Conciseness

The body is entirely code, tables, and actionable checkpoints with zero explanations of concepts Claude already knows — no "a lexer is..." padding, no library recommendations, no filler. Every section (token types, precedence table, AST unions, evaluator dispatch) earns its tokens, matching the score-5 anchor ("lean and efficient; every token earns its place"). Score 4 would require minor over-explanation to trim, and none is present.

Actionability

Every stage has concrete TypeScript — the TokenType enum, the lexer dispatch loop, the precedence table, the AST discriminated unions, and the evaluator switch — but the body snippets are deliberately partial: the lexer omits helper methods, the parser stops at "// Continue down precedence...", and the interpreter switch is missing several cases. This falls between the score-4 anchor ("concrete code with minor gaps") and the score-5 anchor ("fully executable; copy-paste ready"): the gaps are justified by explicit pointers to the full implementations in references/lexer.md and references/parser.md, but the body code alone is not copy-paste runnable.

Workflow Clarity

The pipeline sequence (Source Code → Lexer → Tokens → Parser → AST → Interpreter → Result) is paired with explicit per-stage validation checkpoints: "feed a test string, inspect token stream for correct types/values", "print AST for a simple expression, confirm node shape matches type definitions", and "evaluate a literal or binary expression before wiring up the full pipeline". Error recovery is additionally signaled via references/error-handling.md. This matches the score-5 anchor (clear sequence, explicit validation steps, error recovery); score 4's example only verifies at the end of the flow, which is clearly weaker than this per-stage checkpointing.

Progressive Disclosure

All five referenced files (lexer.md, parser.md, environment.md, error-handling.md, builtins.md) exist, are exactly one level deep (they complement the body rather than chaining to further files), and are signaled both inline at each excerpt ("// Full implementation → references/lexer.md") and in a dedicated Reference Files section with per-file descriptions. Core patterns live in the body, full implementations in references — matching the score-5 anchor (clear overview, well-signaled one-level-deep references, appropriate split).

Specificity

The description lists multiple concrete actions — "Generates lexer and parser implementations, defines AST node structures using discriminated unions, and builds tree-walk interpreter/evaluator patterns with scoped environments" — covering the full pipeline from tokenization to evaluation with no generic filler, matching the comprehensive-coverage anchor. It is clearly above the score-4 anchor, which requires minor gaps in coverage; no action here is vague.

Completeness

It explicitly answers both what ("Generates lexer and parser implementations, defines AST node structures... builds tree-walk interpreter/evaluator patterns") and when, with two concrete "Use when..." clauses ("Use when designing or implementing a programming language, compiler, interpreter, tokenizer, or DSL" and "Use when building a grammar, writing a tokenizer, constructing a syntax tree"). This matches the score-5 anchor exactly; score 4 would require the when-clause to be less explicit or specific.

Trigger Term Quality

Natural trigger terms are comprehensive and include synonyms: "programming language, compiler, interpreter, tokenizer, or DSL (domain-specific language)", "grammar", "syntax tree", "custom language", "tree-walk interpreter/evaluator". This matches the score-5 anchor (comprehensive coverage including synonyms); score 4 requires natural terms to be missing, but the common ways a user would phrase this need are all present.

Distinctiveness Conflict Risk

The language-implementation niche is clear and mostly distinct, but "tokenizer", "grammar", and "syntax tree" carry minor overlap risk with NLP/text-processing and writing-assistant skills, fitting the score-4 anchor ("mostly distinct; minor overlap risk with closely related skills") better than score 5's "minimal conflict risk". Not score 3, because the triggers are firmly anchored in "designing or implementing a programming language... or DSL" rather than being broadly generic.