claude

Haskell bindings to Anthropic's Claude API using servant.

Example usage

{-# LANGUAGE DuplicateRecordFields #-}
{-# LANGUAGE NamedFieldPuns        #-}
{-# LANGUAGE OverloadedLists       #-}
{-# LANGUAGE OverloadedStrings     #-}

import Claude.V1
import Claude.V1.Messages
import Data.Foldable (traverse_)
import qualified Data.Text as Text
import qualified Data.Text.IO as Text.IO
import qualified System.Environment as Environment

main :: IO ()
main = do
    key <- Environment.getEnv "ANTHROPIC_KEY"
    clientEnv <- getClientEnv "https://api.anthropic.com"
    let Methods{ createMessage } = makeMethods clientEnv (Text.pack key) (Just "2023-06-01")

    MessageResponse{ content } <- createMessage _CreateMessage
        { model = "claude-sonnet-4-5-20250929"
        , messages = [ Message{ role = User, content = [ textContent "Hello!" ] } ]
        , max_tokens = 1024
        }

    let display (ContentBlock_Text{ text }) = Text.IO.putStrLn text
        display _ = pure ()
    traverse_ display content

Prompt caching

The bindings support prompt caching at multiple levels:

• top-level request (CreateMessage.cache_control)
• system blocks (SystemBlock.cache_control)
• message/content blocks (Message.cache_control, text/image content block cache control)
• tool definitions (withToolCacheControl)

MessageResponse{ usage } <- createMessage _CreateMessage
    { model = "claude-sonnet-4-5-20250929"
    , max_tokens = 1024
    , cache_control = Just (ephemeralCacheWithTTLDuration "1h")
    , system = Just $ systemBlocks
        [ systemTextBlockCached ephemeralCache "You are a coding assistant."
        ]
    , messages =
        [ Message{ role = User, content = [ textContent "Summarize this file." ] } ]
    }

Setup

Using Nix with Flakes (Recommended)

1. Ensure you have Nix with flakes enabled
2. Copy the sample environment file:

cp .envrc.sample .envrc

3. Edit .envrc with your Anthropic API key
4. Run direnv allow

Manual Setup

cabal build

Environment Variables

Set your Anthropic API key as an environment variable:

# Option 1: Set directly in your shell
export ANTHROPIC_KEY="your-anthropic-api-key"

# Option 2: Using .envrc with direnv (recommended)
cp .envrc.sample .envrc
# Edit .envrc to add your API key
direnv allow

The API key is needed for running the test suite and example programs.

Testing

Run the test suite:

cabal test

Running the Examples

See examples for descriptions.

cabal run claude-example
cabal run claude-stream-example
cabal run claude-tool-example
cabal run claude-vision-example
cabal run claude-tool-search-example
cabal run claude-programmatic-tool-calling-example
cabal run claude-structured-outputs-example

Advanced Examples

• Structured Outputs: Constrain Claude's responses to follow a specific JSON schema, or validate tool parameters with strict mode

• Tool Search Tool: Server-side tool search for efficiently handling large numbers of tools

• Programmatic Tool Calling (PTC): Claude writes and executes code to call multiple tools and aggregate results