Name: xvcl
Rating: 75.2 (1 reviews)
Author: fastly

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.

Quality

93%

Does it follow best practices?

Run evals on this skill

Adds up to 20 points to the overall score

View guide

Securityby

Passed

No findings from the security scan

Quality

Content

86%Weight 40%Scale 1-5

Reviews the quality of instructions and guidance provided to agents. Good implementation is clear, handles edge cases, and produces reliable results.

The body is actionable and well-structured, with executable examples and a clean progressive-disclosure layout that delegates detail to verified reference files. Minor conciseness and feedback-loop gaps keep it just below top marks on two dimensions.

Suggestions

Deduplicate the tables-vs-if-chains advice — state it once (Common Mistakes is the natural home) and reference it from the directive section to save tokens.

Add an explicit fix-and-re-validate loop to the Quick Start (e.g., on lint/simulate failure, fix the .xvcl and re-run `uvx xvcl` then `falco simulate`) to reach the top workflow-clarity anchor.

Trim the restated 'O(1) hash lookups / idiomatic Fastly pattern' sentence that duplicates the directive-summary explanation.

Dimension	Reasoning	Score
Conciseness	Mostly lean and example-driven, assuming competence, but the tables-vs-if-chains guidance is restated (directive summary plus Common Mistakes) and a few sentences could be trimmed.	4 / 5
Actionability	Provides copy-paste-ready commands (uvx xvcl, falco lint/simulate) and complete executable .xvcl examples covering the common constructs and compile/test workflow.	5 / 5
Workflow Clarity	Clear compile→lint→simulate→curl sequence with an explicit warning to run simulate rather than just lint; lacks an explicit fix-and-re-validate feedback loop, so it falls just short of the top anchor.	4 / 5
Progressive Disclosure	Body is a concise overview with a well-organized references table pointing to verified one-level-deep bundle files (all 10 references exist), giving easy navigation.	5 / 5
	Total	18 / 20 Passed

Description

100%Weight 40%Scale 1-5

Based on the skill's description, can an agent find and select it at the right time? Clear, specific descriptions lead to better discovery.

The description is specific, complete, and rich in natural trigger terms, clearly answering both what the skill does and when to use it. It is concise relative to the depth it conveys and occupies a distinct niche.

Dimension	Reasoning	Score
Specificity	Lists multiple concrete capabilities (loops, functions, constants, macros, conditionals, includes) plus the compile action, giving comprehensive coverage rather than just 'several' actions.	5 / 5
Completeness	Explicitly states what it does ('compiles .xvcl files into standard VCL') and gives explicit, multi-clause 'Use when...' trigger guidance with concrete phrases.	5 / 5
Trigger Term Quality	Comprehensive natural terms including the .xvcl file extension, 'writing VCL for Fastly', 'VCL transpiler', and 'VCL development task for Fastly' — exactly what a user needing this skill would say.	5 / 5
Distinctiveness Conflict Risk	Occupies a clear niche (XVCL transpilation for Fastly VCL) with distinct .xvcl/transpiler triggers and an explicit note that the syntax is not in training data, minimizing conflict risk.	5 / 5
	Total	20 / 20 Passed

Validation

100%

Warnings & errors only

Checks the skill against the spec for correct structure and formatting. All validation checks must pass before discovery and implementation can be scored.

Validation — 16 / 16 Passed

Validation for skill structure

No warnings or errors.

Repository: fastly/fastly-agent-toolkit
Path: skills/xvcl/SKILL.md
Commit: d5a8062

Reviewed: 3 days ago

Table of Contents

Discovery Implementation Validation

Is this your skill?

If you maintain this skill, you can claim it as your own. Once claimed, you can manage eval scenarios, bundle related skills, attach documentation or rules, and ensure cross-agent compatibility.