RestSharp is a mature .NET HTTP client library that wraps HttpClient into a fluent request/response API. It is not a standalone application; it ships as NuGet packages consumed by other projects. The core RestClient handles execution through an async pipeline: interceptors, authenticators, request content construction, cookie containers, redirect handling, and response deserialization. RestRequest models parameters (query, header, body, URL segments, cookies, files) and is built fluently through extension methods. Responses are RestResponse<T> with error aggregation and throw helpers, making failure handling explicit and consistent. The architecture is modular: a core library plus optional serializer packages for Newtonsoft.Json, System.Text.Json, XML, and CsvHelper, a DI integration package for Microsoft dependency injection, and a Roslyn source generator that produces immutable and clone variants of configuration types. Authentication is pluggable via IAuthenticator, with built-in Basic, JWT, OAuth1 signature generation, and OAuth2 client-credentials/refresh-token/token authenticators. Serialization is managed by a registry keyed by data format and content type, with automatic body sniffing between JSON and XML. The codebase multi-targets netstandard2.0, net471, net48, net8.0, net9.0, and net10.0, and includes extensive unit and WireMock-based integration tests, benchmarks, and versioned Docusaurus documentation. Code quality is strong: clean separation of concerns, extensive use of extension methods for ergonomics, custom source generation for option immutability, and a comprehensive test suite covering OAuth1 official vectors, redirect behavior, cookies, serializer edge cases, and interceptor ordering. Some legacy surface remains (obsolete hooks, polyfills for old TFMs), but the project tracks current .NET releases and is actively maintained. This is a well-crafted library that remains one of the standard HTTP client abstractions in the .NET ecosystem.