CtrlK
BlogDocsLog inGet started
Tessl Logo

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.

72

Quality

88%

Does it follow best practices?

Impact

No eval scenarios have been run

SecuritybySnyk

Passed

No known issues

SKILL.md
Quality
Evals
Security

Quality

Content

77%

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

This is a strong, highly actionable skill that teaches a novel transpiler syntax not in Claude's training data. Its greatest strength is the concrete, executable examples throughout and the practical gotchas sections that prevent common errors. The main weakness is that the body contains too much inline directive documentation that should be deferred to the referenced xvcl-directives.md, making the SKILL.md longer than necessary for an overview document.

Suggestions

Move the detailed directive syntax examples (constants, loops, functions, macros, conditionals, includes) to the referenced xvcl-directives.md and keep only a brief cheat-sheet table or 1-2 line summaries in SKILL.md to improve conciseness and progressive disclosure.

Remove the duplicate advice about preferring tables over if-chains — it appears both in the Tables section and in Common Mistakes. Keep it in one place only.

DimensionReasoningScore

Conciseness

The skill is mostly efficient and covers a lot of ground, but there's some redundancy — the table pattern advice is stated twice (in the Tables section and again in Common Mistakes), and some explanations like 'Constants are compile-time only' could be tighter. The VCL Gotchas section adds valuable non-obvious knowledge, but the overall document is quite long for a SKILL.md that should be an overview pointing to references.

2 / 3

Actionability

Excellent actionability throughout — executable bash commands for compilation and testing, complete working XVCL examples with proper syntax, concrete code for every directive, and specific patterns like the table-based redirects. The minimal working example is copy-paste ready and the Common Mistakes section provides exact wrong/right patterns.

3 / 3

Workflow Clarity

The Quick Start section provides a clear compile → lint → simulate → test workflow. The instruction 'When the user asks to run locally, always compile and run falco simulate' is an explicit checkpoint. Common Mistakes and VCL Gotchas serve as validation guidance. The workflow is simple enough that the linear sequence with explicit tool commands is sufficient.

3 / 3

Progressive Disclosure

The references table at the bottom is well-organized with clear 'Use when...' guidance, and the xvcl-directives.md reference is properly signaled. However, the SKILL.md itself contains extensive inline directive documentation (constants, loops, functions, macros, conditionals, includes) that largely duplicates what should be in the referenced xvcl-directives.md file. The main body could be significantly shorter by keeping only the most essential syntax and deferring details to references.

2 / 3

Total

10

/

12

Passed

Description

100%

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

This is an excellent skill description that thoroughly covers what the skill does, when to use it, and includes a wide range of natural trigger terms. It is highly specific to the Fastly VCL/XVCL domain, making it clearly distinguishable from other skills. The note that XVCL syntax is not in training data is a smart addition that reinforces why this skill must be selected.

DimensionReasoningScore

Specificity

Lists multiple specific concrete actions: loops, functions, constants, macros, conditionals, includes, transpiling .xvcl files, compiling, testing, reducing code duplication, splitting into modular includes. Very detailed about what the skill does.

3 / 3

Completeness

Clearly answers both 'what' (extends VCL with loops, functions, constants, macros via XVCL transpiler) and 'when' (explicit 'Use when...' clause with extensive trigger scenarios including writing VCL for Fastly, working with .xvcl files, generating repetitive VCL, testing locally, etc.).

3 / 3

Trigger Term Quality

Excellent coverage of natural terms: 'VCL', 'Fastly', '.xvcl files', 'backends', 'routing rules', 'headers', 'loops', 'functions', 'magic numbers', 'VCL configuration', 'compile', 'uvx xvcl', 'falco', 'code duplication'. Also explicitly notes it applies even without mentioning XVCL.

3 / 3

Distinctiveness Conflict Risk

Highly distinctive niche — Fastly VCL and XVCL transpilation is a very specific domain unlikely to conflict with other skills. The description explicitly notes XVCL syntax is not in training data, further clarifying its unique role.

3 / 3

Total

12

/

12

Passed

Validation

100%

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

Validation11 / 11 Passed

Validation for skill structure

No warnings or errors.

Repository
fastly/fastly-agent-toolkit
Reviewed

Table of Contents

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.