Configuration & Environment Variables

Relevant source files

The following files were used as context for generating this wiki page:

• src/config.rs
• README.md
• src/lib.rs
• src/error.rs
• Cargo.toml
• CONTRIBUTING.md

Configuration and environment variables manage how the Resend Rust SDK interacts with the Resend API. The system uses a centralized Config struct to store authentication keys, base URLs, HTTP clients, and rate-limiting states. You can initialize these settings through environment variables for zero-config defaults or use the ConfigBuilder for granular control.

The SDK prioritizes security by ensuring sensitive credentials like API keys are not accidentally exposed in debug logs or error messages. It also implements automated rate limiting to prevent 429 Too Many Requests errors from the server.

Sources: src/config.rs:101-140, README.md:31-40, src/lib.rs:7-25

Environment Variables

The SDK checks specific environment variables during initialization to configure the default client behavior.

Variable	Requirement	Description	Default
RESEND_API_KEY	Required	The API key used for Bearer authentication.	N/A
RESEND_BASE_URL	Optional	Overrides the default Resend API endpoint.	https://api.resend.com
RESEND_RATE_LIMIT	Optional	Sets maximum requests per second (Async only).	9

Sources: README.md:31-40, src/config.rs:113-125

Base URL Resolution

If you provide a base_url through the ConfigBuilder, it takes precedence over the RESEND_BASE_URL environment variable. The SDK ensures that any path prefix in the base URL (such as a proxy path) is preserved when constructing final request URLs.

Sources: src/config.rs:106-118, src/config.rs:205-212

Configuration Architecture

The architecture centers on the Config struct, which is wrapped in an Arc and shared across all service modules (Emails, Contacts, etc.).

Config Struct Components

The Config struct holds the following internal state:

• user_agent: A string formatted as crate_name/crate_version (e.g., resend-rs/0.35.0).
• api_key: The Bearer token for authentication.
• base_url: The target reqwest::Url.
• client: The underlying reqwest::Client.
• limiter: A governor rate limiter used in asynchronous contexts.

Sources: src/config.rs:82-99, src/config.rs:119

Client Customization

You can use ConfigBuilder to inject custom HTTP configurations, which is useful for setting timeouts or using proxies.

let http_client = reqwest::Client::builder()
     .timeout(std::time::Duration::from_secs(10))
     .build()
     .unwrap();

let config = ConfigBuilder::new("re_123456789")
    .base_url("https://my-proxy.com".parse().unwrap())
    .client(http_client)
    .build();

let resend = Resend::with_config(config);

Sources: src/config.rs:24-41, src/config.rs:65-79

Rate Limiting and Quotas

The SDK manages request flow to stay within Resend's API limits. By default, it allows 9 requests per 1.1 seconds. This provides a buffer against the server-side limit of 10 requests per second.

Rate Limiting Logic Flow

Handling 429 Errors

If the server returns a 429 Too Many Requests status, the SDK parses the following headers into an Error::RateLimit variant:

• ratelimit-limit
• ratelimit-remaining
• ratelimit-reset

Sources: src/config.rs:154-173, src/lib.rs:328-334

Feature Flags

Configuration behavior changes based on the feature flags enabled in Cargo.toml.

Feature	Description
blocking	Switches reqwest to the blocking client. Rate limiting is disabled in this mode.
native-tls	Uses system-native TLS (Default).
rustls-tls	Uses the rustls library for TLS.

Sources: Cargo.toml:23-27, README.md:24-29, CONTRIBUTING.md:12-14

Summary

Configuration in the Resend Rust SDK is primarily managed through the Config struct, which can be initialized via environment variables (RESEND_API_KEY, RESEND_BASE_URL) or programmatically via ConfigBuilder. The SDK provides built-in protections including automated rate limiting for async clients and redacted debug formatting for API keys to ensure secure and reliable operation.