Reference documentation

Welcome to the Fastly reference library. Here you'll find comprehensive technical detail about the interfaces exposed by the Fastly platform and products, including complete syntax and examples for functions, variables, CLI commands, and API endpoints.


Via the Fastly API you can perform any of the operations that are possible within the management console, including creating services, domains, and backends, configuring rules or uploading your own application code, as well as account operations such as user administration and billing reports.


Our edge cloud platform evolved from Varnish 2.1.5 and supports a heavily modified and extended variant of Varnish Configuration Language (VCL) for programming custom, high performance edge logic on our VCL services.


Use the Fastly CLI to manage services, scaffold out local development environments and compile your Compute@Edge projects, among a host of other useful features.

Image Optimizer

The Fastly Image Optimizer (IO) allows images to be transformed, resized, and optimized on the fly as they move through the Fastly edge cloud. Use IO query parameters to control the behavior of the optimizer.


Fastly interacts with many common web-standard HTTP headers. We also define a number of custom ones.

HTTP Statuses

Status codes indicate the general category of an HTTP response, and some custom codes are defined or reserved by Fastly.