ein
A safe, simple, extensible, and fast agent harness
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ein book src plugin-authoring
5 files
building-installing.md
model-client-plugins.md
README.md
setup.md
tool-plugins.md
README.md

Plugin Authoring #

Ein's tools and model adapters are WASM components — self-contained binaries compiled to wasm32-wasip2 and loaded by eind at startup. Writing a plugin means writing a Rust crate that implements one of Ein's plugin traits, compiling it to WASM, and dropping the .wasm file in the right directory.

This section covers writing plugins for your own personal use. You don't need to be in the Ein source tree — plugins are standalone Rust projects.

Two plugin types #

Tool plugins #

Tool plugins expose capabilities to the LLM. When the agent decides to use a tool, the server calls the plugin's call method with JSON-encoded arguments and collects the result, which is fed back into the conversation.

Ein ships with four tool plugins: Bash, Read, Write, and Edit. You might write a tool plugin to fetch data from an API, run a custom script, query a database, or do anything else that could be useful in the agent loop.

Tool plugins live in ~/.ein/plugins/tools/.

Model client plugins #

Model client plugins translate between Ein's internal CompletionRequest format and a specific LLM API (Anthropic, OpenAI, etc.). The server calls complete with a serialized request and expects a serialized response back.

You would write a model client plugin to add support for a new API provider, proxy through a custom middleware, or log/transform requests for debugging.

Model client plugins live in ~/.ein/plugins/model_clients/.

Plugin identity #

A plugin's identity is its filename stem. The file ~/.ein/plugins/tools/my_weather.wasm becomes the "my_weather" plugin. This is how the server refers to model implementations in profiles and looks up per-plugin tool policy.

The tool's display name shown to the LLM and in the UI is returned by the name() method — it can differ from the filename.

Loading behavior #

  • Debug builds of eind (i.e., cargo run --bin eind): plugins are loaded from ./target/wasm32-wasip2/debug/. Useful for iterating inside the Ein workspace.
  • Release builds: tool plugins from ~/.ein/plugins/tools/, model clients from ~/.ein/plugins/model_clients/.

For standalone plugins, you'll always install to ~/.ein/plugins/.

Sections #