Breaking Change Detector

What this skill does

This skill directs the agent to compare two versions of a codebase, API schema, or interface definition and systematically identify every change that could break existing callers. It classifies each change by severity, explains why it is breaking, and provides a concrete migration hint so consumers know exactly what they need to update.

Use this before publishing a new package version, before deploying an API change that existing clients depend on, or during a code review to catch unintended contract changes.

How to use

Claude Code / Cline

Copy this file to .agents/skills/breaking-change-detector/SKILL.md in your project root.

Then ask:

• "Use the Breaking Change Detector skill to compare the old and new versions of our API."
• "I changed `src/auth/index.ts`. Use the Breaking Change Detector skill to find any breaking changes."

Provide: the old version (paste, file path, or git ref) and the new version.

Cursor

Add the instructions below to your .cursorrules or paste them into the Cursor AI pane. Then share both the old and new code side by side.

Codex

Paste both versions clearly labeled as "OLD VERSION" and "NEW VERSION" and ask Codex to follow the instructions below.

The Prompt / Instructions for the Agent

When asked to detect breaking changes, follow this process:

Step 1 — Establish what constitutes the public contract

Before comparing, identify what is "public" — i.e., what external callers or consumers depend on:

• For a library: all exported functions, types, interfaces, and constants
• For a REST API: all endpoints (method + path), request body shapes, response shapes, status codes, and headers
• For a database schema: table names, column names, data types, nullability, and unique constraints
• For a TypeScript/JavaScript module: all export statements

Changes to internal/private code are not breaking changes unless they affect something observable from outside.

Step 2 — Categorize each difference

Compare the old and new versions and classify every difference into one of these categories:

Breaking changes (must flag)

• Removed export, endpoint, table, or column
• Renamed export, endpoint, table, or column (removal + add = breaking)
• Changed function signature: removed a parameter, changed a required parameter's type, changed return type in an incompatible way
• Changed REST response: removed a field, changed a field's type, changed a status code that callers branch on
• Added a required field to a request body or function parameter
• Narrowed a type (e.g., string → 'a' | 'b')
• Changed behavior that existing callers rely on (e.g., changed sort order, changed error format)

Non-breaking changes (informational only)

• Added a new optional parameter (with a default value)
• Added a new export, endpoint, or field
• Widened a type (e.g., 'a' | 'b' → string)
• Internal refactoring with identical observable behavior
• Performance improvements
• New optional fields in a response

Step 3 — For each breaking change, produce a migration hint

Every breaking change entry must include:

• What changed (old vs new)
• Why it is breaking (what callers were relying on)
• What callers must do to migrate (specific, actionable)

Step 4 — Format the output

## Breaking Change Report

### Summary
- Breaking changes: N
- Non-breaking changes: N (informational)

---

### Breaking Changes

#### 1. [Short title]
- **Type**: [Removed / Renamed / Signature change / Type change / Behavior change]
- **Location**: `path/to/file.ts` or `POST /api/endpoint`
- **Old**: [What it was before]
- **New**: [What it is now, or "Removed"]
- **Why it breaks**: [What callers were depending on]
- **Migration**: [Exactly what callers need to change]

#### 2. ...

---

### Non-Breaking Changes (Informational)

| # | Location | Change | Notes |
|---|----------|--------|-------|
| 1 | `src/utils.ts` | Added `formatCurrency()` export | New addition, no action needed |

---

### Recommended Version Bump
[Based on semver: MAJOR if any breaking changes exist, MINOR if only additions, PATCH if only bug fixes]

Step 5 — Flag ambiguous cases

If a change is ambiguous (e.g., a behavior change that some callers rely on but it was arguably a bug), flag it explicitly as "Potentially breaking — review required" with context explaining the ambiguity.

Example

Input to Agent:

"Use the Breaking Change Detector skill. Here are the two versions of our user service interface:

>

OLD: ``ts export function getUser(id: string): Promise<{ id: string; name: string; email: string }> export function createUser(name: string, email: string): Promise<{ id: string }> export function deleteUser(id: string): Promise<void> ``

>

NEW: ``ts export function getUser(id: string): Promise<{ id: string; name: string; email: string; role: string }> export function createUser(input: { name: string; email: string; role: string }): Promise<{ id: string }> ``"

Output from Agent:

## Breaking Change Report

>

### Summary - Breaking changes: 2 - Non-breaking changes: 1 (informational)

>

---

>

### Breaking Changes

>

#### 1. createUser signature changed from positional args to object input - Type: Signature change - Location: userService.ts - Old: createUser(name: string, email: string) - New: createUser(input: { name: string; email: string; role: string }) - Why it breaks: All callers passing createUser("Alice", "[email protected]") will break at compile time. Also, role is now required — callers that don't provide it will fail. - Migration: Update all call sites to createUser({ name: "Alice", email: "[email protected]", role: "user" }). Decide on a default role value for existing callers.

>

#### 2. deleteUser removed - Type: Removed - Location: userService.ts - Old: deleteUser(id: string): Promise<void> - New: Not present - Why it breaks: Any caller invoking deleteUser() will get a compile error or runtime error. - Migration: Find a replacement for delete functionality or soft-delete via updateUser. Audit all usages with grep -r "deleteUser".

>

---

>

### Non-Breaking Changes

>

| # | Location | Change | Notes | |---|----------|--------|-------| | 1 | getUser return type | Added role: string field to response | Additive — existing callers can ignore it |

>

### Recommended Version Bump MAJOR — two breaking changes require a major version bump.

Notes

• For REST APIs, also check HTTP status codes — changing a 404 to a 400 can break callers that branch on status codes.
• For database schemas, dropping a column is always breaking even if no application code references it — other services may query directly.
• This skill does not apply the migrations — it produces the report. Use the Migration Guide Writer skill to turn the report into a full migration guide.