Sunshine error 503 in Moonlight: the three causes and how to fix each

Checked October 30, 2026 against Moonlight PC v6.2.0 and Sunshine v2026.914.233613 (latest stable releases) and their documentation. Sources are listed at the end of the page.

The three messages behind a 503

We read Sunshine’s request handler (src/nvhttp.cpp on the master branch) to see exactly when it returns 503. There are three cases:

Cause 1: video capture or encoding failed

This is the one most people see. Sunshine prepares the display before it checks the encoders, because the active GPU can change after a hot-plug, a driver crash or a new primary monitor. If no working display and encoder pair is found, it gives up with a 503.

1. Give it a display. The host monitor needs to be connected and on, or use a dummy plug or a virtual display. On Windows, setting dd_configuration_option to ensure_active lets Sunshine activate an inactive display.
2. Check output_name. If it points at a display that isn’t there any more, capture fails. Look at the list of detected displays in the Sunshine log and pick one that is connected, or clear the setting to use the default.
3. Windows with two GPUs: Desktop Duplication only captures from the GPU driving the display, so the display must be connected to the GPU you want to encode on.
4. Linux with XDG Portal capture: a stale or rejected permission token can stop capture at startup. In one closed Sunshine issue (#5673), the log said “Unable to find display or encoder during startup” and deleting the saved portal_token file fixed it. Sunshine’s docs describe a Troubleshooting → Reset XDG Portal Capture button in the web UI that deletes the saved token and restarts Sunshine so you can approve the prompt again. If the token is stale, current versions try a fallback capture method so you can answer the prompt.
5. Linux with VA-API: if the log shows Could not open codec [h264_vaapi]: Function not implemented, your Mesa build has hardware encoding turned off. Sunshine’s docs explain how to re-enable it.

Cause 2: too many pending pairing sessions

In the current source, Sunshine keeps up to 32 unfinished pairing sessions, and each one expires after 5 minutes (src/nvhttp.h). If you keep retrying pairing without entering the PIN, you can hit the limit. Enter the PIN on the Sunshine web UI’s PIN page (https://localhost:47990 on the host), cancel the other attempts, or wait a few minutes and try once.

Cause 3: no running app to resume

Moonlight thinks a session is still running, but the host has already closed it, for example after a reboot or after the game exited. Go back to the app list and start the app fresh rather than resuming.