diff --git a/apps/builder/__tests__/proxy-public-url.test.ts b/apps/builder/__tests__/proxy-public-url.test.ts
new file mode 100644
index 0000000000..a11c527ec2
--- /dev/null
+++ b/apps/builder/__tests__/proxy-public-url.test.ts
@@ -0,0 +1,72 @@
+// @vitest-environment node
+
+import { NextRequest } from "next/server"
+import { afterEach, describe, expect, test, vi } from "vitest"
+
+vi.mock("@/lib/auth/auth", () => ({
+  auth: { api: { getSession: vi.fn() } },
+}))
+
+vi.mock("better-auth/cookies", () => ({
+  getSessionCookie: vi.fn(() => null),
+}))
+
+vi.mock("next/headers", () => ({
+  headers: vi.fn(async () => new Headers()),
+}))
+
+vi.mock("@/lib/log", () => ({
+  httpLogger: { info: vi.fn(), warn: vi.fn(), error: vi.fn() },
+}))
+
+const originalForcePublicHttps = process.env.FORCE_PUBLIC_HTTPS
+
+afterEach(() => {
+  if (originalForcePublicHttps === undefined) {
+    delete process.env.FORCE_PUBLIC_HTTPS
+  } else {
+    process.env.FORCE_PUBLIC_HTTPS = originalForcePublicHttps
+  }
+})
+
+/**
+ * `/api` is a public route, so `proxy` reaches `attachProxyUrl` without
+ * touching the session. `NextResponse.next({ request: { headers } })` exposes
+ * the overridden request headers as `x-middleware-request-<name>`.
+ */
+async function proxyUrlFor(publicHost: string, requestUrl: string) {
+  delete process.env.FORCE_PUBLIC_HTTPS
+  const { proxy } = await import("@/proxy")
+  const response = await proxy(
+    new NextRequest(requestUrl, {
+      headers: { "x-forwarded-host": publicHost },
+    }),
+  )
+  return response.headers.get("x-middleware-request-x-url")
+}
+
+describe("proxy x-url", () => {
+  test("keeps the port when the public host carries one", async () => {
+    expect(
+      await proxyUrlFor("localhost:3123", "http://internal.test:3000/api/x"),
+    ).toBe("http://localhost:3123/api/x")
+  })
+
+  test("drops the internal port when the public host carries none", async () => {
+    expect(
+      await proxyUrlFor("app.example.com", "http://internal.test:3000/api/x"),
+    ).toBe("http://app.example.com/api/x")
+  })
+
+  test("does not mistake a bracketed IPv6 host for one carrying a port", async () => {
+    expect(await proxyUrlFor("[::1]", "http://internal.test:3000/api/x")).toBe(
+      "http://[::1]/api/x",
+    )
+  })
+
+  test("keeps the port of a bracketed IPv6 host that carries one", async () => {
+    expect(
+      await proxyUrlFor("[::1]:3123", "http://internal.test:3000/api/x"),
+    ).toBe("http://[::1]:3123/api/x")
+  })
+})
diff --git a/apps/builder/src/lib/auth-redirect.ts b/apps/builder/src/lib/auth-redirect.ts
index f0cf0fc6f1..e64491208f 100644
--- a/apps/builder/src/lib/auth-redirect.ts
+++ b/apps/builder/src/lib/auth-redirect.ts
@@ -1,5 +1,6 @@
 import {
   getPublicHostFromRequest,
+  getPublicPortFromRequest,
   getPublicProtocolFromRequest,
 } from "@chatbotx.io/utils"
 import { env } from "@/env"
@@ -67,7 +68,7 @@ export async function rewriteAuthRedirectToPublicHost(
 
   target.host = publicHost
   target.protocol = publicProtocol
-  target.port = ""
+  target.port = getPublicPortFromRequest(request)
 
   const headers = new Headers(response.headers)
   headers.set("location", target.toString())
diff --git a/apps/builder/src/proxy.ts b/apps/builder/src/proxy.ts
index b34b958f47..686433433a 100644
--- a/apps/builder/src/proxy.ts
+++ b/apps/builder/src/proxy.ts
@@ -1,6 +1,7 @@
 import {
   getPublicHostFromRequest,
   getPublicOriginFromRequest,
+  getPublicPortFromRequest,
   getPublicProtocolFromRequest,
 } from "@chatbotx.io/utils"
 import { getSessionCookie } from "better-auth/cookies"
@@ -64,7 +65,7 @@ function attachProxyUrl(request: NextRequest): NextResponse {
   const originUrl = new URL(request.url)
   originUrl.host = getPublicHostFromRequest(request)
   originUrl.protocol = getPublicProtocolFromRequest(request)
-  originUrl.port = ""
+  originUrl.port = getPublicPortFromRequest(request)
 
   const requestHeaders = new Headers(request.headers)
   requestHeaders.set("x-url", originUrl.toString())
diff --git a/packages/utils/__tests__/request.test.ts b/packages/utils/__tests__/request.test.ts
index 7c0dde5dd9..00bba69890 100644
--- a/packages/utils/__tests__/request.test.ts
+++ b/packages/utils/__tests__/request.test.ts
@@ -64,4 +64,52 @@ describe("getPublicUrlFromRequest", () => {
         .protocol,
     ).toBe("https:")
   })
+
+  test("keeps the port when the public host carries one", () => {
+    delete process.env.FORCE_PUBLIC_HTTPS
+
+    expect(
+      getPublicUrlFromRequest(
+        new Request("http://internal.test:3000/callback", {
+          headers: { "x-forwarded-host": "localhost:3123" },
+        }),
+      ).toString(),
+    ).toBe("http://localhost:3123/callback")
+  })
+
+  test("drops the internal port when the public host carries none", () => {
+    delete process.env.FORCE_PUBLIC_HTTPS
+
+    expect(
+      getPublicUrlFromRequest(
+        new Request("http://internal.test:3000/callback", {
+          headers: { "x-forwarded-host": "app.example.com" },
+        }),
+      ).toString(),
+    ).toBe("http://app.example.com/callback")
+  })
+
+  test("does not mistake a bracketed IPv6 host for one carrying a port", () => {
+    delete process.env.FORCE_PUBLIC_HTTPS
+
+    expect(
+      getPublicUrlFromRequest(
+        new Request("http://internal.test:3000/callback", {
+          headers: { "x-forwarded-host": "[::1]" },
+        }),
+      ).toString(),
+    ).toBe("http://[::1]/callback")
+  })
+
+  test("keeps the port of a bracketed IPv6 host that carries one", () => {
+    delete process.env.FORCE_PUBLIC_HTTPS
+
+    expect(
+      getPublicUrlFromRequest(
+        new Request("http://internal.test:3000/callback", {
+          headers: { "x-forwarded-host": "[::1]:3123" },
+        }),
+      ).toString(),
+    ).toBe("http://[::1]:3123/callback")
+  })
 })
diff --git a/packages/utils/src/request.ts b/packages/utils/src/request.ts
index 8f2b17b343..4ab8aa179f 100644
--- a/packages/utils/src/request.ts
+++ b/packages/utils/src/request.ts
@@ -11,10 +11,35 @@ export function getPublicUrlFromRequest(request: Request): URL {
   const url = new URL(request.url)
   url.host = getPublicHostFromRequest(request)
   url.protocol = getPublicProtocolFromRequest(request)
-  url.port = ""
+  url.port = getPublicPortFromRequest(request)
   return url
 }
 
+/**
+ * The port carried by the public host (`localhost:3123` → `"3123"`), or an
+ * empty string when it carries none (`app.example.com`).
+ *
+ * Assigning `URL.host` a value *without* a port leaves the URL's previous port
+ * untouched (per the URL spec), so behind a reverse proxy the internal port
+ * would leak into the public URL. Every caller that assigns `host` must
+ * therefore assign the port as well — clearing it unconditionally instead
+ * would drop the port in local development, where the public host legitimately
+ * is `localhost:3123`.
+ *
+ * Parsing is delegated to the URL parser rather than searching for a `":"`:
+ * the colons inside a bracketed IPv6 literal (`[::1]`) are not port
+ * separators, and an out-of-range or non-numeric port must clear the port
+ * rather than leave the internal one in place.
+ */
+export function getPublicPortFromRequest(request: Request): string {
+  const host = getPublicHostFromRequest(request)
+  try {
+    return new URL(`http://${host}`).port
+  } catch {
+    return ""
+  }
+}
+
 export function getPublicProtocolFromRequest(
   request: Request,
 ): "http" | "https" {