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xvcl

Skill

Extends Fastly VCL with loops, functions, constants, macros, conditionals, and includes via XVCL — a VCL transpiler that compiles .xvcl files into standard VCL. Use when writing VCL for Fastly, working with .xvcl files, generating repetitive VCL (multiple backends, routing rules, headers) with loops

0Testing

Install

git clone https://github.com/fastly/fastly-agent-toolkit.git ~/.claude/skills/xvcl

What is xvcl?

Extends Fastly VCL with loops, functions, constants, macros, conditionals, and includes via XVCL — a VCL transpiler that compiles .xvcl files into standard VCL. Use when writing VCL for Fastly, working with .xvcl files, generating repetitive VCL (multiple backends, routing rules, headers) with loops, defining reusable VCL functions with return values, using compile-time constants instead of magic numbers, or writing any Fastly VCL configuration. XVCL syntax is not in training data so this skill is required. Also applies when writing and testing VCL locally (compile with `uvx xvcl`, test with falco), reducing VCL code duplication, splitting large VCL into modular includes, or doing any VCL development task for Fastly — even without explicitly mentioning XVCL.

What this can do

Capabilities declared in this component's own frontmatter — not inferred.

Inherit all session tools

Declares no tool restrictions — inherits every session tool

~192 tokens of context used while enabled, before you invoke anything

Documentation

README · ~6 min read

Trigger and scope

Trigger on: XVCL, .xvcl files, VCL transpiler, VCL metaprogramming, #const/#for/#def/#inline in VCL context, writing a VCL script, writing VCL and running it locally, or any Fastly VCL writing task.

Do NOT trigger for: debugging existing .vcl files without XVCL, Fastly API/CLI ops, Fastly Compute, or Terraform — even if they mention VCL.

Writing VCL with XVCL

XVCL is a VCL transpiler that adds metaprogramming to Fastly VCL. Write .xvcl files, compile to .vcl, then test with Falco or deploy to Fastly. All XVCL constructs are resolved at compile time — zero runtime overhead.

Quick Start

# Compile (no install needed with uvx)
uvx xvcl main.xvcl -o main.vcl

# Lint the output

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