use-case

What this skill does

> **Version:** 1.0.0 | **Framework:** Jerry Framework | **Constitutional compliance:** P-003 (no recursive subagents), P-020 (user authority), P-022 (no deception)
> **Status:** PROPOSED | **Author:** eng-backend | **Date:** 2026-03-09

---

## Document Sections

| Section | Purpose |
|---------|---------|
| [Document Audience](#document-audience) | Who reads which sections |
| [Purpose](#purpose) | What this skill does and key capabilities |
| [When to Use](#when-to-use) | Activation conditions and anti-patterns |
| [Available Agents](#available-agents) | Agent routing table with decision signals |
| [P-003 Agent Topology](#p-003-agent-topology) | ASCII hierarchy diagram -- multi-agent topology |
| [Invoking an Agent](#invoking-an-agent) | Three invocation modes |
| [Methodology Reference](#methodology-reference) | Cockburn 12-step and Jacobson UC 2.0 summary |
| [Input Requirements](#input-requirements) | Prerequisites per agent |
| [Output Artifacts](#output-artifacts) | Use case artifact and slice formats |
| [Integration Points](#integration-points) | Cross-skill connections |
| [Routing Entry (Priority 13)](#routing-entry-priority-13) | Trigger map entry for mandatory-skill-usage.md |
| [Constitutional Compliance](#constitutional-compliance) | Principle-to-agent mapping |
| [Quick Reference](#quick-reference) | Common workflows and agent selection |
| [Elaboration State Matrix](#elaboration-state-matrix-detail_level-x-realization_level) | 2D valid-state and readiness matrix |
| [References](#references) | File paths and external citations |

---

## Document Audience

| Level | Audience | Sections |
|-------|----------|---------|
| L0 | Stakeholders, project managers | [Purpose](#purpose), [When to Use](#when-to-use), [Quick Reference](#quick-reference) |
| L1 | Developers, analysts using the skill | [Available Agents](#available-agents), [Invoking an Agent](#invoking-an-agent), [Methodology Reference](#methodology-reference), [Input Requirements](#input-requirements), [Output Artifacts](#output-artifacts) |
| L2 | Framework maintainers, reviewers | [P-003 Agent Topology](#p-003-agent-topology), [Integration Points](#integration-points), [Constitutional Compliance](#constitutional-compliance), [References](#references) |

---

## Purpose

The `/use-case` skill provides guided use case authoring and decomposition through two complementary agents that implement industry-standard methodologies:

1. **uc-author** implements Cockburn's 12-step writing process and Jacobson UC 2.0 progressive narrative levels (Briefly Described through Fully Described) for creating structured use case artifacts.
2. **uc-slicer** implements Jacobson UC 2.0 Activities 2, 4, and 5 for decomposing use cases into implementation-ready slices with INVEST criteria verification, and producing the realization interaction sequences that enable downstream `/test-spec` and `/contract-design` consumption.

**Key Capabilities:**

- Progressive elaboration: artifacts evolve through four detail levels (BRIEFLY_DESCRIBED, BULLETED_OUTLINE, ESSENTIAL_OUTLINE, FULLY_DESCRIBED) with explicit prerequisites at each level transition
- Breadth-first authoring (PAT-001): Steps 1-4 for all use cases before depth elaboration of any single use case, preventing missed actors and incorrect goal levels
- Cockburn goal-level classification: SUMMARY (+), USER_GOAL (!), SUBFUNCTION (-) using the sea-metaphor (Cloud/Kite/Sea Level/Fish/Clam)
- Schema-validated output: all artifacts validate against `docs/schemas/use-case-realization-v1.schema.json`
- Five-state slice lifecycle: SCOPED > PREPARED > ANALYZED > IMPLEMENTED > VERIFIED with worktracker Story entity integration
- INVEST criteria verification on every slice before state advancement
- Realization interaction sequences (`$.interactions[]`) that feed directly into `/contract-design` cd-generator and `/test-spec` tspec-generator

---

## When to Use

**Activate this skill when:**

- Writing a new use case from a stakeholder description or actor goal
- Elaborating an existing use case artifact to a higher detail level (e.g., BULLETED_OUTLINE to ESSENTIAL_OUTLINE)
- Decomposing a use case into implementation-ready slices (Jacobson UC 2.0 Activity 2)
- Preparing slices with test cases and enhanced narrative (Activity 4)
- Analyzing slices to produce realization interaction sequences (Activity 5)
- Identifying actors, goals, and system scope for a new system or feature

**NEVER invoke this skill when:**

- Task is generating BDD test specifications from use case artifacts -- Consequence: `/use-case` creates use case artifacts; `/test-spec` consumes them to produce Gherkin Feature files. Using `/use-case` for test generation produces use case artifacts, not test specifications.
- Task is generating API contracts from use case artifacts -- Consequence: `/use-case` produces structured use case artifacts with interaction sequences; `/contract-design` transforms those interactions into OpenAPI specifications.
- Task is adversarial quality review of deliverables -- Consequence: use `/adversary` for quality scoring and adversarial critique.
- Task is requirements engineering with NASA NPR 7123.1D compliance -- Consequence: `/use-case` implements Cockburn/Jacobson methodology, not NPR requirements processes; use `/nasa-se` instead.
- Task is researching or analyzing a problem -- Consequence: `/use-case` is an authoring tool, not a research or analysis methodology; use `/problem-solving` instead.

---

## Available Agents

| Agent | Role | Model | Cognitive Mode | Tool Tier | Output Location | Decision Signal |
|-------|------|-------|----------------|-----------|-----------------|-----------------|
| `uc-author` | Creates and elaborates use case artifacts using Cockburn's 12-step writing process and Jacobson UC 2.0 progressive narrative levels | sonnet | integrative | T2 | `projects/${JERRY_PROJECT}/use-cases/UC-{DOMAIN}-{NNN}-{slug}.md` | "write", "create", "author", "elaborate", "expand", "describe", "draft", "refine", "use case" |
| `uc-slicer` | Decomposes use cases into implementation-ready slices following Jacobson UC 2.0 Activities 2, 4, and 5 | sonnet | systematic | T2 | Same artifact as input (in-place update adding `$.slices[]`, `$.interactions[]`) | "slice", "decompose", "break down", "prepare", "analyze", "realize", "INVEST", "interaction sequence" |

**Default routing:** When intent is ambiguous between authoring and slicing, route to `uc-author` first. Authoring must precede slicing -- `uc-slicer` requires `detail_level >= ESSENTIAL_OUTLINE` which is produced by `uc-author`. When the user says "create a use case and slice it," invoke `uc-author` first, then `uc-slicer` on the output.

---

## P-003 Agent Topology

Both `uc-author` and `uc-slicer` are T2 worker agents. They are invoked independently from MAIN CONTEXT. They do NOT invoke each other. The output of `uc-author` (use case artifact on disk) is consumed by `uc-slicer` via the filesystem -- not via direct agent-to-agent communication.

```
MAIN CONTEXT (orchestrator)
    |
    +-- uc-author (T2 worker) -- via Task tool
    |   Reads: Project context, existing UC artifacts
    |          docs/schemas/use-case-realization-v1.schema.json
    |          skills/use-case/rules/use-case-writing-rules.md
    |          skills/use-case/templates/*.template.md
    |   Writes: UC artifact (.md with YAML frontmatter)
    |
    +-- uc-slicer (T2 worker) -- via Task tool
        Reads: UC artifact (.md) -- produced by uc-author
               docs/schemas/use-case-realization-v1.schema.json
               skills/use-case/rules/use-case-writing-rules.md
        Writes: Updated UC artifact (adds $.slices[], $.interactions[])
                Optional: separate slice documents

Workers do NOT invoke each other.
uc-author output (UC artifact) is consumed by uc-slicer via filesystem.
Cross-agent data flow is mediated by shared artifact files on disk (P-003 compliant).
```

---

## Invoking an Agent

###