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.
75
92%
Does it follow best practices?
Impact
—
No eval scenarios have been run
Passed
No known issues
Quality
Discovery
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 rich trigger terms spanning both specific XVCL terminology and broader Fastly VCL development scenarios. The explicit note that XVCL syntax is not in training data is a smart addition that reinforces why the skill is essential. The broad catch-all for any Fastly VCL task ensures it won't be missed even when users don't mention XVCL directly.
| Dimension | Reasoning | Score |
|---|---|---|
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, etc. via XVCL transpiler) and 'when' with an explicit 'Use when...' clause covering multiple trigger scenarios including writing VCL for Fastly, working with .xvcl files, generating repetitive VCL, and even when XVCL isn't explicitly mentioned. | 3 / 3 |
Trigger Term Quality | Excellent coverage of natural terms: 'VCL', 'Fastly', '.xvcl files', 'backends', 'routing rules', 'headers', 'loops', 'reusable functions', 'magic numbers', 'compile', 'uvx xvcl', 'falco', 'VCL development'. Covers both the XVCL-specific terms and broader Fastly VCL terms users would naturally use. | 3 / 3 |
Distinctiveness Conflict Risk | Highly distinctive niche — XVCL/Fastly VCL transpilation is extremely specific and unlikely to conflict with other skills. The description explicitly notes 'XVCL syntax is not in training data' which further clarifies its unique role. The combination of Fastly, VCL, and .xvcl files creates a very clear, non-overlapping domain. | 3 / 3 |
Total | 12 / 12 Passed |
Implementation
85%Reviews the quality of instructions and guidance provided to agents. Good implementation is clear, handles edge cases, and produces reliable results.
This is a high-quality skill that effectively teaches a novel transpiler syntax (XVCL) that Claude wouldn't know from training data. Its strengths are strong actionability with complete, executable examples throughout, clear workflow sequencing from compilation to testing, and excellent progressive disclosure with a well-organized reference table. The main weakness is minor redundancy in a few places (repeated warnings about bare constants and table patterns), but overall token efficiency is good for the breadth of content covered.
| Dimension | Reasoning | Score |
|---|---|---|
Conciseness | The skill is mostly efficient and covers a lot of ground, but some content is repeated (e.g., the table pattern advice appears twice — in the Tables with Loops section and again in Common Mistakes; the bare constant warning appears in both the Constants section and Common Mistakes). Some sections like the VCL Gotchas, while useful, add significant length. Overall it's reasonably lean for the breadth covered but could be tightened. | 2 / 3 |
Actionability | Excellent actionability throughout — the Quick Start has copy-paste-ready bash commands, the Minimal Working Example is a complete compilable .xvcl file, every directive section includes executable syntax examples, and the compilation section provides concrete CLI invocations with option tables. The common mistakes section gives specific wrong vs. right patterns. | 3 / 3 |
Workflow Clarity | The workflow is clearly sequenced: write .xvcl → compile with uvx → lint with falco → simulate with falco → test with curl. The Quick Start establishes this pipeline immediately. The note about 'run locally' meaning compile AND simulate (not just lint) is an excellent validation checkpoint. The common mistakes section serves as an error-prevention checklist. | 3 / 3 |
Progressive Disclosure | The SKILL.md provides a comprehensive but scannable overview with inline examples for each directive, then clearly references 10 separate topic-specific files in a well-organized table with 'Use when...' guidance. References are one level deep with clear signaling. The project structure section aids navigation. Note: bundle files weren't provided so path accuracy can't be verified, but the structure is sound. | 3 / 3 |
Total | 11 / 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.
Validation — 11 / 11 Passed
Validation for skill structure
No warnings or errors.
- Repository
- fastly/fastly-agent-toolkit
- Commit
e0f4205
- 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.