diff --git a/src/TestWSServer.zig b/src/TestWSServer.zig
index 6d28acf3..35e9e29e 100644
--- a/src/TestWSServer.zig
+++ b/src/TestWSServer.zig
@@ -95,6 +95,22 @@ fn handleClient(client: posix.socket_t) void {
     const key_end = std.mem.indexOfScalarPos(u8, request, key_line_start, '\r') orelse return;
     const key = request[key_line_start..key_end];
 
+    // Capture the request's Cookie header value (if any) so the test
+    // fixture can ask for it back via the `get-cookie` command. Copy out
+    // of `request` because the WS frame loop below reuses `buf` for
+    // incoming frames, invalidating the slice. Buffer is sized to match
+    // `buf` since any cookie that fit in the request fits here.
+    var cookie_buf: [4096]u8 = undefined;
+    var cookie_len: usize = 0;
+    const cookie_header = "Cookie: ";
+    if (std.mem.indexOf(u8, request, cookie_header)) |cookie_start| {
+        const value_start = cookie_start + cookie_header.len;
+        const value_end = std.mem.indexOfScalarPos(u8, request, value_start, '\r') orelse value_start;
+        const value = request[value_start..value_end];
+        cookie_len = @min(value.len, cookie_buf.len);
+        @memcpy(cookie_buf[0..cookie_len], value[0..cookie_len]);
+    }
+
     // Compute accept key
     var hasher = std.crypto.hash.Sha1.init(.{});
     hasher.update(key);
@@ -128,7 +144,7 @@ fn handleClient(client: posix.socket_t) void {
 
         // Handle commands or echo
         if (frame.opcode == 1) { // Text
-            handleTextMessage(client, frame.payload) catch break;
+            handleTextMessage(client, frame.payload, cookie_buf[0..cookie_len]) catch break;
         } else if (frame.opcode == 2) { // Binary
             handleBinaryMessage(client, frame.payload) catch break;
         }
@@ -233,12 +249,20 @@ const RecvBuffer = struct {
     }
 };
 
-fn handleTextMessage(client: posix.socket_t, payload: []const u8) !void {
+fn handleTextMessage(client: posix.socket_t, payload: []const u8, cookie_header: []const u8) !void {
     // Command: force-close - close socket immediately without close frame
     if (std.mem.eql(u8, payload, "force-close")) {
         return error.ForceClose;
     }
 
+    // Command: get-cookie - send the Cookie header value the upgrade
+    // request carried (empty string if none). Used by the cookie-on-
+    // upgrade regression test.
+    if (std.mem.eql(u8, payload, "get-cookie")) {
+        try sendFrame(client, 1, "cookie:", cookie_header);
+        return;
+    }
+
     // Command: send-large:N - send a message of N bytes
     if (std.mem.startsWith(u8, payload, "send-large:")) {
         const size_str = payload["send-large:".len..];
diff --git a/src/browser/HttpClient.zig b/src/browser/HttpClient.zig
index 81c4dc14..6eb3611e 100644
--- a/src/browser/HttpClient.zig
+++ b/src/browser/HttpClient.zig
@@ -1247,10 +1247,12 @@ pub const Request = struct {
             .origin_url = self.cookie_origin,
             .is_navigation = self.resource_type == .document,
         });
-        const written = aw.written();
-        if (written.len == 0) return null;
+        if (aw.written().len == 0) {
+            return null;
+        }
         try aw.writer.writeByte(0);
-        return written.ptr[0..written.len :0];
+        const written = aw.written();
+        return written.ptr[0 .. written.len - 1 :0];
     }
 
     pub fn deinit(self: *const Request) void {
diff --git a/src/browser/URL.zig b/src/browser/URL.zig
index 620e63a2..f2650164 100644
--- a/src/browser/URL.zig
+++ b/src/browser/URL.zig
@@ -464,8 +464,8 @@ pub fn getProtocol(raw: [:0]const u8) []const u8 {
     return raw[0 .. pos + 1];
 }
 
-pub fn isHTTPS(raw: [:0]const u8) bool {
-    return std.mem.startsWith(u8, raw, "https:");
+pub fn isSecure(raw: [:0]const u8) bool {
+    return std.mem.startsWith(u8, raw, "https:") or std.mem.startsWith(u8, raw, "wss:");
 }
 
 pub fn getHostname(raw: [:0]const u8) []const u8 {
diff --git a/src/browser/tests/net/websocket.html b/src/browser/tests/net/websocket.html
index 2c2e315b..e85813b2 100644
--- a/src/browser/tests/net/websocket.html
+++ b/src/browser/tests/net/websocket.html
@@ -579,3 +579,169 @@
   });
 }
 </script>
+
+<script id=cookies_on_upgrade type=module>
+{
+  // Cookies set on the page should ride along with the WebSocket
+  // upgrade request as a Cookie header — same as any other same-origin
+  // HTTP request. Without this, cookie-authenticated WS endpoints
+  // (Phoenix LiveView, anything session-gated) reject the upgrade.
+  //
+  // RFC 6455 §4.1: the upgrade is an HTTP/1.1 request and follows
+  // normal HTTP cookie semantics.
+  // RFC 6265 §5.4: cookies attach to "any HTTP request" matching
+  // the cookie's domain/path; ports are not part of cookie scope, so
+  // a cookie set on 127.0.0.1:9582 attaches to a 127.0.0.1:9584 WS.
+  // Set with path=/ so the cookie applies to the WS server's "/" path
+  // (without an explicit Path attribute, default-path uses the page's
+  // directory which doesn't match the WS endpoint).
+  document.cookie = 'ws_token=session_xyz; path=/';
+
+  // Sanity-check the cookie did land in the jar before opening the WS.
+  testing.expectEqual(true, document.cookie.includes('ws_token=session_xyz'));
+
+  const state = await testing.async();
+  let received = [];
+
+  let ws = new WebSocket('ws://127.0.0.1:9584/');
+
+  ws.addEventListener('open', () => {
+    ws.send('get-cookie');
+  });
+
+  ws.addEventListener('message', (e) => {
+    received.push(e.data);
+    ws.close();
+  });
+
+  ws.addEventListener('close', () => {
+    state.resolve();
+  });
+
+  await state.done(() => {
+    testing.expectEqual(['cookie:ws_token=session_xyz'], received);
+  });
+
+  // Clean up so other tests on this fixture don't see this cookie.
+  document.cookie = 'ws_token=; path=/; Max-Age=0';
+}
+</script>
+
+<script id=cookies_on_upgrade_path_scoped_out type=module>
+{
+  // Path scoping: a cookie scoped to /restricted MUST NOT attach to
+  // a WS upgrade for "/". Guards against the patch over-sending cookies.
+  // Each test starts by clearing all cookies on path=/ since this
+  // fixture's <script> blocks share a single page/cookie jar.
+  for (const pair of document.cookie.split(';')) {
+    const name = pair.split('=')[0].trim();
+    if (name) document.cookie = `${name}=; path=/; Max-Age=0`;
+  }
+  document.cookie = 'restricted=nope; path=/restricted';
+
+  const state = await testing.async();
+  let received = [];
+
+  let ws = new WebSocket('ws://127.0.0.1:9584/');
+
+  ws.addEventListener('open', () => { ws.send('get-cookie'); });
+  ws.addEventListener('message', (e) => { received.push(e.data); ws.close(); });
+  ws.addEventListener('close', () => { state.resolve(); });
+
+  await state.done(() => {
+    // Server should report an empty Cookie header.
+    testing.expectEqual(['cookie:'], received);
+  });
+
+  document.cookie = 'restricted=; path=/restricted; Max-Age=0';
+}
+</script>
+
+<script id=cookies_on_upgrade_samesite_strict type=module>
+{
+  // SameSite=Strict on a same-site upgrade (origin and target share
+  // host 127.0.0.1) MUST attach. The patch passes is_navigation=false;
+  // the cookie jar's SameSite logic only blocks Strict cookies when
+  // the request is cross-site, regardless of navigation status. This
+  // test pins the same-site path so a regression that breaks same-site
+  // detection on WS upgrades fails loudly.
+  for (const pair of document.cookie.split(';')) {
+    const name = pair.split('=')[0].trim();
+    if (name) document.cookie = `${name}=; path=/; Max-Age=0`;
+  }
+  document.cookie = 'strict_token=s; path=/; SameSite=Strict';
+
+  const state = await testing.async();
+  let received = [];
+
+  let ws = new WebSocket('ws://127.0.0.1:9584/');
+
+  ws.addEventListener('open', () => { ws.send('get-cookie'); });
+  ws.addEventListener('message', (e) => { received.push(e.data); ws.close(); });
+  ws.addEventListener('close', () => { state.resolve(); });
+
+  await state.done(() => {
+    testing.expectEqual(['cookie:strict_token=s'], received);
+  });
+
+  document.cookie = 'strict_token=; path=/; Max-Age=0';
+}
+</script>
+
+<script id=cookies_on_upgrade_empty_jar type=module>
+{
+  // With no matching cookies, no Cookie header should be added.
+  // The patch has a `if (cookie_buf.written().len > 0)` guard;
+  // this test exercises the false branch so a regression that
+  // sends an empty `Cookie: ` header is caught.
+  for (const pair of document.cookie.split(';')) {
+    const name = pair.split('=')[0].trim();
+    if (name) document.cookie = `${name}=; path=/; Max-Age=0`;
+  }
+
+  const state = await testing.async();
+  let received = [];
+
+  let ws = new WebSocket('ws://127.0.0.1:9584/');
+
+  ws.addEventListener('open', () => { ws.send('get-cookie'); });
+  ws.addEventListener('message', (e) => { received.push(e.data); ws.close(); });
+  ws.addEventListener('close', () => { state.resolve(); });
+
+  await state.done(() => {
+    // Server's get-cookie returns "cookie:" with whatever Cookie header
+    // value it captured — empty string means no header was sent.
+    testing.expectEqual(['cookie:'], received);
+  });
+}
+</script>
+
+<script id=cookies_on_upgrade_multiple type=module>
+{
+  // Multiple matching cookies must be joined with "; " per RFC 6265 §5.4.
+  for (const pair of document.cookie.split(';')) {
+    const name = pair.split('=')[0].trim();
+    if (name) document.cookie = `${name}=; path=/; Max-Age=0`;
+  }
+  document.cookie = 'a=1; path=/';
+  document.cookie = 'b=2; path=/';
+
+  const state = await testing.async();
+  let received = [];
+
+  let ws = new WebSocket('ws://127.0.0.1:9584/');
+
+  ws.addEventListener('open', () => { ws.send('get-cookie'); });
+  ws.addEventListener('message', (e) => { received.push(e.data); ws.close(); });
+  ws.addEventListener('close', () => { state.resolve(); });
+
+  await state.done(() => {
+    // Order in the Cookie header isn't strictly specified by RFC 6265
+    // but the jar iterates insertion order, so a=1 lands first.
+    testing.expectEqual(['cookie:a=1; b=2'], received);
+  });
+
+  document.cookie = 'a=; path=/; Max-Age=0';
+  document.cookie = 'b=; path=/; Max-Age=0';
+}
+</script>
diff --git a/src/browser/webapi/net/WebSocket.zig b/src/browser/webapi/net/WebSocket.zig
index 4d2b589f..c80b520f 100644
--- a/src/browser/webapi/net/WebSocket.zig
+++ b/src/browser/webapi/net/WebSocket.zig
@@ -133,9 +133,29 @@ pub fn init(url: []const u8, protocols: [][]const u8, exec: *const Execution) !*
 
     var headers = try http_client.newHeaders();
     errdefer headers.deinit();
+    var has_extra_headers = false;
+
     if (protocols.len > 0) {
         const header = try std.fmt.allocPrintSentinel(arena, "Sec-WebSocket-Protocol: {s}", .{try std.mem.join(arena, ", ", protocols)}, 0);
         try headers.add(header);
+        has_extra_headers = true;
+    }
+
+    {
+        var buf: std.Io.Writer.Allocating = .init(arena);
+        try exec.session.cookie_jar.forRequest(resolved_url, &buf.writer, .{
+            .is_http = true,
+            .is_navigation = false,
+            .origin_url = exec.url.*,
+        });
+        if (buf.written().len > 0) {
+            try buf.writer.writeByte(0);
+            const written = buf.written();
+            try conn.setCookies(written.ptr[0 .. written.len - 1 :0]);
+        }
+    }
+
+    if (has_extra_headers) {
         try conn.setHeaders(&headers);
     }
 
diff --git a/src/browser/webapi/storage/Cookie.zig b/src/browser/webapi/storage/Cookie.zig
index 0e023705..b7c721ea 100644
--- a/src/browser/webapi/storage/Cookie.zig
+++ b/src/browser/webapi/storage/Cookie.zig
@@ -577,7 +577,7 @@ pub const Jar = struct {
         const target = PreparedUri{
             .host = URL.getHostname(target_url),
             .path = URL.getPathname(target_url),
-            .secure = URL.isHTTPS(target_url),
+            .secure = URL.isSecure(target_url),
         };
         const same_site = try areSameSite(opts.origin_url, target.host);
 
diff --git a/src/browser/webapi/storage/CookieStore.zig b/src/browser/webapi/storage/CookieStore.zig
index 4d679e54..cf365537 100644
--- a/src/browser/webapi/storage/CookieStore.zig
+++ b/src/browser/webapi/storage/CookieStore.zig
@@ -76,7 +76,7 @@ fn onCookieChanged(ctx: *anyopaque, data: *const Notification.CookieChanged) !vo
     const target = Cookie.PreparedUri{
         .host = URL.getHostname(doc_url),
         .path = URL.getPathname(doc_url),
-        .secure = URL.isHTTPS(doc_url),
+        .secure = URL.isSecure(doc_url),
     };
     if (target.host.len == 0) return;
 
@@ -353,7 +353,7 @@ fn matchCookies(
     const target = Cookie.PreparedUri{
         .host = URL.getHostname(url_resolved),
         .path = URL.getPathname(url_resolved),
-        .secure = URL.isHTTPS(url_resolved),
+        .secure = URL.isSecure(url_resolved),
     };
     if (target.host.len == 0) return error.SecurityError;
 
@@ -408,7 +408,7 @@ fn storeCookie(exec: *const Execution, init: CookieInit) !void {
         if (std.mem.indexOfAny(u8, d, ";\r\n\x00") != null) return error.InvalidCookieDomain;
     }
 
-    const is_https = URL.isHTTPS(url);
+    const is_https = URL.isSecure(url);
     // Per spec, SameSite=None requires Secure. CookieStore additionally
     // marks any cookie written from an HTTPS document as Secure.
     const secure = is_https or init.sameSite == .none;
diff --git a/src/cdp/domains/network.zig b/src/cdp/domains/network.zig
index 5a33df88..864c72f5 100644
--- a/src/cdp/domains/network.zig
+++ b/src/cdp/domains/network.zig
@@ -233,7 +233,7 @@ fn getCookies(cmd: *CDP.Command) !void {
         urls.appendAssumeCapacity(.{
             .host = try Cookie.parseDomain(cmd.arena, url, null),
             .path = try Cookie.parsePath(cmd.arena, url, null),
-            .secure = URL.isHTTPS(url),
+            .secure = URL.isSecure(url),
         });
     }
 
diff --git a/src/cdp/domains/storage.zig b/src/cdp/domains/storage.zig
index dac20e10..7e38c132 100644
--- a/src/cdp/domains/storage.zig
+++ b/src/cdp/domains/storage.zig
@@ -153,7 +153,7 @@ pub fn setCdpCookie(cookie_jar: *CookieJar, param: CdpCookie) !void {
         const domain = try Cookie.parseDomain(a, param.url, param.domain);
         const path = if (param.path == null) "/" else try Cookie.parsePath(a, null, param.path);
 
-        const secure = if (param.secure) |s| s else if (param.url) |url| URL.isHTTPS(url) else false;
+        const secure = if (param.secure) |s| s else if (param.url) |url| URL.isSecure(url) else false;
 
         break :blk Cookie{
             .arena = arena,