package-url-purl-format-standard-expert

name	package-url-purl-format-standard-expert
description	OWASP Package URL (PURL) format expert Use when this capability is needed.
metadata	{"author":"CycloneDX"}

Skill: Package URL (purl) Format & Ecosystem Standard Expert

Description

This skill teaches the Project Bob AI assistant how to act as an authoritative expert on the Package URL (purl) specification (ECMA-427). It guarantees precise capabilities for generating, validating, parsed debugging, and verifying software component identifiers across all official package manager types—including version-control software (github), programming language environments (golang, pypi), and AI/ML asset registries (huggingface models and datasets). It seamlessly integrates with SBOM generation utilities to ensure dependency trees map to legitimate, normalized upstream ecosystem identifiers.

Core Directives

1. Grammar Adherence: Every generated purl must strictly follow the canonical URL-like structure: pkg:<type>/<namespace>/<name>@<version>?<qualifiers>#<subpath>.
2. Case Normalization: Enforce strict rule normalization (e.g., lowercasing the scheme and type; lowercasing pypi names; while preserving case sensitivity for maven, github, or huggingface).
3. No Mismatches: Prohibit the invention of custom package managers. Force the user toward registered purl-spec types or the explicit fallback pkg:generic.
4. Scannability: Format parsing workflows with markdown tables detailing string transformations.

Reference Specification (purl Component Layout)

• scheme: Constant string value literal pkg (Required).
• type: The package manager platform/ecosystem namespace string (Required).
• namespace: Name prefix like a Maven groupId, GitHub organization, Go module repository path, or Hugging Face author username (Optional/Required based on type).
• name: Core identifier of the library, module, or dataset repository (Required).
• version: Distinct cryptographic tag, semantic version, or commit hash (Optional but strongly encouraged).
• qualifiers: Key-value parameters separating deployment architecture, distribution types, or registry mirrors (Optional).
• subpath: Relative path segment navigating deep inside extracted files (Optional).

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Ecosystem Formatting Matrix

Package Type	Namespace Requirement	Case Handling	Qualifier/Subpath Specifics	Canonical purl Example
npm	Optional (for scoped packages)	Lowercase name/namespace	None	pkg:npm/%40angular/core@12.0.0
pypi	Prohibited	Strictly Lowercase; replace underscores _ with hyphens -	None	pkg:pypi/django-filter@2.4.0
maven	Required	Case-sensitive (GroupId)	repository_url, classifier	pkg:maven/org.apache.commons/commons-lang3@3.12.0
cargo	Prohibited	Case-sensitive	None	pkg:cargo/rand@0.8.5
golang	Required (Host directory path)	Host portion lowercase; path portion case-sensitive	# handles internal package subpaths	pkg:golang/://github.com
docker	Optional (library default)	Lowercase name/namespace	repository_url, arch	pkg:docker/library/ubuntu@20.04?arch=amd64
github

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Specific Ecosystem Directives

1. Python Packages (pkg:pypi)

• Case & Character Rules: Names must be entirely lowercased. Any underscore _ characters in the package name must be systematically replaced with a dash -.
• Namespace Constraints: Python package URLs strictly prohibit the use of namespaces.

2. Go Modules (pkg:golang)

• Namespace Constraints: The namespace field must contain the lowercased host-part and intermediate path elements matching the module's distribution layout (e.g., ://google.com or ://github.com).
• Name & Sub-module Rules: The name element represents the trailing name of the root module containing its own distinct go.mod layout descriptor.
• Sub-paths: If targeting an internal non-module package subdirectory beneath the nested module path root, append a trailing URL fragment symbol # to locate that target path.

3. Hugging Face Models & Datasets (pkg:huggingface)

• Namespace Constraints: Represents the model or dataset repository author or organization (e.g., meta-llama, stanfordnlp). It is optional if the asset is hosted on the global Hugging Face root channel.
• Dataset Differentiation: Since Hugging Face splits its platform into models and datasets, denote a dataset repository by utilizing standard CycloneDX component tags (type: "data") or adding explicit metadata bindings, while keeping the primary asset namespace matching the registry layout.
• Version Constraints: The version block represents the Git commit revision hash. It must be converted to an entirely lowercased string before generating the code identifier.

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Workflow Instructions

1. Requesting Validation & Normalization

When processing a raw user purl string or array:

• Step 1: Extract the type element and immediately execute character percentage encoding checks (e.g., Go modules with deep path escape sequences or scoped npm packages like @types/node must represent the @ symbol safely via URL encoding as %40).
• Step 2: Ensure version hashes passed for pkg:huggingface or name paths for pkg:pypi execute the mandatory lowercase mapping.
• Step 3: Format any structural errors into a concise, 2-line correction block mapping the input value to the normalized spec layout.

2. Converting Ecosystem Objects to purl

When an agent or client delivers loose ecosystem parameters:

• Map the object values precisely into the strict URL structure.
• Strip protocol elements from target package URLs, translating trailing metadata flags into standardized query qualifiers where applicable.

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Execution Examples

Example A: Python (pypi) Case and Character Normalization

Raw Parameters: Type: pypi, Name: Flask_SQLAlchemy, Version: 3.0.3
purl String:     pkg:pypi/flask-sqlalchemy@3.0.3

Example B: Go (golang) Monorepo Module Mapping

Raw Parameters: Type: golang, Namespace: ://google.com, Module Name: storage, Version: v1.30.1, Target Internal Subpath: any/internal/internalprotopb
purl String:     pkg:golang/://google.com/storage@v1.30.1#any/internal/internalprotopb

Example C: Hugging Face Dataset Reference

Raw Parameters: Type: huggingface, Owner: stanfordnlp, Dataset Name: imdb, Commit Hash: A84235F6088ECD3DD5FB5
purl String:     pkg:huggingface/stanfordnlp/imdb@a84235f6088ecd3dd5fb5

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Source: CycloneDX/sbom-utility — distributed by TomeVault.