Edge Case Discovery

Edge Case Discovery

What this skill does

This skill directs the agent to systematically work through a function, API endpoint, or user flow and enumerate all edge cases and boundary conditions that could cause incorrect behavior, errors, or security issues. It applies a structured checklist across multiple dimensions (data types, boundary values, state, concurrency, authorization) and outputs a prioritized list of cases to test or guard against.

Use this before writing tests, during code review, when designing a new feature, or when a production bug makes you wonder "what else could go wrong here."

How to use

Claude Code / Cline

Copy this file to .agents/skills/edge-case-discovery/SKILL.md in your project root.

Then ask:

• "Use the Edge Case Discovery skill on the transferFunds function."
• "Find edge cases for the POST /api/orders endpoint using the Edge Case Discovery skill."
• "What edge cases should I handle in the user registration flow? Use the Edge Case Discovery skill."

Provide the function signature, the code, or a description of the flow.

Cursor

Add the instructions below to your .cursorrules or paste them into the Cursor AI pane before sharing the function or flow you want analyzed.

Codex

Paste the function or flow description and ask Codex to follow the instructions below to generate the edge case list.

The Prompt / Instructions for the Agent

When asked to discover edge cases, apply the following checklist systematically. Not every dimension applies to every input — skip the ones that clearly don't apply and note why.

Dimension 1 — Numeric inputs

For every numeric input or value in the function/flow:

• Zero (0)
• Negative numbers
• Maximum safe integer / maximum allowed value
• Minimum allowed value
• Values just above and just below each boundary (boundary value analysis)
• Non-integer values when integers are expected
• Infinity / NaN (for floating-point)
• Very large numbers that could cause overflow or performance issues

Dimension 2 — String inputs

For every string input:

• Empty string ("")
• Whitespace-only string (" ", "\t", "\n")
• Maximum length string (at or beyond any length limit)
• Single character
• Unicode characters (emoji, RTL text, accented characters)
• SQL injection payloads: ' OR 1=1 --
• XSS payloads: <script>alert(1)</script>
• Null bytes and control characters
• Strings that look like other types: "null", "undefined", "true", "123"

Dimension 3 — Collections (arrays, lists, maps)

For every collection input:

• Empty collection ([], {})
• Single item
• Very large collection (performance/timeout risk)
• Duplicate items (if uniqueness is expected)
• Items in unexpected order
• Null or undefined items within the collection
• Collection with all identical values

Dimension 4 — Null and missing values

• null where an object is expected
• undefined where a value is expected
• Missing optional fields in a request body
• Fields present but set to null explicitly
• Partially populated objects (some required fields missing)

Dimension 5 — State and ordering

• Running the operation twice (idempotency: does it produce duplicates?)
• Running the operation on already-completed state (e.g., canceling a canceled order)
• Running operations out of expected order
• Concurrent calls (two users doing the same operation at the same time)
• Calling a cleanup/delete before the resource is fully initialized
• Interrupted operations (what happens if the process crashes halfway?)

Dimension 6 — Authorization and ownership

• Accessing a resource belonging to another user
• Accessing a deleted or expired resource
• Using an expired or revoked token
• Accessing with insufficient permissions (role too low)
• Accessing with no authentication at all
• Privilege escalation attempts (passing someone else's ID in the request body)

Dimension 7 — External dependencies

• Third-party API returns an error
• Third-party API times out
• Third-party API returns an unexpected response shape
• Database is unavailable or returns a constraint violation
• File system is full or read-only

Format the output

## Edge Cases for [Function/Endpoint/Flow]

### Critical (must handle — will cause data loss, security issue, or crash)
1. [Edge case] — [Why it's critical] — [What currently happens / what should happen]

### High (should handle — causes incorrect behavior or poor UX)
1. [Edge case] — [Expected behavior]

### Medium (nice to handle — minor issues or uncommon paths)
1. [Edge case] — [Expected behavior]

### Out of scope / acceptable to ignore
1. [Edge case] — [Why it's acceptable to not handle]

### Suggested test cases
[List the top 5–7 cases to write as unit/integration tests, in order of priority]

Example

Input to Agent:

"Use the Edge Case Discovery skill on this function:

async function transferFunds(fromAccountId: string, toAccountId: string, amount: number) {
  const from = await Account.findById(fromAccountId);
  const to = await Account.findById(toAccountId);
  from.balance -= amount;
  to.balance += amount;
  await from.save();
  await to.save();
}
```"

Output from Agent:

Edge Cases for transferFunds

Critical

1. amount is negative — Caller passes -100, which silently reverses the transfer direction. Should throw a validation error.
2. fromAccountId === toAccountId — Transferring to the same account: balance unchanged but operation succeeds silently. May indicate a bug in the caller.
3. Insufficient balance — from.balance goes negative. No guard exists. Should check balance before deducting.
4. Race condition on concurrent transfers — Two simultaneous transfers from the same account can both read the same balance and both succeed, causing a negative balance. Requires a database transaction with row-level locking.
5. from.save() succeeds but to.save() fails — Money is deducted from sender but never credited to receiver. The whole operation must be wrapped in a database transaction that rolls back on failure.

High

6. fromAccountId or toAccountId not found — Account.findById returns null, and from.balance throws Cannot read properties of null. Add existence checks.
7. amount is 0 — Technically valid but likely a bug in the caller. Consider throwing a validation error.
8. amount is Infinity or NaN — JavaScript will not throw; balance becomes Infinity or NaN silently.

Suggested test cases

1. Transfer with insufficient balance → expect error thrown, balances unchanged
2. Transfer to self → expect error thrown
3. Transfer with negative amount → expect validation error
4. Either account not found → expect error thrown, no balance changes
5. to.save() throws → expect from balance is rolled back

Notes

• Run this skill before writing tests, not after — it's faster to discover edge cases analytically than by writing tests and waiting for them to fail.
• For APIs, always run Dimension 6 (authorization) — authorization edge cases are the most commonly missed and the most dangerous.
• Not every edge case needs a test — some can be handled with a single defensive guard at the function boundary.