Compared
Letta is where the agent lives. This is storage it can call.
The Letta README opens with "Build stateful agents with memory that can learn and improve over time." It says the current source is letta-ai/letta-code, including the agent harness, the app server and the runtime used by the desktop and web apps. The archive branch is described as a retired API server.
What they say they are
Install, in their README, is npm install -g @letta-ai/letta-code, then a terminal UI, an app server, or a cloud. That was fetched from github.com/letta-ai/letta on 30 September 2026. FluctlightDB is not an agent loop, a terminal UI, or a place the model edits its own blocks. It is the directory an agent in some other loop writes to.
When to pick which
Pick Letta when you want the runtime and you are not attached to a framework you already have. Pick FluctlightDB when the loop already exists — yours, LangChain’s, or the Agents SDK’s handlers — and you want a brain it can call. The short form is on the comparison .