Context memory
What the LLM literally sees this turn. Bounded by the model's context window (32K, 200K, 2M depending on model). Every token here costs money.
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Session memory
The conversation history + any state accumulated during this session. Persists as long as the session is alive. Can be much larger than context — you sample from it.
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Long-term memory
Cross-session knowledge about the user or domain. Persists indefinitely. Retrieved by query (semantic search, key lookup).