Anthropic Provider

Claude 4, Sonnet, Opus, and extended thinking models

The Anthropic provider supports all Claude models including Claude 4, Sonnet, Opus, Haiku, and extended thinking.

Configuration

provider · string · required

Set to "anthropic"

api_key · string · required

Anthropic API key. Get yours at console.anthropic.com

base_url · string

Custom base URL for proxies (defaults to https://api.anthropic.com)

model · string · required

Model name: claude-sonnet-4, claude-opus-4, etc.

temperature · number

Sampling temperature (0.0-1.0). Defaults to 0.7.

max_tokens · number

Maximum output tokens. Defaults to 8192.

Example Configuration

{
  "provider": "anthropic",
  "model": "claude-sonnet-4",
  "api_key": "sk-ant-...",
  "temperature": 0.7
}

OAuth Setup Tokens

Anthropic supports OAuth setup tokens (from anthropic.com/setup):

{
  "provider": "anthropic",
  "model": "claude-sonnet-4",
  "api_key": "sk-ant-oat01-..."
}

OAuth tokens use Bearer authentication instead of x-api-key:

Authorization: Bearer sk-ant-oat01-...

Supported Models

• Claude 4: claude-sonnet-4, claude-opus-4, claude-haiku-4
• Claude 3.5: claude-3-5-sonnet-20241022, claude-3-5-haiku-20241022
• Claude 3: claude-3-opus-20240229, claude-3-sonnet-20240229, claude-3-haiku-20240307
• Extended thinking: Models with reasoning capabilities (thinking content)

Capabilities

Feature	Support
Streaming	Yes
Function Calling	Yes
Vision (images)	Yes
System Messages	Yes (top-level system field)
Tool Calls	Yes (native)

Authentication

The Anthropic provider supports two authentication methods:

1. Standard API Key

x-api-key: sk-ant-api-...
anthropic-version: 2023-06-01

2. OAuth Setup Token

Authorization: Bearer sk-ant-oat01-...
anthropic-version: 2023-06-01
anthropic-beta: oauth-2025-04-20

API keys are read from:

1. api_key field in config
2. ANTHROPIC_API_KEY environment variable

Message Format

Anthropic uses a different message format than OpenAI:

• System messages go in top-level system field (not in messages array)
• Roles are user and assistant (tool messages use user role)
• Messages are objects with role and content

{
  "model": "claude-sonnet-4",
  "max_tokens": 8192,
  "system": "You are a helpful assistant",
  "messages": [
    {"role": "user", "content": "Hello"},
    {"role": "assistant", "content": "Hi there!"}
  ]
}

Tool Calls

Anthropic uses a native tool_use content block format:

{
  "content": [
    {"type": "text", "text": "Let me check that"},
    {"type": "tool_use", "id": "call_1", "name": "shell", "input": {"cmd": "ls"}}
  ]
}

Vision Support

Claude supports image input via content_parts:

{
  "role": "user",
  "content": [
    {"type": "text", "text": "What's in this image?"},
    {"type": "image", "source": {"type": "base64", "media_type": "image/png", "data": "iVBOR..."}}
  ]
}

Custom Base URL

For proxies or self-hosted endpoints:

{
  "provider": "anthropic",
  "model": "claude-sonnet-4",
  "api_key": "sk-ant-...",
  "base_url": "https://proxy.example.com"
}

Or use anthropic-custom: prefix:

{
  "provider": "anthropic-custom:https://proxy.example.com",
  "model": "claude-sonnet-4",
  "api_key": "sk-ant-..."
}

Code Example

From src/providers/anthropic.zig:

pub const AnthropicProvider = struct {
    credential: ?[]const u8,
    base_url: []const u8,
    allocator: std.mem.Allocator,

    const DEFAULT_BASE_URL = "https://api.anthropic.com";
    const API_VERSION = "2023-06-01";

    pub fn init(allocator: std.mem.Allocator, api_key: ?[]const u8, base_url: ?[]const u8) AnthropicProvider {
        const url = if (base_url) |u| trimTrailingSlash(u) else DEFAULT_BASE_URL;
        return .{
            .credential = api_key,
            .base_url = url,
            .allocator = allocator,
        };
    }
};

Error Handling

The provider classifies common Anthropic API errors:

• error.RateLimited — 429 rate limit exceeded
• error.InvalidApiKey — Authentication failed
• error.ApiError — Generic API error
• error.NoResponseContent — Empty response

Related

• Providers Overview
• OpenAI Provider
• OpenRouter Provider