Cersei

Cersei is a Rust SDK for building coding agents. It provides the building blocks of a production coding agent as composable library functions: tool execution, streaming from language model providers, subagent orchestration, persistent memory backed by an embedded graph database, skills and MCP integration. Developers can use it to build a replacement for a terminal coding assistant, embed an agent inside an existing application, or assemble something new. It is aimed at Rust developers and teams who want control over their agent stack rather than a finished product. The SDK is modular, so you can take the whole package or individual crates for types, providers, tools, the agent loop, memory, hooks and MCP. Documented features include built in coding tools with code and AST intelligence, multimodal input, embeddings, context compression, sandboxes and virtual machines for isolated execution, sessions and background tasks, and configurable system prompts and permission policies. A newer module, AgentRL, supports self evolving agents that trace their own failures, test candidate fixes in isolated sandboxes, promote the one that passes verification and register it as a reusable tool, with sub agents written in an AgentTemplate language. An accompanying Abstract CLI and a workflows API are also documented. Cersei is free and open source under the MIT license, with code on GitHub. You bring your own model provider keys; there is no hosted service or paid tier. Cersei is part of Atlas, which describes itself as an open agent stack, and the documentation includes published benchmarks comparing it with other agent frameworks.