This is an old release — see the latest release for typed structured output, derived tool schemas, and current model ids. Everything below still applies to 0.1.0.

A Rust SDK for Anthropic's Claude API that can't panic on you.

unwrap, expect, panic!, and todo! are denied at compile time in the library. A malformed or unexpected response is always an Error you handle — never a panic in your service.

cargo add crimson-crab

use crimson_crab::model_ids::CLAUDE_OPUS_4_8;
use crimson_crab::prelude::*;
#[tokio::main]
async fn main() -> crimson_crab::Result<()> {
    let client = Client::from_env()?;              // reads ANTHROPIC_API_KEY
    let request = MessagesRequest::builder()
        .model(CLAUDE_OPUS_4_8)
        .max_tokens(1024)
        .messages(vec![MessageParam::user("Explain Rust's borrow checker in one line.")])
        .build()?;
    println!("{}", client.messages().create(&request).await?.text());
    Ok(())
}

Client is Clone + Send + Sync and shares one connection pool — build it once and drop it in your axum state or server struct. No Arc, no Mutex.

What's covered

The full Messages API: streaming (hand-rolled SSE that accumulates to a final Message), tool use, extended thinking, prompt caching, structured outputs, token counting, the Models API, and Message Batches (create / poll / cancel / stream JSONL results).

Three design decisions

Forward-compatible types. Every tagged enum has a catch-all variant and model is an open string, so a new model or a content-block type Anthropic adds is a value you can match on — not a deserialization error, and not a release you have to wait for.

No Tokio in the public API. Streaming returns a plain futures_core::Stream, so it drops into whatever runtime you already run. The crate also compiles to wasm32-unknown-unknown.

Hardened against hostile input. Typed errors per status with retry-after capture, retries with exponential backoff and full jitter matching the official SDKs' policy, capped retry-after, bounded allocations, and an idle-timeout design so streaming is never cut short by a total-request timeout.

Verified in this release

191 tests (wiremock integration suites plus serde round-trips against real API fixtures) · zero clippy warnings on --all-targets --all-features · MSRV 1.75 · MIT OR Apache-2.0.

Links

📖 docs.rs · 📦 crates.io · 🌐 Site · 🦀 MCP server template

Claude-only, and deep on purpose. If you need multi-provider, rig and genai are genuinely good and worth your time.

crimson-crab is an independent open-source project and is not affiliated with Anthropic.