skillscore 0.9.0

Lint and score AI agent skills (SKILL.md) against the official Claude, Codex, and Antigravity authoring guides. Offline, deterministic CLI.

skillscore

skillscore statically analyzes any AI agent skill (a SKILL.md manifest and its folder) and produces a 0 to 100 quality score, a letter grade, and a list of actionable findings, scored against the official skill-authoring guides from Anthropic (Claude), Google (Antigravity), and OpenAI (Codex). It is offline, deterministic, and built for CI.

What is skillscore?

skillscore is a skill linter, SKILL.md validator, and agent-skill quality checker. Agent skills are an open standard: a folder with a SKILL.md (YAML frontmatter plus a Markdown body) and optional references/, examples/, scripts/, and assets/ subfolders, used by Claude Code, Codex, Antigravity, Gemini CLI, and Cursor.

Because an agent keeps every skill's name and description in its context budget permanently, a vague or malformed skill is worse than no skill. skillscore catches exactly those problems before a skill ships, and it never leaves your machine.

How it works

A manifest goes in, a score comes out. Every step runs locally.

The parser reads the frontmatter and body, the rule engine runs 26 checks grouped into 7 categories, and the scorer normalizes the result to a 0 to 100 score with a letter grade. There is no network call anywhere in that path.

Quickstart

# Install
dart pub global activate skillscore

# Score a single skill (any name, any location)
skillscore path/to/SKILL.md

# Score every skill in a folder or monorepo
skillscore path/to/skills/

# Score several specific skills in one command
skillscore skill-a/ skill-b/ skill-c/

# Pick a target ruleset
skillscore my-skill/ --target claude

# Machine-readable output for CI and dashboards
skillscore my-skill/ --format json

# Gate CI: fail the build if any skill scores below 80
skillscore skills/ --min-score 80

Scoring, and the rubric

100 points, seven categories, each rule tagged with the authoring guide it comes from.

Each rule awards full, partial, or zero points. Category G (safety) is a penalty of up to -15 that applies only when a skill ships scripts or terminal commands. Profiles that exclude a rule (for example --target claude excludes the Codex-specific B4) are normalized back to a 0 to 100 scale, so scores stay comparable across targets.

Grades: A is 90 and up, B is 80 and up, C is 70 and up, D is 60 and up, F is below 60.

The full rubric (all 26 rules)

Rule	Title	Pts	Severity	Targets	Source
A1_frontmatter_present	YAML frontmatter delimited by ---	4	error	all	Anthropic
A2_name_format	name at most 64 chars, lowercase / digits / hyphens	4	error	all	Anthropic
A3_name_reserved_words	name avoids "anthropic" and "claude"	3	error (claude) / info	all	Anthropic
A4_description_present	description present, at most 1024 chars	4	error	all	Anthropic
A5_frontmatter_keys	Only recognized keys, no typos ("did you mean")	2	warning	all	Anthropic
B1_description_what	States WHAT (opens with an action verb)	6	warning	all	Anthropic
B2_description_when	States WHEN ("use when ...")	6	warning	all	Anthropic
B3_third_person	Written in third person	5	warning	all	Anthropic
B4_frontloaded_triggers	Concrete keywords in the first 60 chars	4	warning	codex, universal	Codex
B5_boundary_clause	Has a "do not use" boundary	4	warning (antigravity) / info	antigravity, universal	Antigravity
B6_description_truncation	Self-contained within 250 chars (Claude routing)	3	warning	claude, universal	Anthropic
C1_body_length	Body at most 500 lines (linear to 0 at 1000)	6	warning	all	Anthropic
C2_explainer_bloat	No definitions of common knowledge	5	warning	all	Anthropic
C3_excessive_optionality	No long "or" chains	4	info	all	Anthropic
D1_progressive_disclosure	Depth split into references / examples	5	info	all	Anthropic
D2_one_level_links	Reference links one level deep	5	warning	all	Anthropic
D3_reference_toc	Long reference files have a TOC	5	info	all	Anthropic
E1_anti_patterns	States anti-patterns explicitly	6	warning	all	Flutter
E2_workflow_checklist	Checklist or numbered workflow	5	warning	all	Anthropic
E3_feedback_loop	Validate, fix, repeat loop	5	warning	all	Anthropic
E4_code_example	At least one fenced code example	4	warning	all	Anthropic
F1_time_sensitive	No date-anchored statements that rot	4	warning	all	Anthropic
F2_forward_slashes	Paths use forward slashes only	3	error	all	Anthropic
F3_consistent_terminology	No synonym mixing (conservative)	3	info	all	Anthropic
G1_safety_section	Scripts / commands need a Safety section	-8	error	antigravity, universal	Antigravity
G2_script_docs	Bundled scripts are documented	-7	warning	all	Anthropic

Run skillscore rules for the live table, or skillscore explain <rule-id> for any rule's rationale, fix, and source.

Every finding cites the guide it comes from, and skillscore explain prints the rationale and the fix:

One manifest, four guides

The same SKILL.md can be scored through four different authoring guides. Pick a lens with --target; the default universal is the union of all of them.

A skill that passes universal is portable across all four runtimes, because universal activates every guide's rules at once. Each rule stays tagged with its origin, so a finding always tells you which guide it comes from.

Scoring a whole monorepo

Pass a folder or several paths and skillscore walks the tree, finds every SKILL.md (case-insensitive), scores each one, and prints a summary. Overlapping paths are deduplicated; if one path is bad, the rest still score and the bad one is reported as a warning.

Token budget

Every scorecard shows the BPE token cost of a skill, split by the two scopes in which agent runtimes load SKILL.md content:

  Tokens  description (permanent)    67 gpt-4   ~74 claude
          full manifest (active)   1474 gpt-4  ~1622 claude

Permanent is the per-prompt cost: the agent loads the description field on every call so it knows which skills exist. Active is the per-invocation cost, paid only when the agent decides to use the skill.

Counts use the cl100k_base BPE vocabulary (exact for GPT-4 and Codex). The Claude estimate adds a calibrated 10% overhead, and it appears in --format json under a tokens key for dashboards and CI.

Validated against the Anthropic count_tokens API

The 10% Claude estimate was checked against the official Anthropic count_tokens API across all 31 skills in google/skills:

Metric	Value
Skills validated	31 (all of google/skills)
Mean actual Claude overhead vs cl100k	+10.2%
Median	+10.0%
Range	+0% to +20% (varies with keyword density)

Descriptions dense with trigger keywords run toward +18 to +20%; clean prose runs toward 0 to 6%.

Catching frontmatter typos (rule A5)

The SKILL.md frontmatter is a fixed set of keys (name, description, license, allowed-tools, metadata, version), and YAML gives you no protection when you misspell one. Write descrption: and YAML happily accepts it as an unknown field while the real description goes missing. The skill still loads, but with empty metadata it is invocable only by name and is never auto-triggered. Strict validators, including Anthropic's own skill-creator, reject any unexpected key outright.

Rule A5_frontmatter_keys catches this. It flags every top-level key outside the recognized set, and when the key is a near-miss for a real one (within an edit distance of two) it tells you which key you meant:

The two findings together tell the whole story: A4 reports the field is gone, and A5 points at the typo that swallowed it. Design details:

• Custom fields are welcome, under metadata. Only top-level keys are checked, so anything nested inside a metadata: map is yours to name freely and is never flagged.
• No false suggestions. A genuinely unrecognized key such as author is flagged without a misleading "did you mean", because it is not close to any real key. (Move it under metadata.)
• No double-counting. When the frontmatter is missing or malformed entirely, A5 stays silent and lets A1 own that failure.
• Fully offline and deterministic. The "did you mean" suggestion is a local Levenshtein comparison. No network, no model.

A full QA record for this rule, every case run against the compiled binary with screenshot evidence, lives in docs/qa/a5/.

Fix it automatically with --fix

When a finding has a safe, mechanical correction, skillscore marks it [fixable] and can apply it in place. A misspelled key is the first such fix: skillscore <path> --fix renames descrption: to description:, then re-scores so the report and the exit code reflect the corrected file.

One typo drops the skill to a D (the real description is missing, so the description rules all fail); --fix recovers it to a perfect A. The fix is deterministic and idempotent, preserves your line endings, and only ever touches a key that has a confident "did you mean" match. Keys with no near match (move them under metadata yourself) are left untouched, never guessed at.

Eval harness

Static linting tells you a skill is well-formed. The eval harness tells you whether queries actually route to it, the thing that matters once a skill is deployed. Three commands, one workflow, no API key.

# 1. Scaffold 20 queries from the skill's description
skillscore eval init my-skill/

# 2. Review and extend the generated queries
cat my-skill/evals.json

# 3. Run the eval, fully offline, no API key, no cost
skillscore eval run my-skill/

eval init reads the description and derives 20 queries: 10 that should trigger the skill and 10 that should not. eval validate checks the suite has both classes and sane thresholds. eval run scores each query 3 times and passes it when the trigger-rate clears (or, for non-trigger queries, stays under) the 0.5 threshold.

How the offline scoring heuristic works

eval run uses a local heuristic, no model call and no network. It scores each query by matching content words against three regions pulled from the description:

Region	Source	Role
Trigger terms	the "Use when ..." clause, scaffold words stripped	what activates the skill
Boundary terms	the "Do not use ..." clause	what the skill excludes
What terms	the first sentence of the description	the skill's primary capability

All text is lowercased, tokenized, stop-word filtered, and suffix-stemmed before comparison.

Boundary exclusivity. A boundary term penalizes a query only when it does not also appear in the trigger or what regions. This stops a shared noun (for example pdf in "Do not use for scanned PDFs") from falsely blocking a trigger query that legitimately mentions the same noun.

Wave noise. A small deterministic offset cycles through roughly plus or minus 7% across successive calls, so a borderline query may trigger on two runs of three and not the other, modeling the natural variance of a real model.

What PASS and FAIL mean. A trigger query passes when its triggered count is at least trigger_threshold x runs_per_query (default 2 of 3); a non-trigger query passes when it stays below that. The heuristic measures textual alignment with the skill's declared intent, not live model routing, so use it to catch obvious description problems early.

Output formats and CI

Three renderers, one flag. pretty (the default) is the colored scorecard; json is a stable machine shape for dashboards; sarif is a valid SARIF 2.1.0 document that GitHub code scanning renders as inline PR annotations.

# .github/workflows/skills.yml
- name: Lint agent skills
  run: |
    dart pub global activate skillscore
    skillscore skills/ --min-score 80 --no-color

--min-score fails the build when any skill scores below the threshold, --strict promotes warnings to failures, and the exit codes are designed for pipelines: 0 all good, 1 a quality gate failed, 2 a usage error.

The same three steps drop into any runner. The CI/CD guide has copy-paste configs for ten platforms (GitHub Actions, GitLab CI, CircleCI, Jenkins, Azure Pipelines, Bitbucket, Travis, Drone, Google Cloud Build, and pre-commit), a reusable GitHub Action (uses: sayed3li97/skillscore@v1), a pre-commit hook, and a container image, with a real GitHub Actions run and its SARIF findings in the Security tab.

Commands and flags

skillscore <path> [<path> ...]     Score one or more manifests, folders, or trees
skillscore rules                   List every rule: id, points, severity, targets, source
skillscore explain <rule-id>       Print a rule's rationale, the fix, and its source guide
skillscore eval init <path>        Scaffold evals.json from the skill's description
skillscore eval validate <path>    Validate and summarize evals.json
skillscore eval run <path>         Run trigger-rate evals offline (no API key)
skillscore --version
skillscore --help

Flag	Values	Default	Purpose
--target	claude | antigravity | codex | universal	universal	Which guide's ruleset to apply
--format	pretty | json | sarif	pretty	Output format (SARIF renders in code-review tools)
--min-score <n>	0 to 100	unset	Exit non-zero if any skill scores below n
--fix	flag	off	Apply safe auto-fixes in place (rename a misspelled key), then re-score
--strict	flag	off	Treat warning-level findings as failures
--quiet	flag	off	Print only the score line per skill
--no-color	flag	off	Disable ANSI colors

Exit codes: 0 every skill met the gate. 1 a skill is below --min-score, or --strict found an error or warning, or an eval run failed. 2 a usage error (bad path, unreadable file, unknown rule, invalid flag).

Editor integration

Prefer to score inside your IDE? The Skillscore VS Code extension wraps this CLI and adds inline diagnostics, hover tooltips with the fix and rule id, a sidebar score panel, and a live status-bar indicator. It works in VS Code, Antigravity IDE, VSCodium, and Cursor.

Install from the VS Marketplace or Open VSX.

Library use

skillscore is also a Dart library:

import 'package:skillscore/skillscore.dart';

void main() {
  final doc = SkillParser().parseFile('my-skill/SKILL.md');
  final result = Scorer(RuleRegistry()).score(doc, Target.universal);
  print('${result.score}/100 ${result.grade}');
}

FAQ

What is an agent skill? A folder with a SKILL.md manifest (YAML frontmatter plus Markdown instructions) that teaches an AI agent a repeatable task. Optional subfolders hold references, examples, scripts, and assets.

Does it work with Claude Code, Codex, Antigravity, Gemini CLI, and Cursor? Yes. They share the SKILL.md format. Score against one vendor's rules with --target, or use the default universal profile that a portable skill should pass everywhere.

Is it offline? Completely. Both the linter and the eval harness read local files only and make no network calls. Output is deterministic given the same input.

Does my skill have to be named a certain way? No. skillscore is name-agnostic: the frontmatter name, the folder name, and the file name are independent. Unusual and non-ASCII folder names are handled, though rule A2 will tell you if the name field itself breaks the official format.

What happens with malformed frontmatter? No crash. The relevant A-category errors are reported and every other rule that can still run does, so you always get a score.

How does skillscore compare?

• Vendor skill validators verify only schema validity (name format, description present). skillscore additionally scores quality: discoverability, conciseness, structure, instruction design, hygiene, and safety, with a cited source per rule.
• Generic Markdown linters (markdownlint, Vale) check prose style, not skill semantics. They do not know what a frontmatter description needs for an agent to find the skill.
• Asking an LLM to review a skill is non-deterministic and unsuitable for a CI gate. skillscore is static, reproducible, and exits with pipeline-friendly codes. The two combine well.

Contributing

A new rule is one class plus one registration. See CONTRIBUTING.md for the walkthrough and the project's design principles: every rule cites its source guide, output is deterministic, everything is offline, and nothing assumes a skill's name. Use the "Propose a new rule" issue template to suggest one.

License

Apache-2.0. See CHANGELOG.md for release history.