The body is lean: it jumps straight into DO/DON'T rules, code, and a severity table without explaining what diagnostics are or how Rust works, so every token earns its place.

Provides copy-paste-ready Rust examples (Diagnostic::new, Label::new, add_message, Suggestions::patch, Patch::new) plus good/bad message pairs and a when-to-use severity table — concrete and executable.

This is a single-purpose patterns skill where the create-then-annotate-diagnostic flow is unambiguous across the examples; no destructive or batch operation requires a validation checkpoint.

SKILL.md is a concise overview with a clearly signaled one-level-deep reference to references/guidelines.md (verified to exist), each annotated with its contents, keeping navigation shallow and discoverable.

Lists multiple concrete items a user works with — 'error messages, warnings, Labels, Messages, Severity levels, Patches, Suggestions' — matching the anchor for several specific concrete actions.

States what it does ('HashQL diagnostic writing patterns using hashql-diagnostics crate') and explicitly when to use it via an explicit 'Use when ...' clause, satisfying both what and when.

It includes natural terms ('error messages', 'warnings', 'improving diagnostic quality') but leans heavily on capitalized crate type names (Label, Message, Severity, Patch) that read as API jargon rather than words users naturally say.

The HashQL-specific crate niche and diagnostic-only scope give it a clear, distinct trigger set unlikely to overlap with other skills.