seldon lets you interact with Apple's on-device LLM Foundation models on macOS. It's a minimal native macOS chat app built with SwiftUI as a Swift Package. It has a UI, terminal REPL and single shot command mode as well as supporting tool calling.

Save money and privacy by using a local model, and save resources by using a built-in LLM without requiring third party applications.

By @singe

swift build -c release

Build artifacts with make:

make binary        # dist/seldon-macos-<arch>
make bundle        # dist/seldon-macos-<arch>-with-tools.tar.gz
make app           # dist/Seldon-macos-<arch>.app.zip
make release-assets

Run the .app or run the binary.

seldon

This launches a native window (Seldon Chat).

Interactive terminal mode:

seldon --cli

--cli streams the model output as it is generated.

Single-shot headless mode (for scripts):

seldon --prompt "What is psychohistory?"

--prompt prints a single final response (no streaming), then exits.

Optional sampling temperature for CLI modes (0.0 to 2.0):

seldon --cli --temperature 0.2
seldon --prompt "Tell me where to find the second foundation" --temperature=0.7

The same --temperature value is also applied to GUI mode if you launch seldon with that flag.

Pass a tools YAML file to enable Foundation Models tool calling:

seldon --tools tools.example.yaml
seldon --cli --tools tools.example.yaml
seldon --prompt "search the web for hari seldon and describe the top result" --tools tools.example.yaml

Example config is included at tools.example.yaml with three basic tools:

• web_search implemented by tools/web_search.py
• fetch_url implemented by tools/fetch_url.py
• calculator implemented by tools/calculator.py

Change the system prompt passed to the LLM with --system:

seldon --system "you are Hari Seldon"

This will work for the GUI, cli or prompt mode.

• The app uses Apple's on-device Foundation Models API when available.
• In this environment, LanguageModelSession is available on macOS 26.0+.
• On older macOS versions (or SDKs without FoundationModels), the UI still runs and shows a clear unsupported message when you send a prompt.