# ADR-0013: Skill model — additive instruction+tool bundles

**Status:** Accepted — 2026-06-10

## Context

mort's "skills" are reusable capabilities (a prompt fragment plus tools)
attached to different agents at runtime. majordomo needs the same shape
without mort specifics: attachable to any agent, on demand, composing
additively and predictably.

## Decision

- The **contract** is the three-method `agent.Skill` interface (Name /
  Instructions / Tools). It lives in package agent — the consumer of the
  contract — so skill→agent and agent→skill never both happen (no import
  cycle, and third parties can satisfy it without importing package skill).
- Package **skill** is the standard constructor: `skill.New(name,
  WithInstructions(...), WithTools(...)/WithToolbox(...))`. Instruction-only
  skills (nil toolbox) are legal — agent treats Tools()==nil as "no tools".
- **Composition is additive and ordered:** instructions append to the
  agent's system prompt in attachment order, separated by blank lines;
  skill tools join the merged toolset with the same loud duplicate-name
  policy as toolboxes (ADR-0012). No hooks, no resources, no lifecycle —
  none of mort's skills need them, and out-of-scope discipline says don't
  build for hypotheticals (they'd be an additive interface upgrade later).
- Two ready-made example skills ship as subpackages and double as the
  pattern reference: `skill/clock` (time awareness; injectable clock) and
  `skill/calc` (exact arithmetic via a hand-rolled recursive-descent
  evaluator — no eval, no dependencies).

## Consequences

- A skill instance is reusable across agents simultaneously (it is
  read-only after construction).
- mort's skill registry maps onto plain values: build `*skill.Skill`s,
  attach per agent at invocation time.