Anthropic Adapter

The Anthropic Messages API chat adapter.

AnthropicChatAdapter implements ChatProvider, translating behest's provider-neutral types to Anthropic's Messages API and back.

The full files are src/adapt/anthropic/chat.rs and src/adapt/anthropic/convert.rs.

API

pub struct AnthropicChatAdapter {
    id: ProviderId,
    http: HttpClient,
    config: ProviderHttpConfig,
}

impl AnthropicChatAdapter {
    pub fn new(config: ProviderHttpConfig) -> Result<Self, ProviderError>;
}

#[async_trait]
impl ChatProvider for AnthropicChatAdapter {
    fn id(&self) -> ProviderId;
    fn capabilities(&self) -> ProviderCapabilities;
    async fn complete(&self, request: ChatRequest) -> ProviderResult<ChatResponse>;
    async fn stream(&self, request: ChatRequest) -> ProviderResult<ChatStream>;
}

Capabilities

ProviderCapabilities {
    chat: true,
    chat_stream: true,
    tool_calls: true,
    vision: true,
    structured_output: false,
    context_overflow_retry: true,
}

Anthropic does not support structured output (JSON mode). structured_output: false means ModelRouter rejects requests that require it.

Request building

convert.rs maps ChatRequest → Anthropic's CreateMessageRequest:

• Message::System → top-level system field (Anthropic's system prompt is separate from messages).
• Message::User / Message::Assistant → MessageParam with role and content.
• ContentPart::ImageUrl → ImageBlockSource with base64-encoded data.
• ToolSpec → Tool with input_schema (Anthropic's term for JSON Schema parameters).
• ToolChoice::Auto → { type: "auto" }; ToolChoice::Required → { type: "any" }.

Tool use

Anthropic's tool calling uses tool_use and tool_result content blocks, not a separate tool_calls array. The adapter maps:

• Anthropic tool_use block → ContentPart::ToolCall.
• Anthropic tool_result block → ContentPart::ToolResult.

Response parsing

convert.rs maps Anthropic's CreateMessageResponse → ChatResponse:

• content blocks → Message::Assistant with ContentPart values.
• stop_reason → FinishReason ("end_turn" → Stop, "tool_use" → ToolCalls, "max_tokens" → Length).
• usage → TokenUsage (Anthropic reports input_tokens and output_tokens separately).

Streaming

Anthropic's streaming uses SSE with event: types:

• content_block_start — a new content block (text or tool_use) is beginning.
• content_block_delta — a text delta or tool_use input_json_delta.
• content_block_stop — the block is complete.
• message_delta — stop_reason and usage.
• message_stop — stream end.

The adapter's StreamAccumulator merges these into a complete Message::Assistant.

Error mapping

HTTP status	Anthropic error type	ProviderError
401	authentication_error	Authentication
429	rate_limit_error	RateLimited
529	overloaded_error	Overloaded
400	invalid_request_error	BadRequest
—	network error	Transport

Feature gate

[dependencies]
behest = { version = "0.4", features = ["anthropic"] }

Registered under "provider.anthropic.chat" in default_factory_registry().

See also

• ChatProvider — the port.
• ProviderConfig — the configuration.
• OpenAI Adapter — the other concrete adapter.