diff --git a/browse/src/terminal-agent.ts b/browse/src/terminal-agent.ts
index 2e39d99e4..0b9dc0698 100644
--- a/browse/src/terminal-agent.ts
+++ b/browse/src/terminal-agent.ts
@@ -598,12 +598,10 @@ function buildServer() {
         // first that matches a known token.
         const protoHeader = req.headers.get('sec-websocket-protocol') || '';
         let token: string | null = null;
-        let acceptedProtocol: string | null = null;
         for (const raw of protoHeader.split(',').map(s => s.trim()).filter(Boolean)) {
           const candidate = raw.startsWith('gstack-pty.') ? raw.slice('gstack-pty.'.length) : raw;
           if (validTokens.has(candidate)) {
             token = candidate;
-            acceptedProtocol = raw;
             break;
           }
         }
@@ -632,13 +630,13 @@ function buildServer() {
         // sessionsById so /internal/restart and (Commit 3) re-attach
         // lookups can find it.
         const sessionId = validTokens.get(token) ?? null;
+        // No explicit Sec-WebSocket-Protocol echo: Bun >= 1.3 auto-echoes the
+        // first offered protocol in the 101 response, so setting the header
+        // here produced a DUPLICATE header — strict clients (Chromium, python
+        // websockets) reject the handshake per RFC 6455 and the sidebar
+        // terminal could never connect. Verified on Bun 1.3.6.
         const upgraded = server.upgrade(req, {
           data: { cookie: token, sessionId },
-          // Echo the protocol back so the browser accepts the upgrade.
-          // Required when the client sends Sec-WebSocket-Protocol — the
-          // server MUST select one of the offered protocols, otherwise
-          // the browser closes the connection immediately.
-          ...(acceptedProtocol ? { headers: { 'Sec-WebSocket-Protocol': acceptedProtocol } } : {}),
         });
         return upgraded ? undefined : new Response('upgrade failed', { status: 500 });
       }
diff --git a/browse/test/terminal-agent-integration.test.ts b/browse/test/terminal-agent-integration.test.ts
index cdcbe8de5..c4a72c019 100644
--- a/browse/test/terminal-agent-integration.test.ts
+++ b/browse/test/terminal-agent-integration.test.ts
@@ -227,6 +227,45 @@ describe('terminal-agent: PTY round-trip via real WebSocket (Cookie auth)', () =
     expect(resp.headers.get('sec-websocket-protocol')).toBe(`gstack-pty.${token}`);
   });
 
+  test('upgrade response contains exactly ONE Sec-WebSocket-Protocol header', async () => {
+    // RFC 6455: the server MUST select at most one subprotocol. Bun >= 1.3
+    // auto-echoes the first offered protocol in server.upgrade(), so a
+    // manual echo on top of that produced TWO Sec-WebSocket-Protocol
+    // headers — and strict clients (Chromium, python websockets) reject the
+    // handshake, leaving the sidebar terminal permanently disconnected.
+    //
+    // Headers.get() normalizes duplicates away, so this test handshakes
+    // over a raw socket and counts header lines in the response head.
+    const token = 'dup-proto-token-must-be-at-least-seventeen-chars';
+    await grantToken(token);
+
+    const head = await new Promise<string>((resolve, reject) => {
+      const req =
+        'GET /ws HTTP/1.1\r\n' +
+        `Host: 127.0.0.1:${agentPort}\r\n` +
+        'Connection: Upgrade\r\n' +
+        'Upgrade: websocket\r\n' +
+        'Sec-WebSocket-Version: 13\r\n' +
+        'Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==\r\n' +
+        `Sec-WebSocket-Protocol: gstack-pty.${token}\r\n` +
+        'Origin: chrome-extension://test-extension-id\r\n' +
+        '\r\n';
+      let buf = '';
+      const socket = require('net').connect(agentPort, '127.0.0.1', () => socket.write(req));
+      socket.setTimeout(5000, () => { socket.destroy(); reject(new Error('handshake timeout')); });
+      socket.on('data', (chunk: Buffer) => {
+        buf += chunk.toString('utf8');
+        const end = buf.indexOf('\r\n\r\n');
+        if (end !== -1) { socket.destroy(); resolve(buf.slice(0, end)); }
+      });
+      socket.on('error', reject);
+    });
+
+    expect(head).toContain('101');
+    const protoLines = head.split('\r\n').filter(l => l.toLowerCase().startsWith('sec-websocket-protocol:'));
+    expect(protoLines).toEqual([`Sec-WebSocket-Protocol: gstack-pty.${token}`]);
+  });
+
   test('Sec-WebSocket-Protocol auth: rejects unknown token even with valid Origin', async () => {
     const resp = await fetch(`http://127.0.0.1:${agentPort}/ws`, {
       headers: {