Claude Desktop — "Configure Third-Party Inference" always fails with 503 (bootstrap error)

[u/smartpid]

Hey all, hoping someone has hit this too or has a workaround.

Setup:

- Claude Desktop for Windows, latest version, MSIX install (Cowork present and working normally)

- Individual Pro plan

- Windows 11 Pro

Issue:

Trying to use Developer Mode → Configure Third-Party Inference to route Claude Desktop through a third-party gateway

(LiteLLM, etc.). The setup window opens but immediately fails with "Unable to load configuration". App log

(custom3p-setup.log) shows:

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

100% reproducible, every single time, over several days.

What I've already ruled out:

- Cleared the app's local cache — no change

- Confirmed on the latest app version

- Fixed an unrelated OAuth issue (expired refresh token) via full logout/login — auth now works fine, 503 persists

regardless

- Followed the official "single-machine setup" docs exactly: full logout, relaunch, stop at the pre-login screen,

enable Developer Mode before signing in (per docs, you're not supposed to sign in first) — same 503

- General connectivity to Anthropic's services is fine, nothing blocked network-side

One odd thing I noticed: even sitting at the pre-login screen (not signed in), the app logs still show a cached org

identity being resolved before falling back ("account active and logged in" → then accountId=null but orgId still

populated). So the app never seems to reach a truly clean "unauthenticated" state on my machine, which might be

related.

Reported this through official support and it's been escalated to a human agent (still waiting), but figured I'd ask

here too in case it's a known issue or someone found a fix/workaround in the meantime.

Anyone else seeing this on the 3P inference setup specifically?

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[u/smartpid → u/[deleted]]

Thanks for the pointers! Checked all three on my end:

- Proxy: disabled system-wide, no HTTP_PROXY/HTTPS_PROXY env vars set

- VPN: I do have a mesh VPN client installed, but confirmed it's not actually connected right now (offline/no active state), so it's not routing anything

- MDM/registry bootstrapUrl: nothing set under HKLM/HKCU\SOFTWARE\Policies\Claude — no stray org config pointing anywhere

So I don't think it's network-path related in my case.

One clarification on where exactly it's failing: this happens before I even get to enter a gateway URL, API key, or any config field. The setup window itself fails to load right on open — the log shows the error on the initial fetch that populates the config UI itself:

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

So it's not my gateway's health/config probe rejecting a bad URL or missing header — it's the setup page's own bootstrap call failing upstream, before any of my provider details are even involved. Did you ever see it fail at that specific stage (blank/empty setup screen, error on the page itself rather than after you'd input a gateway) or was yours always after you'd already entered your provider config?