A simple-reflex agent selects actions based on the agent's current perception of the world and not based on past perceptions. It can handle a full observation environment.
A model-based-reflex agent is designed to deal with partial accessibility. They do this by keeping track of the part of the world it can see now. It does this by keeping an internal state that depends on what it has seen before so it holds information on the unobserved aspects of the current state. It can handle a partial observation environment.
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