Component Model — layer decision and fat-to-thin refactor

The operational side of the repository's component model: place new logic in the right layer, and pull overgrown components back into theirs. The canonical definitions — the four layers, the placement table, the thin-executor pattern — live in .claude/kb/shared/component-model.md; this skill applies that model, it does not restate it.

When to use

• Deciding whether new functionality should be an agent, a skill, a command, a KB file, or a template.
• Planning a new component before authoring it — the authoring flows load this skill first as their layer-decision gate.
• A component has grown fat — an agent body full of process steps, a command re-implementing methodology inline — and needs restructuring.
• Reviewing a change that spans component types and looks misplaced.

Skip if

• The layer is already decided and you are authoring the component — go straight to create-skill, create-agent, or /create-kb.
• The change is content-only inside an existing component (fixing a step in a skill, updating a KB fact) — no layer question exists.

The decision procedure

1. Read the canonical model. Open .claude/kb/shared/component-model.md — the four layers, the "where does new logic go" table, and the thin-executor pattern are defined there, and every classification below is grounded in that file. Read it before classifying, not after.

2. Classify the new logic. Ask, in order, and stop at the first yes:

   • Does it say how to do something — process steps, question frameworks, quality-gate checklists, a verification procedure? → skill.
   • Does it define who does the work — identity, tool scope, model tier, stop conditions, escalation targets, non-negotiable policies? → agent (frontmatter plus thin shell).
   • Does it define where the user starts — argument surface, mode selection, sequencing of existing pieces? → command.
   • Is it a fact other components must agree on — reference truth, standards, domain deep dives? → KB file; if it is an artifact's required shape, it is a template the skill points at.

   A piece of work that answers yes more than once spans layers — go to step 3.

3. Split multi-layer work along layer lines. Do not pick the biggest layer and stuff everything into it. A new capability normally lands as a skill plus a one-line loading instruction in the agent or command that uses it — not as a fat agent. A new entrypoint lands as a thin command that sequences existing agents and skills, never one that re-implements them. Worked example — "add a code-review capability" splits four ways: the review method is a skill; the reviewer identity and tool scope are agent frontmatter; the user-facing entrypoint is a thin command; the severity taxonomy every component must agree on is a KB fact.

4. Route to the authoring resource.

   Chosen layer	Author it with
   Skill	create-skill — repo naming, placement, frontmatter, ship checklist; defers to upstream skill-creator for general craft
   Agent	create-agent
   KB domain	/create-kb; add --validated (the kb-build skill) when every claim must be source-verified
   Command	no authoring skill — keep it thin: parse arguments, pick the mode, load the skill or delegate to the agent
   Template	own it in the template directories (.claude/sdd/templates/, .claude/kb/_templates/) and have skills reference it

Misclassification smells

Concrete tells that logic sits in the wrong layer, with the fix:

Refactoring a fat component

The operational version of the recipe in the KB — run it when a component (usually an agent) has absorbed methodology that belongs a layer down:

1. Split the body into methodology (process steps, frameworks, checklists) versus executor shell (identity, non-negotiable policies, escalation).
2. Create the skill — named <domain>-<verb> per create-skill conventions — and move the methodology into it; point at templates for artifact shapes instead of re-inlining them.
3. Shrink the agent to identity + policies + one loading instruction: read the skill and execute it.
4. Keep agent frontmatter byte-identical unless the contract itself is changing — the agent router regenerates from frontmatter, so an accidental frontmatter diff silently changes routing.
5. Re-point every other copy — commands, contracts, docs — at the new skill, keeping only contract-grade facts where they are.
6. Verify with a rebuild — run the plugin build and confirm the router output and mirrored skills change only where intended.

The SDD workflow components are the reference implementation of the end state: thin *-agent executors, sdd-* skills carrying the methodology, and thin phase commands sequencing them.

References