OpenCode Desktop 1.18.20 – ChatGPT Plus OAuth fails with “Unexpected server error” and “Token exchange failed: 403”

Dear OpenCode Team,

I am experiencing a persistent OpenAI / ChatGPT Plus OAuth authorization issue in OpenCode Desktop on Windows.

Environment:

OS: Windows 11 x64
OpenCode: Desktop application
OpenCode version: 1.18.20
Architecture: x64
Electron: 42.3.3
Node.js: 24.15.0

The issue happens when I try to connect OpenAI / ChatGPT Plus through the browser OAuth flow.

The browser-side authorization appears to complete successfully. The browser shows that OpenCode has been connected to ChatGPT successfully.

However, after returning to OpenCode Desktop, the UI displays:

授权失败：Unexpected server error. Check server logs for details.

In English:

Authorization failed: Unexpected server error. Check server logs for details.

I then checked the OpenCode server log. The actual backend error is:

Error: Token exchange failed: 403

I have seen this error repeatedly during multiple authorization attempts.

Example log entries:

2026-08-21T08:19:47.612Z
Error: Token exchange failed: 403

2026-08-21T08:20:35.364Z
Error: Token exchange failed: 403

2026-08-21T08:40:12.442Z
Error: Token exchange failed: 403

There was also another related error:

Error: Failed to initiate device authorization

In some previous attempts, I also encountered:

OAuth callback timeout - authorization took too long

The current problem can be reproduced as follows:

Open OpenCode Desktop.
Go to the OpenAI provider authentication page.
Choose ChatGPT Plus / Pro browser authorization.
Complete the login and authorization in the browser.
The browser reports that OpenCode has successfully connected to ChatGPT.
Return to OpenCode Desktop.
OpenCode displays:
Authorization failed: Unexpected server error. Check server logs for details.
The server log records:
Token exchange failed: 403

I have already tried the following:

Confirmed that OpenCode is already updated to the latest version, 1.18.20.
Restarted OpenCode completely.
Repeated the OAuth process several times.
Closed old browser authorization pages and started a fresh authorization attempt.
Reset the OpenCode authentication file:
C:\Users\43590.local\share\opencode\auth.json

to:

{}

and then retried authentication.

Enabled a global VPN.
Confirmed that browser-side ChatGPT authorization itself succeeds.

The issue still remains.

Relevant log paths on my machine:

C:\Users\43590.local\share\opencode\log\opencode.log

Desktop logs:

C:\Users\43590\AppData\Roaming\ai.opencode.desktop\logs

OpenCode configuration directory:

C:\Users\43590.config\opencode

My understanding of the failure is:

OpenCode starts OAuth
→ Browser login and authorization succeed
→ Authorization code is returned
→ OpenCode attempts to exchange the authorization code for tokens
→ Token endpoint returns HTTP 403
→ OpenCode Desktop shows “Unexpected server error”

Could you please help confirm whether this is a known issue in OpenCode Desktop 1.18.20 on Windows?

I would especially like to know:

Why the token exchange request returns HTTP 403 even though browser authorization succeeds.
Whether this is related to OpenCode Desktop proxy handling.
Whether OpenCode Desktop requires explicit HTTP_PROXY / HTTPS_PROXY settings even when the VPN is running in global mode.
Whether there is a known workaround for ChatGPT Plus / Pro OAuth.
Whether this is a current OpenCode OAuth bug or an OpenAI-side compatibility issue.

I can provide the complete opencode.log, desktop logs, and any other debugging information if needed.

Thank you for your help.

Best regards