A lightweight, pure-Rust client library for Anthropic's Messages API. it's blocking (no async runtime required), has a tiny dependency tree, and leans on serde for everything on the wire.

What it does

• send messages and receive typed responses from Claude
• stream server-sent events incrementally
• count input tokens without generating a completion
• tool-use workflows with proper error handling
• retries with exponential backoff
• a neutral error taxonomy so you can tell failure categories apart

Install

still pre-release and not on crates.io yet, so it's a git dependency:

[dependencies]
anthropical = { git = "https://github.com/xandwr/anthropical" }

requires Rust 1.96.0 or later.

Usage

send a blocking message:

let client = Anthropic::from_env()?;
let response = client
    .message("claude-opus-4-8")
    .system("You are concise.")
    .user("Say hello in one word.")
    .max_tokens(64)
    .send()?;

stream a response:

let stream = Anthropic::from_env()?
    .message("claude-opus-4-8")
    .user("Count to five.")
    .stream()?;

for event in stream {
    let event = event?;
    // process event
}

count tokens:

let tokens = Anthropic::from_env()?
    .message("claude-opus-4-8")
    .user("How many tokens is this?")
    .count_tokens()?;

Dependencies

kept deliberately small: ureq (HTTP with rustls), serde/serde_json, and thiserror.

Status

early-stage with an unstable API surface. contributions and issues welcome.