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js-perf-investigation

Structured performance opportunity investigation for SpiderMonkey (the Firefox JavaScript engine). Use this skill when the user wants to investigate JS engine performance, profile SpiderMonkey, find optimization opportunities, write performance patches, or evaluate benchmark regressions. Trigger on mentions of: profiling JS, SpiderMonkey performance, JIT optimization, benchmark regression analysis, shell benchmarking, or any request to make JS workloads faster. The methodolgy is described mostly for the JS shell but can be adapted to browser investigation.

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, domain-specific skill that provides a rigorous, evidence-driven methodology for SpiderMonkey performance investigation. Its greatest strengths are actionability (concrete commands, executable code, specific flags) and workflow clarity (four well-sequenced phases with explicit validation checkpoints and feedback loops). The main weaknesses are moderate verbosity in places and the length of the document which could benefit from splitting some content into referenced files.

Suggestions

Consider moving the full Python statistical analysis script and the anti-patterns section into separate reference files to reduce the main skill's length and improve progressive disclosure.

Trim explanatory text that justifies 'why' to Claude (e.g., 'Statistical profilers need sufficient samples to produce meaningful data — short runs produce noisy profiles where real hotspots are hard to distinguish from sampling noise') — Claude understands sampling theory.

DimensionReasoningScore

Conciseness

The skill is generally well-written and most content earns its place, but there's some verbosity — e.g., explaining what bad hypotheses look like, explaining why debug builds distort profiles, and some redundant reminders about --strict-benchmark-mode. The anti-patterns section partially duplicates guidance already given in the phases. However, given the complexity of the domain, most content is justified.

2 / 3

Actionability

The skill provides fully executable bash commands, concrete C++ instrumentation examples, a complete Python statistical analysis script with dependency management, specific mozconfig settings, and exact CLI flags. Every phase has copy-paste-ready code and commands.

3 / 3

Workflow Clarity

The four-phase methodology is clearly sequenced with explicit validation checkpoints: profile before patching, instrument to confirm hypotheses before writing code, run test suites before committing, re-profile after patching to confirm expected effect. The feedback loops (e.g., borderline p-values → more runs, re-validate after instrumentation) are well-defined. Safety evaluation with test suites is explicit.

3 / 3

Progressive Disclosure

The skill references 'references/advanced-tools.md' and a 'profiler-analysis' skill for deeper content, which is good. However, no bundle files are provided, so we can't verify these references exist. The document itself is quite long (~300 lines) and some sections (like the full Python script or the anti-patterns list) could potentially be split into reference files. The structure within the document is well-organized with clear headers though.

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 a strong skill description that clearly defines its niche (SpiderMonkey performance investigation), lists concrete actions, and provides explicit trigger guidance with natural keywords. It uses proper third-person voice and includes both 'Use this skill when...' and 'Trigger on mentions of:' clauses, making it easy for Claude to select appropriately. The only minor note is a typo ('methodolgy') but that doesn't affect functional quality.

DimensionReasoningScore

Specificity

Lists multiple specific concrete actions: investigate JS engine performance, profile SpiderMonkey, find optimization opportunities, write performance patches, evaluate benchmark regressions. Also specifies the domain (SpiderMonkey/Firefox JS engine) and context (JS shell, adaptable to browser).

3 / 3

Completeness

Clearly answers both 'what' (structured performance opportunity investigation for SpiderMonkey) and 'when' (explicit 'Use this skill when...' clause and 'Trigger on mentions of:' with specific trigger terms).

3 / 3

Trigger Term Quality

Excellent coverage of natural trigger terms: 'profiling JS', 'SpiderMonkey performance', 'JIT optimization', 'benchmark regression analysis', 'shell benchmarking', 'make JS workloads faster'. These are terms a user working in this domain would naturally use.

3 / 3

Distinctiveness Conflict Risk

Highly distinctive — targets a very specific niche (SpiderMonkey/Firefox JS engine performance investigation). The combination of SpiderMonkey, JIT optimization, and benchmark regression analysis makes it extremely unlikely to conflict with other skills.

3 / 3

Total

12

/

12

Passed

Validation

90%

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

Validation10 / 11 Passed

Validation for skill structure

CriteriaDescriptionResult

allowed_tools_field

'allowed-tools' contains unusual tool name(s)

Warning

Total

10

/

11

Passed

Repository
mozilla/enterprise-firefox
Reviewed

Table of Contents

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