Pychess External Engine

Analyse positions on Pychess with native engines running on your own computer. Use your own UCI-compatible engine for its supported variants. The required pyffish and pyffish-alice libraries validate rules and moves; they do not choose your analysis engine.

Version 0.2.0 registers your local engine with a Pychess account token and requires the matching Pychess frontend/server integration.

Install and run

Install uv and obtain your engine executable. Create an engine token at Pychess Account → External engines and save it to a local file. Run:

uvx --python 3.13 pychess-external-engine@0.2.0 \
  --engine /path/to/your-uci-engine --token-file /path/to/your-token

Engines and NNUE networks are supplied locally. For Alice, configure a compatible engine and any network it needs as described in the usage guide.

Connect to Pychess

1. Leave the app running after it reports that your engine is registered.
2. Open Pychess analysis engine settings and select your engine by name.
3. Allow local network access if your browser asks, then enable analysis.

The default address is http://127.0.0.1:9670. Stop the app with Ctrl+C; restarting refreshes its registration automatically. Select the browser engine to switch back; the remove button removes an external registration. Only one tab can use the native engine at a time. Account tokens stay in the local app; search traffic stays local.

Linux x86_64 native transport and integrated UI are verified with Chromium 154, Edge 155 and Firefox 155. Firefox permission Allow was verified; explicit denial remains a manual check. Safari and other operating systems remain unverified. Connection controls require Pychess's external-engine frontend integration.

For configuration, resource limits and troubleshooting, see the usage guide. See compatibility for tested engines and variants.

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