Letta (formerly MemGPT)

$10M seed, 21K GitHub stars. Agents self-edit memory blocks via system calls. MemFS uses git-backed markdown. Letta Code (2026) for persistent agents with model-transfer. Pricing: Free OSS / $20/mo Pro / $100 Max Lite / $200 Max / $20 API + usage. Strengths: full open-source self-hosting (no vendor dependency), academic rigor, greenfield agent platform. Trade: heavyweight to integrate (replacement, not bolt-on), no cross-tool managed story unless you build it yourself.