Use Case Spec

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Use Case Specification

Instructions

Create or update use case specification documents for $ARGUMENTS in docs/use_cases/. Each use case describes a complete interaction between an actor and the system to achieve a goal.

File naming (do this exactly)

One file per use case, written to docs/use_cases/UC-XXX-<kebab-case-name>.md where:

• UC-XXX is the use case's three-digit ID (e.g. UC-001).
• <kebab-case-name> is the use case name taken verbatim from the use case diagram (docs/use_cases.puml), lowercased with spaces replaced by hyphens. Do not paraphrase, expand, or reorder the words.

Scope: one or many use cases

• If the task names a single use case (e.g. "write UC-001 Place Order"), produce only that one file. Do not create specs for other use cases in the diagram.
• If the task asks for "all use cases" or names several, produce one file per use case:
  • UC-XXX IDs come from the diagram and never repeat.
  • BR-XXX business-rule IDs are unique within their own file only and restart at BR-001 in every file — the use case is the namespace. When referring to a rule of another use case, qualify it with the use case id (e.g. "UC-005 BR-002"), never by the bare rule id.

DO NOT

• Write vague or incomplete scenarios
• Skip numbering steps in the Main Success Scenario
• Omit alternative flows for error conditions
• Invent an alternative flow only to fill the section — when no step has a meaningful alternative or exception condition, say so with a placeholder (see workflow step 9)
• Leave postconditions undefined
• Describe the reaction to a failure ("System displays an error message") as a failure postcondition — that is a step of the alternative flow; failure postconditions are guarantees that hold on every unsuccessful end
• Write a technical step (validate, load, persist) as if it were a user goal — see workflow step 2
• Write an event or an actor action as a precondition ("User clicks New Order") — that is the trigger
• Write a precondition that the use case establishes or evaluates itself ("A room is available for the requested dates" when the dates are entered in step 4 and availability is checked in step 5) — that is a step and an alternative flow
• Mix multiple use cases in one document
• Use technical implementation details in the flow steps

Template

Use references/use-case.md as the document structure, and see references/example.md for a complete worked example — actor-focused steps, alternative flows that reference specific step numbers, and success postconditions paired with failure postconditions written as minimum guarantees. These paths are relative to the folder containing this SKILL.md, not to the project root.

The normative definition of the format — including the German variant and the tolerances of the AI Unified Process Studio structured editor — is references/format-spec.md. A machine check of both the structure and the rules of this skill is bundled as scripts/validate_use_case.py.

Status values

Step writing guidelines

Steps describe what the actor and system achieve, never how it is implemented. Keep out protocol and infrastructure terms (SMTP, JWT, bcrypt, hashing, SQL/INSERT/SELECT, HTTP verbs, class and exception names) — those belong in the implementation, not the specification.

Workflow

1. Read the docs/requirements.md and docs/use_cases.puml, and docs/glossary.md when it exists. Use the glossary's terms for actors, business objects, and states, and never a synonym listed in its Avoid column; a new domain term the use case needs goes into the glossary (see /requirements).

2. Determine the set of use cases to document (one, several, or all in the diagram — see "Scope" above). Take each UC-XXX ID and name from the diagram. Before writing, ask of each one: is this use case a complete goal that the primary actor would recognize as valuable? A subfunction ("Validate METAR", "Load NOTAM", "Persist Result") is a step of a larger user goal ("Determine Airport Suitability"), and a summary ("Manage Flight Operations") spans several. A scenario that hands the work over to another role, waits for an outside event or a deadline, or runs branches in parallel for different actors is a summary, too: its parts are separate user goals, and the flow between them is a business process in BPMN (docs/processes/, written by /business-process), not a section of the use case. Do not rename, merge, or split use cases yourself — the ids belong to the diagram. Write the specification, then tell the user which use case looks like a subfunction or a summary, name the user goal it belongs to, and hand off to /use-case-diagram. A subfunction that the diagram draws as an <<include>> shared by several use cases is intended; leave it.

3. Clarify before writing. Scan the sources for each use case with references/clarify-checklist.md and ask the user the questions whose answer changes the specification and has no reasonable default — at most five per use case, the most important first, each with a recommended option. Write the answers into the steps, flows, and rules they affect, never into a questions section. Decide everything else with the best default and report it as an assumption (step 16). When no one can answer — a pipeline run, a host without a way to ask, or the user said not to ask — take the recommended option for every question and report it as an assumption as well. Never ask about implementation choices; they do not belong in a use case.

4. Use TodoWrite to track progress — one item per use case file.

5. For each use case, derive the filename with the rule in "File naming" above.

6. Write the Overview section: Use Case ID, primary actor, secondary actors, goal, trigger, and a Status from the "Status values" list above. The primary actors are the roles that pursue the goal. A use case often has one, but it may have several: list them comma-separated (**Primary Actor:** Front Desk Clerk, Guest) when each of them can start the use case on its own and pursues the same goal through the same main success scenario — never joined with "or" or "and", and never "System". When two roles pursue different goals or need different scenarios, they are two use cases. Name concrete roles from the requirements and the glossary rather than a generic "User" when the requirements distinguish roles. Secondary actors are the roles and external systems that support the use case or provide information or services to it (e.g. **Secondary Actors:** Weather Service, Flight Planning System). Name them so an implementation treats them as outside the system's responsibility, not as something to build. Omit the **Secondary Actors:** line when the use case has none. The **Trigger:** line (German documents: **Auslösendes Ereignis:**, never **Auslöser:**, which labels alternative flows) names the event that starts the use case: an actor's request (Dispatcher requests an airport suitability assessment), a point in time (End of each business day), or a message from an external system (Payment Service reports a chargeback). It happens at a moment; a precondition is a state that is already true. Test it by asking "when does this happen?" — User is logged in has no moment and is a precondition. The trigger may coincide with step 1, but step 1 does not repeat it word for word. When docs/requirements.md exists, add a **Requirements:** line after the Status line: one Markdown link to the catalog whose link text lists the requirement ids — at least the functional requirements (FR-*) this use case realizes, plus the non-functional requirements (NFR-*) and constraints (C-*) it must respect, e.g. **Requirements:** [FR-001, NFR-004, C-003](../requirements.md). List ids only, never copy requirement text — requirements.md stays the source of truth. /spec-review uses the line to find requirements no use case covers and ids that do not exist. Omit the line only when there is no docs/requirements.md.

7. Define preconditions — verifiable facts that must be true before the use case starts. They are states the system has already established (often by another use case), never events or actor actions, and the use case does not check them again. A precondition must not describe a condition that is established or evaluated during the use case: when the fact depends on input the actor gives in a step ("a room is available for the requested dates"), when a step checks it, or when an alternative flow handles its violation, it is not a precondition but a condition to handle in the Main Success Scenario and its alternative flow. Keep the stable state it rests on instead (Room inventory is configured).

8. Write the Main Success Scenario as numbered steps (start at 1, no gaps), alternating actor action and system response, ending with the goal achieved.

9. Analyze every Main Success Scenario step for meaningful alternative or exception conditions — can the actor decide differently, can the input be invalid, can a check fail, can an external system refuse or not answer? Document every extension you identify (error conditions, optional paths, exceptional situations); most real use cases have two or more. Each one must:

   • name a Trigger that references a specific main-scenario step number, written as (step N) (e.g. Payment is declined (step 7)); and
   • end with either Use case continues at step N. or Use case ends.

   Do not invent an alternative flow solely to satisfy the template. When the analysis finds no meaningful alternative, replace the template flow with an italic placeholder that states the result, e.g. _None — no step of the main success scenario can fail or branch._ The validator accepts the placeholder as a deliberate statement and warns only when the section is left empty.

10. Define postconditions for both success and failure (both subsections non-empty).