
- * body byte-for-byte.
+ * admission, so wrapping after that point would not propagate. Rebuilt from the
+ * public accessors (url, method, headers, redirect, body stream), which
+ * preserves method, url and body byte-for-byte. It must NOT be
+ * `new Request(request, init)`: Next.js hands handlers without a `dynamic`
+ * export a Proxy around the NextRequest (`proxyNextRequest`), and the Request
+ * constructor reads the input's private `#state`, which throws through a Proxy.
  *
  * Controller recovery downstream (`getDeadlineController`) is two-layered:
  * the combined signal object (fast path — same object when nothing rebuilds),
@@ -338,7 +341,15 @@ export function withDeadlineSignal(request: Request): {
   // admission rebuilds, which both copy headers but mint new signal objects.
   const token = `dl-${Date.now().toString(36)}-${(deadlineTokenSeq += 1)}`;
   headers.set(DEADLINE_TOKEN_HEADER, token);
-  const wrappedReq = new Request(request, { signal: combined, headers });
+  const hasBody = request.method !== "GET" && request.method !== "HEAD";
+  const wrappedReq = new Request(request.url, {
+    method: request.method,
+    headers,
+    body: hasBody ? request.body : null,
+    redirect: request.redirect,
+    signal: combined,
+    duplex: "half",
+  } as RequestInit & { duplex: "half" });
   deadlineControllers.set(combined, deadlineController);
   deadlineControllersByToken.set(token, new WeakRef(deadlineController));
   deadlineTokenByController.set(deadlineController, token);
diff --git a/tests/unit/early-stream-keepalive.test.ts b/tests/unit/early-stream-keepalive.test.ts
index 8fc8dda131e4..eb735aa8774f 100644
--- a/tests/unit/early-stream-keepalive.test.ts
+++ b/tests/unit/early-stream-keepalive.test.ts
@@ -692,3 +692,38 @@ test("deadline token registry returns to its original size after N requests", as
     "a released token must not resolve through the header fallback"
   );
 });
+
+// Next.js hands App Router handlers without a `dynamic` export a Proxy around the
+// NextRequest (`proxyNextRequest`). Private class fields do not pass through a
+// Proxy, so `new Request(proxiedRequest, init)` threw "Cannot read private member
+// #state" and every /v1/chat/completions, /v1/messages and /v1/responses call
+// returned 500. The wrap must rebuild from public accessors instead.
+test("withDeadlineSignal accepts a Proxy-wrapped request (Next.js proxyNextRequest)", async () => {
+  const client = new AbortController();
+  const inner = new Request("http://localhost/v1/chat/completions", {
+    method: "POST",
+    headers: { "Content-Type": "application/json", "X-Test": "kept" },
+    body: '{"model":"auto"}',
+    signal: client.signal,
+  });
+  const proxied = new Proxy(inner, {
+    get: (target, prop) => Reflect.get(target, prop, target),
+  });
+
+  const { wrappedReq, deadlineController } = withDeadlineSignal(proxied);
+
+  assert.equal(wrappedReq.method, "POST");
+  assert.equal(wrappedReq.url, "http://localhost/v1/chat/completions");
+  assert.equal(wrappedReq.headers.get("x-test"), "kept");
+  assert.equal(await wrappedReq.text(), '{"model":"auto"}');
+  assert.equal(getDeadlineController(wrappedReq), deadlineController);
+  assert.equal(wrappedReq.signal.aborted, false);
+  client.abort();
+  assert.equal(wrappedReq.signal.aborted, true, "client abort must still reach the wrapped signal");
+});
+
+test("withDeadlineSignal keeps a body-less GET request valid", () => {
+  const { wrappedReq } = withDeadlineSignal(new Request("http://localhost/v1/models"));
+  assert.equal(wrappedReq.method, "GET");
+  assert.equal(wrappedReq.body, null);
+});