External backend: UI actions fail with "Error: Failed to fetch" even though /config works

[Gandalf-Le-Dev]

Summary

When Goose Desktop is configured to use an external backend, the connection appears successful, but UI actions (notably provider setup flows) fail with:

• Error: Failed to fetch

The backend is reachable and returns valid responses for config/session/catalog endpoints.

Environment

• Goose Desktop: latest (as of 2026-03-06)
• goosed server: goose-server 1.27.0
• External backend host: remote dev environment (not localhost)
• External backend URL used in Desktop settings: http://dev.company.net:12604
• Secret key configured and valid

Reproduction

1. Run goosed agent remotely in a dev environment.
2. Expose it through a gateway/reverse proxy (no SSH tunnel), endpoint:
   • http://dev.company.net:12604
3. In Goose Desktop:
   • Enable Use external server
   • Set URL to http://dev.company.net:12604
   • Set secret key
   • Restart Desktop
4. Open provider UI (welcome/provider setup / add provider modal).

Actual behavior

• UI shows Error: Failed to fetch on provider-related actions.
• But direct HTTP checks from client and server succeed.

Expected behavior

If external backend URL is accepted and reachable, Desktop UI should operate fully (provider catalog, config reads/writes, etc.), not fail with generic fetch errors.

Diagnostics

The external backend is healthy:

curl -s http://dev.company.net:12604/status -H "x-secret-key: test"
# ok

[lifeizhou-ap]

will be in the release V1.29.0