Interface SkillSpec

Hint for the argument (frontmatter). Ignored when inputs is set — inputs derive the argument-hint instead.

Freeform instruction body (linear/unstructured skills). Use this OR steps.

Execution context. "fork" runs the skill's body as the task inside a forked SUBAGENT (its own context window) instead of inline in the main conversation (Anthropic's context: fork frontmatter). This is the ONLY setting under which a skill gains a real call→return boundary — so it's the prerequisite for declaring an output Result contract (see output). Omit for the default inline execution.

Short description (used in frontmatter).

Whether to disable model invocation (frontmatter flag).

Typed inputs — compile to argument-hint + a ## Arguments section.

Max lines for an inline fenced code block before compilation errors, forcing the script into a file referenced via file() (default 20). Keeps big scripts out of the skill body (token budget + progressive disclosure). Set 0 to disable.

Skill name (used in frontmatter).

The skill's typed railway outcome — the SAME Result<ok, err> contract a subagent declares with result(okShape, errShape). Valid ONLY with context: "fork": a forked skill runs as a subagent, so it has the call→return boundary a typed outcome needs (compile errors if output is set without context: "fork"). When valid, compiles to a ## Output contract with a vigiles:ok / vigiles:err block — parseable (parseAgentResult) and testable (assertAgentOk) via the existing subagent rail. An INLINE skill has no return, so a typed outcome there is a category error — hence the gate. See research/spec-syntax-and-railway-scope.md.

Declare this skill's purity floor — compile rejects a tool contract looser than it. "pure" allows only read-only tools; "bounded" also allows decidable side-effecting tools (Write, Edit, …) but bars Bash / unknown-effect / inherits-all; "dangerously-unrestricted" (or omitting it) enforces nothing. NOTE: "pure"/"bounded" require an explicit read-only tools list — an absent list inherits ALL tools and is a violation.

Terminal postcondition — the skill is "done" only when this gate passes. Compiles to a ## Result section + a vigiles:result marker.

Gated pipeline steps. When set, the skill compiles to a ## Steps checklist with a deterministic gate per step. Use this OR body.

The allowed-tools contract for this skill. Each entry must be a known built-in tool or an MCP tool (mcp__server__tool). Omit to inherit all tools. When purity is "pure"/"bounded", the declared tools are checked against that floor — compile rejects a tool looser than the declared level.