use-case— agent skill

What this skill does

Produces a complete use case following the Cockburn/RUP format. A use case describes a goal-directed interaction between an actor and the system — capturing not just the happy path, but the extension flows (error, alternative, and exception paths) that user stories typically omit.

When to use it

• User asks to "write a use case" or needs more rigorous specification than a user story.
• The interaction involves complex branching, multiple actors, or system-to-system calls.
• The requirement must satisfy a compliance, audit, or contractual standard that demands formal documentation.
• User is breaking down a user story that is too complex to estimate because its edge cases are unknown.
• Use cases are being collected for a system specification or requirements-document.

Key concepts

Use case vs. user story

Use case components

Extension notation

Extensions are written as [step number][condition]:

3a. Payment is declined:
    3a1. System displays "Payment failed" message.
    3a2. User is offered the option to retry or cancel.
    3a3. If user cancels, go to step 9 (failure).

Instructions

1. Assign an ID and name. Use the format UC-NNN. Name with active verb + object.

1. State the goal. One sentence: what the primary actor wants to achieve.

1. Identify actors.

• Primary actor: who triggers the use case.
• Secondary actors: systems, services, or roles that participate but don't initiate.

1. Write preconditions. List the conditions that must be true for the use case to start. Be specific — not "user is logged in" but "user has an active session with at least Reader role".

1. Write the main success scenario. Numbered steps, each in the form "Actor does something" or "System does something". Include all steps from trigger to goal achieved.

• Keep steps at the same level of abstraction.
• One action per step.
• Do not embed conditions in the main flow — move them to extensions.

1. Write extensions. For each step that can deviate, write the condition and the sub-flow. Reference back to the main flow where it resumes.

1. Write postconditions. What is true after success? What is true after failure/cancellation?

1. List business rules. Numbered, referenced from the relevant steps.

1. Produce the output using the format below.

Output format

## Use Case: [UC-NNN] — [Name]

| Field | Detail |
| --- | --- |
| **ID** | UC-NNN |
| **Name** | [Active verb + noun] |
| **Goal** | [What the primary actor wants to achieve] |
| **Primary actor** | [Role or persona] |
| **Secondary actors** | [System / service / role] |
| **Preconditions** | [Numbered list] |
| **Postconditions (success)** | [System state after success] |
| **Postconditions (failure)** | [System state after failure or cancellation] |

---

### Main success scenario

1. [Actor] [action].
2. System [response].
3. [Actor] [action].
4. System validates [X] against [BR-01].
5. System [response].
6. Goal achieved: [outcome].

---

### Extensions

**3a.** [Condition at step 3]:
  - 3a1. System [response].
  - 3a2. [Actor] [option]. Use case resumes at step 3 / ends in failure.

**4a.** [Condition at step 4]:
  - 4a1. System displays [error].
  - 4a2. Use case ends in failure.

---

### Business rules

| ID | Rule |
| --- | --- |
| BR-01 | [Rule text] |

---

### Notes

- [Assumptions, open questions, or related use cases]

Examples

Example 1 — Place order

Input: "Write a use case for placing an order on an e-commerce site."

Expected output: UC-001 with primary actor "Authenticated customer", secondary actors "Payment gateway", "Inventory service". Main scenario: browse → add to cart → checkout → enter payment → confirm. Extensions: 4a (out of stock), 5a (payment declined), 5b (payment timeout). Business rule: order total must not exceed credit limit.

Example 2 — Reset password

Input: "Use case for password reset via email link."

Expected output: UC-002. Precondition: user has a registered email. Main flow: request → email sent → link clicked → new password entered → confirmed. Extensions: 3a (link expired), 4a (password does not meet policy). Postcondition (success): user session active with new credentials. Postcondition (failure): password unchanged.

Notes

• Use cases describe what the system does, not how. Avoid implementation detail in steps ("System calls the /auth/reset endpoint" → "System sends a password reset email").
• Keep the main success scenario to 10 steps or fewer. If it is longer, the use case is likely two use cases — split at the natural boundary.
• Extension steps that lead to the same outcome as another extension can share a reference (e.g., "3a3. Use case ends as per 4a.").
• This skill pairs with write-user-story (a use case can be decomposed into stories), acceptance-test-plan (each extension is a test scenario), and requirements-document (use cases are listed in the functional requirements section).