# Thread Safety

URL: /milo-cli/docs/about/thread-safety/
Section: about
Tags: about, concurrency, free-threading

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Milo is designed for Python 3.14t with PYTHON_GIL=0. The core rule is simple: reducers do not mutate shared state or perform I/O. Effects run in sagas, Cmd thunks, command handlers, or explicit boundary code. Important boundaries: Reducers are pure functions. State should be immutable or treated as immutable. Store dispatch serializes action processing. Sagas and commands run on an executor and report back through actions. Terminal cleanup belongs to the app runtime, not reducers. See App Lifecycle for the runtime flow.

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Metadata:
- Author: lbliii
- Word Count: 81
- Reading Time: 1 minutes