🦀 crimson-crab

The production-grade Rust SDK for Anthropic's Claude API.

Everything the Claude API can do, in idiomatic Rust — streaming, tool use, extended thinking, prompt caching, and batches — with wire-faithful, zero-surprise types and bulletproof retries. The definitive Rust client for building on Claude: depth beats breadth.

191 tests · zero clippy warnings · a panic-free library (unwrap/expect/panic denied at compile time) · MSRV 1.75 · MIT OR Apache-2.0

• Docs: docs.rs/crimson-crab
• Site: singhpratech.github.io/crimson-crab
• Source: github.com/singhpratech/crimson-crab

Install

cargo add crimson-crab

Streaming and batch results are plain futures_core::Streams. To drive them with .next(), add the StreamExt extension trait:

cargo add futures-util

30-second quickstart

use crimson_crab::model_ids::CLAUDE_OPUS_4_8;
use crimson_crab::prelude::*;

#[tokio::main]
async fn main() -> crimson_crab::Result<()> {
    // Reads ANTHROPIC_API_KEY from the environment.
    let client = Client::from_env()?;

    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()?;

    let message = client.messages().create(&request).await?;
    println!("{}", message.text());
    Ok(())
}

Client is Clone + Send + Sync and shares one connection pool, so build it once and store it in your axum state, your MCP server struct, or a plain field — no Arc, no Mutex, no manual bounds.

Why crimson-crab

• Wire-faithful types — your code never breaks on new models. Types mirror the API field-for-field; there are no renamed concepts or leaky abstractions to relearn, and no adapter layer to lag behind a release.
• Forward-compatible enums — future models work day one. Every wire enum (content blocks, stream events, deltas, stop reasons, tool definitions, cache TTLs, thinking configs) carries an Unknown catch-all that preserves the raw JSON and re-serializes it unchanged instead of erroring. A response from a model the SDK has never heard of — including future Fable- and Mythos-class models — deserializes cleanly and round-trips verbatim.
• A tokio-free public API — runs anywhere. The public surface exposes futures_core::Stream, not runtime-specific types; tokio is a dev-dependency only. The same builder code compiles for native and wasm32-unknown-unknown on default features.
• Official-SDK-parity retries — production-grade out of the box. Connection errors, timeouts, 408/409/429, and 5xx are retried with full-jitter exponential backoff (0.5s base, 8s cap) and honor retry-after — capped at 60s so a hostile or broken server can't park your retry loop for hours. Streaming requests retry only before the first byte.
• Streaming that never truncates mid-generation. The client uses an idle read timeout rather than a total-request deadline, so a long-but-actively-flowing SSE response is never cut off just because total elapsed time crossed a limit.
• Tested against real API fixtures. Every content block and stream event from the wire reference has a serde round-trip test, and every endpoint has wiremock coverage — 191 tests, zero clippy warnings, and a library that denies unwrap/expect/panic so it cannot panic on you in production.

Feature coverage

Claude-specific features are first-class here because Claude is the only API this crate targets. Building against several model vendors? A multi-provider framework will serve you better — rig and genai are genuinely good. crimson-crab is for teams who have chosen Claude and want the whole surface, exactly as Anthropic ships it.

Streaming

Iterate typed events as they arrive; the stream accumulates a complete Message for you in the background.

use crimson_crab::prelude::*;
use futures_util::StreamExt;

# async fn run(client: &Client, request: &MessagesRequest) -> crimson_crab::Result<()> {
let mut stream = client.messages().stream(request).await?;
while let Some(event) = stream.next().await {
    if let StreamEvent::ContentBlockDelta {
        delta: ContentDelta::TextDelta { text },
        ..
    } = event?
    {
        print!("{text}");
    }
}
// After draining, the accumulated `Message` is identical in shape to a
// non-streaming response.
if let Some(message) = stream.final_message() {
    println!("\n[stop_reason: {:?}]", message.stop_reason);
}
# Ok(())
# }

Relaying text deltas (SSE bodies, channels, web handlers)

MessageStream is Send + Unpin and crimson_crab::Error is Send + Sync + std::error::Error, so a streaming response drops straight into an axum Sse body (or any channel) with no Box::pin and no wrapper error type. Map the event stream down to plain String deltas with filter_map:

use crimson_crab::prelude::*;
use futures_util::StreamExt;

# async fn run(client: &Client, request: &MessagesRequest) -> crimson_crab::Result<()> {
// A `Stream<Item = crimson_crab::Result<String>>` of plain text deltas, ready to
// hand to an `axum` `Sse` body, a channel, or any consumer.
let text_deltas = client
    .messages()
    .stream(request)
    .await?
    .filter_map(|event| async move {
        match event {
            Ok(StreamEvent::ContentBlockDelta {
                delta: ContentDelta::TextDelta { text },
                ..
            }) => Some(Ok(text)),
            // A late/in-stream error surfaces as an `Err` item — forward it as a
            // final `event: error` frame instead of dropping the connection.
            Err(e) => Some(Err(e)),
            Ok(_) => None,
        }
    });

forward(text_deltas).await;
# Ok(())
# }
# async fn forward<S>(_deltas: S)
# where
#     S: futures_util::Stream<Item = crimson_crab::Result<String>>,
# {
# }

Tool use (manual agentic loop)

message.into_param() converts a response Message straight into a request MessageParam — the two tool_use blocks echoed verbatim — so the "append the assistant turn, then a user message of tool results" contract is two lines with no lossy serde round-trip. Parallel tool calls need no special handling: just iterate message.content.

use crimson_crab::prelude::*;
use crimson_crab::types::ToolResultBlockParam;

# async fn run(client: &Client, mut messages: Vec<MessageParam>, tool: Tool) -> crimson_crab::Result<()> {
loop {
    let request = MessagesRequest::builder()
        .model("claude-opus-4-8")
        .max_tokens(1024)
        .messages(messages.clone())
        .tool(tool.clone()) // `.tool(_)` appends any `Into<ToolUnion>`; `.tools(vec)` replaces
        .build()?;
    let message = client.messages().create(&request).await?;

    if message.stop_reason != Some(StopReason::ToolUse) {
        println!("{}", message.text());
        break;
    }

    // Answer every tool call. `ContentBlock` is in the prelude, so matching
    // `ToolUse` needs no extra import.
    let mut results = Vec::new();
    for block in &message.content {
        if let ContentBlock::ToolUse(call) = block {
            match run_tool(&call.name, &call.input) {
                // Success: the discoverable `ContentBlockParam::tool_result` helper.
                Ok(output) => results.push(ContentBlockParam::tool_result(&call.id, output)),
                // Failure: surface it to the model with `is_error: true`.
                Err(why) => results.push(ContentBlockParam::ToolResult(
                    ToolResultBlockParam::error(&call.id, why),
                )),
            }
        }
    }

    // Echo the assistant turn back verbatim, then one user message of results.
    messages.push(message.into_param());
    messages.push(MessageParam::user(results));
}
# Ok(())
# }
// Your tool dispatch. Note `std::result::Result<_, _>`: see "Imports & the prelude".
# fn run_tool(_name: &str, _input: &serde_json::Value) -> std::result::Result<String, String> {
#     Ok("tool output".to_string())
# }

Prompt caching & token budgeting

The simplest caching path: a plain-string system prompt plus a top-level cache_control, which auto-places one breakpoint on the last cacheable block — no per-block wiring.

use crimson_crab::prelude::*;

# async fn run(client: &Client) -> crimson_crab::Result<()> {
let request = MessagesRequest::builder()
    .model("claude-opus-4-8")
    .max_tokens(256)
    .system("A long, reusable system prompt worth caching…")
    .cache_control(CacheControl::ephemeral()) // or `CacheControl::ephemeral_with_ttl(CacheTtl::OneHour)`
    .messages(vec![MessageParam::user("Restate rule one.")])
    .build()?;

let message = client.messages().create(&request).await?;
let usage = &message.usage;
println!(
    "fresh input: {}  written to cache: {:?}  read from cache: {:?}",
    usage.input_tokens,
    usage.cache_creation_input_tokens,
    usage.cache_read_input_tokens,
);
# Ok(())
# }

Token accounting for cost reports. The three input buckets are disjoint: input_tokens counts only the uncached input, while cache_creation_input_tokens and cache_read_input_tokens are separate. Total input tokens = input_tokens + cache_creation_input_tokens + cache_read_input_tokens. Never add the cache buckets into input_tokens — that double-bills the cached prefix.

Need the number before you spend on generation? Derive a count request from the same messages request — no rebuilding the prompt twice:

# use crimson_crab::prelude::*;
# async fn run(client: &Client, request: &MessagesRequest) -> crimson_crab::Result<()> {
let count = client.messages().count_tokens(&request.as_count_request()).await?;
println!("this request will cost {} input tokens", count.input_tokens);
# Ok(())
# }

For fine-grained control you can attach a breakpoint to an individual block instead: build a TextBlockParam (in crimson_crab::types), set cache_control, and pass it as a system block or message content — see examples/prompt_caching.rs.

Message Batches

The whole submit → poll → stream → tally pipeline. BatchRequestItem::from_request turns a MessagesRequest into a batch entry with no hand-rolled JSON; BatchStatus and BatchRequestCounts are typed for progress display; and results() decodes the JSONL stream line-by-line, tolerating blank lines and a missing trailing newline.

use crimson_crab::api::{BatchRequestItem, BatchResultOutcome, BatchStatus};
use crimson_crab::prelude::*;
use futures_util::StreamExt;

# async fn run(client: &Client, request: &MessagesRequest) -> crimson_crab::Result<()> {
// Submit, each entry tagged with your own custom id.
let items = vec![BatchRequestItem::from_request("row-1", request)?];
let batch = client.batches().create(&items).await?;

// Poll until the batch reaches a terminal state. (A built-in `poll_until_ended`
// helper is on the v0.2 roadmap; until then, loop with your runtime's timer and
// your own deadline guard.)
let batch = loop {
    let current = client.batches().get(&batch.id).await?;
    if current.processing_status == BatchStatus::Ended {
        break current;
    }
    tokio::time::sleep(std::time::Duration::from_secs(30)).await;
};

// Results arrive in any order — key them by `custom_id`, never by position.
let mut results = client.batches().results(&batch.id).await?;
while let Some(result) = results.next().await {
    let result = result?;
    match result.result {
        BatchResultOutcome::Succeeded(ok) => {
            println!("{}: {}", result.custom_id, ok.message.text());
        }
        BatchResultOutcome::Errored(err) => {
            // `err.error` is the raw error envelope (a `serde_json::Value`).
            println!("{}: errored: {}", result.custom_id, err.error);
        }
        BatchResultOutcome::Canceled(_) | BatchResultOutcome::Expired(_) => {}
        // `BatchResultOutcome` is `#[non_exhaustive]`; the wildcard keeps you
        // forward-compatible with outcome types added in a future release.
        _ => {}
    }
}
# Ok(())
# }

Imports & the prelude

use crimson_crab::prelude::*; is the fastest way to get the common types — Client, MessagesRequest, MessageParam, StreamEvent, ContentDelta, ContentBlock, ContentBlockParam, Tool, ToolChoice, StopReason, CacheControl, and more.

One thing worth knowing: the prelude also re-exports the crate's Result<T> and Error type aliases, which shadow std::result::Result / std::error::Error inside a glob import. If you write a two-type-argument Result<T, E> in the same scope — common with axum handlers, thiserror, or macro-heavy crates like rmcp — it resolves to the one-argument alias and fails with a confusing E0107 ("type alias takes 1 generic argument but 2 were supplied"). Two easy fixes:

• Fully-qualify it: std::result::Result<String, String> (as in the tool-loop example above); or
• Skip the glob and import exactly what you need. Curated paths: crimson_crab::Client, request/response types under crimson_crab::api::* (e.g. MessagesRequest), and the wire types under crimson_crab::types::* (e.g. MessageParam, TextBlockParam, ToolResultBlockParam).

Platform support

The crate compiles for native targets and wasm32-unknown-unknown on default features — no feature juggling. On wasm, reqwest resolves to the browser fetch backend and TLS features are ignored, so you do not need to disable rustls-tls; cargo tree -i tokio --target wasm32-unknown-unknown prints nothing.

Two honest caveats for edge/browser deployments: the retry loop cannot sleep on wasm (there are no threads), so on that target retries fire without backoff and do not observe retry-after — the browser applies its own backpressure. Streaming type-checks on wasm but is best-effort and not exercised in a headless browser in CI. For a 429-sensitive edge worker, cap max_retries accordingly.

More examples

Runnable programs live in examples/: basic, streaming, tool_use, thinking, prompt_caching, and structured_output. Run one with:

ANTHROPIC_API_KEY=sk-ant-... cargo run --example streaming

Build a Claude-powered MCP server

Want a ready-to-run Model Context Protocol server backed by Claude? Start from the template — a Rust MCP server (rmcp + crimson-crab) you can spin up in one click:

→ crimson-crab-mcp-template — click Use this template, or:

cargo generate --git https://github.com/singhpratech/crimson-crab-mcp-template

Roadmap

Shipping fast and iterating; spec fidelity and release cadence are the whole point. On deck for v0.2+: the Files API (beta), schemars-derived tool schemas with a #[tool] macro, typed tool-input deserialization, a tool-runner loop helper, a batches().poll_until_ended() convenience, a parse::<T>() structured-output helper, the Admin/Usage API, and Vertex/Bedrock transports. The model field is an open string everywhere, so any model Anthropic ships works today with zero changes.

Minimum supported Rust version

MSRV is 1.75. Raising it is a minor-version change.

Semver policy

The wire enums are #[non_exhaustive] and carry Unknown catch-all variants, so a minor release may add a new known variant (for a feature Anthropic ships) without breaking your build. Match with a wildcard _ arm on SDK enums to stay forward-compatible across minor versions.

License

Licensed under either of MIT or Apache-2.0 at your option.

🦀 Crimson Crab · crimson-crab is an independent open-source project and is not affiliated with Anthropic.