diff --git a/docker-compose.yml b/docker-compose.yml
index ff1a3e6..aaa74c3 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -15,6 +15,11 @@ services:
       # AUTH_PASSWORD: change-me
       # SESSION_SECRET: long-random-string-at-least-32-chars
       # SESSION_TTL_HOURS: "168"
+      # Cookie Secure: auto (default) follows http/https per request — use for WireGuard IP + public HTTPS.
+      # COOKIE_SECURE: auto
+      # Force: always | never (or true | false)
+      # Behind a TLS-terminating reverse proxy, keep TRUST_PROXY on so X-Forwarded-Proto is honored.
+      # TRUST_PROXY: "true"
     volumes:
       - ./data:/app/data
     restart: unless-stopped
diff --git a/src/server/auth.js b/src/server/auth.js
index 1bd1950..d687f7b 100644
--- a/src/server/auth.js
+++ b/src/server/auth.js
@@ -7,6 +7,10 @@ const PUBLIC_AUTH_PATHS = new Set([
   "POST /api/auth/logout",
 ]);
 
+/**
+ * @typedef {"auto"|"always"|"never"} CookieSecureMode
+ */
+
 /**
  * @typedef {object} AuthConfig
  * @property {boolean} enabled - Whether AUTH_PASSWORD is set.
@@ -14,8 +18,63 @@ const PUBLIC_AUTH_PATHS = new Set([
  * @property {string} password - Expected password when enabled.
  * @property {string} secret - HMAC secret for session cookies.
  * @property {number} ttlMs - Session lifetime in milliseconds.
+ * @property {CookieSecureMode} cookieSecureMode - How to set the Secure cookie flag.
  */
 
+/**
+ * Parses COOKIE_SECURE for dual HTTP/HTTPS access (e.g. WireGuard IP + public domain).
+ * - unset / "auto": Secure follows the request scheme (https → Secure)
+ * - "true" / "1" / "always": always Secure
+ * - "false" / "0" / "never": never Secure
+ * @returns {CookieSecureMode} Cookie Secure policy.
+ */
+function parseCookieSecureMode() {
+  const raw = (process.env.COOKIE_SECURE || "").trim().toLowerCase();
+  if (raw === "true" || raw === "1" || raw === "always") {
+    return "always";
+  }
+  if (raw === "false" || raw === "0" || raw === "never") {
+    return "never";
+  }
+  return "auto";
+}
+
+/**
+ * Whether this request was made over HTTPS (direct TLS or reverse-proxy proto).
+ * With Fastify trustProxy enabled, request.protocol reflects X-Forwarded-Proto.
+ * @param {import("fastify").FastifyRequest} request - Incoming request.
+ * @returns {boolean} True when the client connection is HTTPS.
+ */
+function requestIsHttps(request) {
+  if (request.protocol === "https") {
+    return true;
+  }
+  const forwarded = request.headers["x-forwarded-proto"];
+  if (typeof forwarded === "string" && forwarded.length > 0) {
+    return forwarded.split(",")[0].trim().toLowerCase() === "https";
+  }
+  if (Array.isArray(forwarded) && forwarded.length > 0) {
+    return String(forwarded[0]).split(",")[0].trim().toLowerCase() === "https";
+  }
+  return false;
+}
+
+/**
+ * Resolves the Secure flag for a Set-Cookie on this request.
+ * @param {import("fastify").FastifyRequest} request - Incoming request.
+ * @param {AuthConfig} config - Auth config.
+ * @returns {boolean} Whether the cookie should include Secure.
+ */
+function resolveCookieSecure(request, config) {
+  if (config.cookieSecureMode === "always") {
+    return true;
+  }
+  if (config.cookieSecureMode === "never") {
+    return false;
+  }
+  return requestIsHttps(request);
+}
+
 /**
  * Reads auth settings from the environment.
  * @returns {AuthConfig} Parsed config (enabled=false when AUTH_PASSWORD is empty).
@@ -41,6 +100,7 @@ function loadAuthConfig() {
     password,
     secret,
     ttlMs: Math.round(ttlHours * 60 * 60 * 1000),
+    cookieSecureMode: parseCookieSecureMode(),
   };
 }
 
@@ -130,16 +190,17 @@ function sessionFromRequest(request, config) {
 /**
  * Cookie options for setting/clearing the session.
  * @param {AuthConfig} config - Auth config.
+ * @param {import("fastify").FastifyRequest} request - Request used for Secure auto-detect.
  * @param {number} [maxAgeMs] - Max-Age in ms (omit when clearing).
  * @returns {import("@fastify/cookie").CookieSerializeOptions} Cookie options.
  */
-function sessionCookieOptions(config, maxAgeMs) {
+function sessionCookieOptions(config, request, maxAgeMs) {
   /** @type {import("@fastify/cookie").CookieSerializeOptions} */
   const opts = {
     path: "/",
     httpOnly: true,
     sameSite: "lax",
-    secure: process.env.NODE_ENV === "production",
+    secure: resolveCookieSecure(request, config),
   };
   if (typeof maxAgeMs === "number") {
     opts.maxAge = Math.floor(maxAgeMs / 1000);
@@ -197,7 +258,11 @@ function registerAuth(app, config) {
 
     const expiresAt = Date.now() + config.ttlMs;
     const token = createSessionToken(config.username, config.secret, expiresAt);
-    reply.setCookie(COOKIE_NAME, token, sessionCookieOptions(config, config.ttlMs));
+    reply.setCookie(
+      COOKIE_NAME,
+      token,
+      sessionCookieOptions(config, request, config.ttlMs),
+    );
 
     return {
       ok: true,
@@ -207,8 +272,8 @@ function registerAuth(app, config) {
     };
   });
 
-  app.post("/api/auth/logout", async (_request, reply) => {
-    reply.clearCookie(COOKIE_NAME, sessionCookieOptions(config));
+  app.post("/api/auth/logout", async (request, reply) => {
+    reply.clearCookie(COOKIE_NAME, sessionCookieOptions(config, request));
     return { ok: true, authenticated: false };
   });
 
@@ -240,8 +305,11 @@ module.exports = {
   COOKIE_NAME,
   createSessionToken,
   loadAuthConfig,
+  parseCookieSecureMode,
   parseSessionToken,
   registerAuth,
+  requestIsHttps,
+  resolveCookieSecure,
   sessionCookieOptions,
   sessionFromRequest,
   verifyPassword,
diff --git a/src/server/index.js b/src/server/index.js
index f716f86..77ee970 100644
--- a/src/server/index.js
+++ b/src/server/index.js
@@ -29,7 +29,12 @@ async function main() {
   const db = openDatabase(DATA_DIR);
   const queue = new DownloadQueue(db);
   const authConfig = loadAuthConfig();
-  const app = Fastify({ logger: true });
+  // Honor X-Forwarded-Proto / X-Forwarded-Host when behind a reverse proxy so
+  // session cookies can set Secure automatically on the public HTTPS domain
+  // while still working over plain HTTP (e.g. WireGuard IP).
+  const trustProxyRaw = (process.env.TRUST_PROXY || "true").trim().toLowerCase();
+  const trustProxy = !(trustProxyRaw === "false" || trustProxyRaw === "0");
+  const app = Fastify({ logger: true, trustProxy });
 
   // Allow POST/PATCH with Content-Type: application/json and an empty body
   app.addContentTypeParser(
@@ -90,9 +95,10 @@ async function main() {
   app.log.info(`Data dir: ${DATA_DIR}`);
   app.log.info(`Watch cron: ${WATCH_CRON}`);
   app.log.info(`Watch title interval: ${String(WATCH_TITLE_INTERVAL_HOURS)}h (one title per tick)`);
+  app.log.info(`Trust proxy: ${trustProxy}`);
   app.log.info(
     authConfig.enabled
-      ? `Auth: enabled (user=${authConfig.username})`
+      ? `Auth: enabled (user=${authConfig.username}, cookieSecure=${authConfig.cookieSecureMode})`
       : "Auth: disabled (set AUTH_PASSWORD to enable)",
   );
 }