mem0 solves the 'I want to sell memory to my users' problem. It is a managed platform where each user or org gets isolated memory that summarizes over time and deduplicates semantically. You call it via REST API and it handles the vector DB, summarization, and scaling. The tradeoff is you are on their infrastructure and their API contract. Best for SaaS products where memory is a feature your customers understand they are buying.
Zep solves the 'I want to self-host a memory server' problem. It is a clean separation of concerns: your agent talks to a service that manages long-term memory, handles retrieval, and chains retrieval into agentic workflows. You deploy it in your own environment, own the data, and can modify the memory pipeline as your use case evolves. Best for teams who need control, scale their own infra, and want memory as a reusable service other agents can share.
Letta solves the 'I want memory built into my agent framework' problem. It is not a separate service. Memory, tools, and reasoning are one stack. Persistence is automatic, the agent lifecycle is managed for you, and the whole thing runs locally or in a container. Best for teams shipping agents where memory is a design constraint from the start, not a feature bolted on later. For most teams shipping today, the answer is not either or. Use mem0 if memory is a SaaS feature, use Zep if you need a memory server you control, use Letta if you are already in its ecosystem and memory belongs in the agent framework. The three address different architectural questions.