diff --git a/frontend/.env.example b/frontend/.env.example
index aeb458e5b..afd5b3cfb 100644
--- a/frontend/.env.example
+++ b/frontend/.env.example
@@ -20,3 +20,12 @@
 # Defaults to localhost — only override for non-local deployments.
 # DEER_FLOW_INTERNAL_GATEWAY_BASE_URL="http://localhost:8001"
 # DEER_FLOW_TRUSTED_ORIGINS="http://localhost:3000,http://localhost:2026"
+
+# Extra hosts allowed to load Next.js dev-server resources (`/_next/*`, fonts,
+# HMR). Development only — production builds ignore it.
+# Needed to open `pnpm dev` / `make docker-start` on anything other than
+# localhost, such as a LAN address or a proxied hostname: without it Next.js
+# answers those requests with 403, so the page renders server-side but never
+# hydrates and no interaction (including the login form) works.
+# Comma-separated; a full URL is reduced to its host.
+# DEER_FLOW_DEV_ALLOWED_ORIGINS="192.168.1.10,*.local"
diff --git a/frontend/AGENTS.md b/frontend/AGENTS.md
index 33ba03568..1040ceed8 100644
--- a/frontend/AGENTS.md
+++ b/frontend/AGENTS.md
@@ -128,6 +128,8 @@ NEXT_PUBLIC_LANGGRAPH_BASE_URL=http://localhost:8001/api
 
 Leave these unset for the standard `make dev` / Docker flow, where nginx serves the public `/api/langgraph/*` prefix and rewrites it to Gateway's native `/api/*` routes.
 
+To reach a dev server on anything other than localhost — a LAN address, or a proxied hostname — list the host in `DEER_FLOW_DEV_ALLOWED_ORIGINS` (comma-separated; a full URL is reduced to its host). It feeds Next's `allowedDevOrigins`, which gates `/_next/*`, fonts, and HMR. Without it those requests get a 403 and the page renders server-side but never hydrates, so nothing on it — including the login form — responds. Development only; production builds ignore it.
+
 ## Resources
 
 - [LangGraph Documentation](https://langchain-ai.github.io/langgraph/)
diff --git a/frontend/next.config.js b/frontend/next.config.js
index 7007d59fc..c1b984c2f 100644
--- a/frontend/next.config.js
+++ b/frontend/next.config.js
@@ -3,6 +3,7 @@
  * for Docker builds.
  */
 import "./src/env.js";
+import { getAllowedDevOrigins } from "./src/dev-origins.js";
 
 function getInternalServiceURL(envKey, fallbackURL) {
   const configured = process.env[envKey]?.trim();
@@ -25,6 +26,7 @@ const config = {
     defaultLocale: "en",
   },
   devIndicators: false,
+  allowedDevOrigins: getAllowedDevOrigins(),
   async rewrites() {
     const rewrites = [];
     const gatewayURL = getInternalServiceURL(
diff --git a/frontend/src/dev-origins.js b/frontend/src/dev-origins.js
new file mode 100644
index 000000000..b092a9a6d
--- /dev/null
+++ b/frontend/src/dev-origins.js
@@ -0,0 +1,59 @@
+/**
+ * Hosts allowed to load Next.js dev-server resources (`/_next/*`,
+ * `/__nextjs_font/*`, HMR) from an origin other than the one `pnpm dev` was
+ * started on.
+ *
+ * Next.js answers those requests with 403 unless the host is listed, so a dev
+ * stack opened on a LAN address or a proxied hostname serves the SSR HTML but
+ * never hydrates: the page renders and nothing on it responds.
+ *
+ * Dev-only — Next ignores `allowedDevOrigins` in production builds.
+ */
+
+/**
+ * Reduce one entry to the bare host `allowedDevOrigins` matches against.
+ *
+ * Next matches on host alone, so an entry that still carries a scheme, port, or
+ * path matches nothing and leaves the caller with the same 403 they were trying
+ * to fix. Accept the URL people naturally copy out of the address bar.
+ *
+ * @param {string} value
+ * @returns {string} bare host, or `""` if the entry was empty
+ */
+function normalizeHost(value) {
+  let host = value.trim();
+  if (!host) return "";
+
+  host = host.replace(/^[a-z][a-z0-9+.-]*:\/\//i, "");
+  host = host.replace(/[/?#].*$/, "");
+
+  const bracketedIpv6 = /^\[([^\]]+)\](?::\d+)?$/.exec(host);
+  if (bracketedIpv6) return bracketedIpv6[1];
+
+  // A bare IPv6 literal has several colons and no port to strip; only a single
+  // colon can be a `host:port` separator.
+  if ((host.match(/:/g) ?? []).length === 1) {
+    host = host.replace(/:\d+$/, "");
+  }
+  return host;
+}
+
+/**
+ * Parse a comma-separated host list into the shape `allowedDevOrigins` expects.
+ *
+ * @param {string | undefined} raw
+ * @returns {string[]}
+ */
+export function parseAllowedDevOrigins(raw) {
+  return (raw ?? "").split(",").map(normalizeHost).filter(Boolean);
+}
+
+/**
+ * Read the configured hosts from the environment.
+ *
+ * @param {Record<string, string | undefined>} [env]
+ * @returns {string[]}
+ */
+export function getAllowedDevOrigins(env = process.env) {
+  return parseAllowedDevOrigins(env.DEER_FLOW_DEV_ALLOWED_ORIGINS);
+}
diff --git a/frontend/tests/unit/dev-origins.test.ts b/frontend/tests/unit/dev-origins.test.ts
new file mode 100644
index 000000000..382772330
--- /dev/null
+++ b/frontend/tests/unit/dev-origins.test.ts
@@ -0,0 +1,69 @@
+import { describe, expect, test } from "@rstest/core";
+
+import { getAllowedDevOrigins, parseAllowedDevOrigins } from "@/dev-origins";
+
+describe("parseAllowedDevOrigins", () => {
+  test("returns an empty list when unset or empty", () => {
+    expect(parseAllowedDevOrigins(undefined)).toEqual([]);
+    expect(parseAllowedDevOrigins("")).toEqual([]);
+    expect(parseAllowedDevOrigins("   ")).toEqual([]);
+  });
+
+  test("splits a comma-separated list and trims each entry", () => {
+    expect(parseAllowedDevOrigins(" 192.168.1.10 , dev.example.com ")).toEqual([
+      "192.168.1.10",
+      "dev.example.com",
+    ]);
+  });
+
+  test("drops empty entries from trailing or doubled commas", () => {
+    expect(parseAllowedDevOrigins("a.example,,b.example,")).toEqual([
+      "a.example",
+      "b.example",
+    ]);
+  });
+
+  test("reduces a pasted URL to the bare host Next matches on", () => {
+    expect(parseAllowedDevOrigins("http://192.168.1.10:2026")).toEqual([
+      "192.168.1.10",
+    ]);
+    expect(parseAllowedDevOrigins("https://dev.example.com/")).toEqual([
+      "dev.example.com",
+    ]);
+    expect(parseAllowedDevOrigins("http://dev.example.com/login?x=1")).toEqual([
+      "dev.example.com",
+    ]);
+  });
+
+  test("preserves wildcard patterns", () => {
+    expect(parseAllowedDevOrigins("*.local, *.example.com")).toEqual([
+      "*.local",
+      "*.example.com",
+    ]);
+  });
+
+  test("strips the port from a bracketed IPv6 host without mangling the address", () => {
+    expect(parseAllowedDevOrigins("[::1]:2026")).toEqual(["::1"]);
+    expect(parseAllowedDevOrigins("http://[fe80::1]:3000")).toEqual([
+      "fe80::1",
+    ]);
+  });
+
+  test("leaves a bare IPv6 literal intact", () => {
+    // Several colons and no port to strip — treating the last group as a port
+    // would corrupt the address.
+    expect(parseAllowedDevOrigins("fe80::1")).toEqual(["fe80::1"]);
+  });
+});
+
+describe("getAllowedDevOrigins", () => {
+  test("reads DEER_FLOW_DEV_ALLOWED_ORIGINS", () => {
+    expect(
+      getAllowedDevOrigins({ DEER_FLOW_DEV_ALLOWED_ORIGINS: "192.168.1.10" }),
+    ).toEqual(["192.168.1.10"]);
+  });
+
+  test("defaults to an empty list, keeping localhost-only the default", () => {
+    expect(getAllowedDevOrigins({})).toEqual([]);
+  });
+});