Claw Code API Client: LLM Provider Integration and Streaming

The claw code API client lives in rust/crates/api/src/client.rs and serves as the sole interface between the agent runtime and the Anthropic LLM backend. It wraps a reqwest::Client with authentication, base URL configuration, and a configurable retry policy. Every message sent to and received from the Claude API flows through this crate.

AnthropicClient

The AnthropicClient struct is the central type. It holds the HTTP client, authentication source, base URL, and retry configuration. Two primary methods handle all API communication:

• send_message(request) — Non-streaming call. Sends a MessageRequest and returns a fully materialized MessageResponse.
• stream_message(request) — Streaming call. Returns a MessageStream that wraps the raw reqwest::Response with an SseParser. Call next_event() to yield Option<StreamEvent> values incrementally.

Constants

Authentication

The claw code API client supports multiple authentication modes through the AuthSource enum:

Two factory methods construct the client from environment state:

• from_env() — Reads ANTHROPIC_API_KEY and ANTHROPIC_AUTH_TOKEN from the environment.
• from_env_or_saved() — Also checks for saved OAuth credentials at ~/.claude/credentials.json (or $CLAUDE_CONFIG_HOME/credentials.json).

OAuth PKCE Flow

For browser-based authentication, the claw code API client implements the full OAuth PKCE (Proof Key for Code Exchange) flow:

1. generate_pkce_pair() — Creates a cryptographic code verifier and S256 challenge.
2. loopback_redirect_uri(port) — Constructs the redirect URI pointing to a local callback listener.
3. The user is directed to the browser for authorization.
4. A local server captures the callback with the authorization code.
5. exchange_oauth_code(config, request) — Exchanges the code for tokens.
6. refresh_oauth_token(config, request) — Refreshes expired tokens.

Credentials are persisted as an OAuthTokenSet containing access_token, refresh_token, expires_at, and scopes. The save operation uses atomic temp-file rename to prevent corruption.

API Types

MessageRequest

The request payload sent to the Messages API:

MessageResponse

Streaming and SSE Parsing

The claw code API client processes streaming responses using Server-Sent Events (SSE). The SseParser handles raw byte chunks from the HTTP response:

• push(chunk) — Feed a raw byte chunk into the parser buffer.
• finish() — Signal that the stream has ended.
• parse_frame — Processes event: and data: lines, ignoring ping frames and [DONE] sentinels.

StreamEvent Enum

Usage Tracking

Every response includes a Usage struct that tracks token usage across four dimensions:

The total_tokens() method returns the sum of all four fields.

Retry Policy

The claw code API client automatically retries failed requests for transient HTTP errors. The following status codes are considered retryable:

Retries use exponential backoff starting at 200ms, capped at 2 seconds, with a maximum of 2 retry attempts by default.

Error Types

The ApiError enum covers every failure mode in the API client: