Conciseness

Every sentence carries non-obvious, project-specific facts — e.g. "registered in standardHandlers() before the block handler", "the stateless Run entrypoint used by the hook runner", and the ctx.Err() vs interp.ExitStatus(n) semantics. No generic shell/Go padding; anchor 5 (every token earns its place).

Actionability

Each step ships copy-paste-adaptable code: the `case "mycommand":` snippet, the exact handler signature `func handleMyCommand(ctx context.Context, args []string, stdin io.Reader, stdout, stderr io.Writer) error`, and the ctx.Err() polling loop. The common case (adding a builtin) is fully covered with specific file paths.

Workflow Clarity

A clear six-step numbered sequence with rationale at each step, but no explicit verification checkpoint (nothing about how to test that the new builtin works). Fits anchor 4 (clear sequence, minor validation gaps); the destructive/batch cap does not apply since this is additive code with compiler feedback.

Progressive Disclosure

The skill is self-contained with no bundle files and no external references, organized into clean sections (How Builtins Work, Adding a New Builtin, Existing Builtins table). Per the rubric's simple-skill guidance, well-organized sections with no nested references score 5.

Specificity

Names three concrete actions ("creating a new shell builtin command", "editing an existing one", "understand how commands are intercepted") plus concrete paths like internal/shell/, matching the 'several specific actions with minor gaps' anchor. Not 5 because it never states what the skill itself provides (handler wiring, code patterns).

Completeness

An explicit "Use when..." clause with three distinct triggers is present, and the 'what' is embedded in the gerund phrases rather than stated as a standalone capability sentence. Fits anchor 4 (both present, one could be more explicit — here the what); not 5 since there is no leading what-statement as in the anchor-5 example.

Trigger Term Quality

Good natural keyword coverage — "shell builtin", "command", "Crush", "intercepted", "embedded shell" — all terms a user would plausibly say. Not 5 because common phrasings like "add a command to the shell" or "bash tool" are absent.

Distinctiveness Conflict Risk

"Crush (internal/shell/)", "shell builtin", and "embedded shell" carve out a clear niche with distinct triggers and minimal overlap with generic shell or command skills. Matches anchor 5.