[BUG] Claude Desktop – "Configure third-party inference" setup page fails with 503 (bootstrap request)

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) and this hasn't been reported yet
- [x] This is a single bug report (please file separate reports for different bugs)
- [x] I am using the latest version of Claude Code

### What's Wrong?

Issue:
Opening the third-party inference setup page (app://localhost/setup-desktop-3p) always fails with "Unable to load
configuration". Log excerpt (custom3p-setup.log):
[bootstrap] Eager fetch failed: Bootstrap request failed: 503 Service Unavailable

Troubleshooting already performed (issue persists in every case):
1. Cleared Claude-3p profile cache (Cache, Code Cache, GPUCache, Dawn caches) — no config/credentials touched.
2. Confirmed app is on latest version (1.52386.0).
3. Found and fixed an unrelated OAuth issue (refresh token invalid_grant / session_stale_relogin) via full logout
("sign out of all devices") + fresh login. Auth now works correctly.
4. Despite a fully valid session and clean cache, the bootstrap call for setup-desktop-3p still returns 503 on every
attempt, across multiple days/sessions.
5. General network connectivity to Anthropic/claude.ai confirmed working (no proxy/firewall block detected).

### What Should Happen?

Request:
Please check server-side status of the third-party-inference bootstrap endpoint, and confirm whether this account/org
is provisioned/entitled for this feature (the menu item is visible under Developer Mode, but the backend consistently
rejects the config fetch with 503).

### Error Messages/Logs

```shell
[bootstrap] Eager fetch failed: Bootstrap request failed: 503 Service Unavailable
```

### Steps to Reproduce

Steps to reproduce:

1. Open Claude Desktop (v1.52386.0, Windows 11).
2. Sign in normally with your Anthropic account.
3. Enable Developer Mode in Claude Desktop settings.
4. From the menu, open "Configure third-party inference".
5. The setup window opens at app://localhost/setup-desktop-3p.
6. The page fails to load and shows: "Unable to load configuration", with a toast: "Unable to update saved
configurations".
7. Clicking "Retry" reproduces the same error every time.

Expected result: the third-party inference configuration page loads normally.

Actual result: page fails to load; custom3p-setup.log shows:
[bootstrap] Eager fetch failed: Bootstrap request failed: 503 Service Unavailable

Additional notes:
- Reproducible 100% of the time, across multiple sessions/days.
- Persists after: clearing the app cache, confirming latest app version, and a full sign-out/sign-in (which did fix a
separate, unrelated OAuth invalid_grant issue found in the logs).
- General connectivity to claude.ai / Anthropic services is otherwise working fine.

### Claude Model

Not sure / Multiple models

### Is this a regression?

No, this never worked

### Last Working Version

_No response_

### Claude Code Version

Claude 1.52386.0 (1003ca) 2026-09-10T05:55:17.000Z

### Platform

Anthropic API

### Operating System

Windows

### Terminal/Shell

Windows Terminal

### Additional Information

_No response_

Start by reviewing the reported bootstrap failure in custom3p-setup.log and the reproduction steps for app://localhost/setup-desktop-3p. Check the third-party-inference bootstrap endpoint and account or organization provisioning status; done means the setup page loads and Retry no longer produces the 503 configuration error.

Written by the indexing model from the issue text.