api-breaking-change-detector

name: api-breaking-change-detector description: "Cross-references C# Web API controllers/DTOs against their TypeScript/JavaScript consumers (React, Angular, Vue, Svelte, Node.js, or hand-written/auto-generated HTTP clients like Fetch, Axios, NSwag) to catch contract drift in both directions: backend changes that break client applications (renamed/removed JSON keys, new required parameters, status code shifts) and frontend code sending fields the backend no longer reads. Works directly against source code, not exported OpenAPI spec files. Use when the user asks to check for breaking API changes, verify frontend/backend contract sync, or audit a DTO/controller change against its TypeScript/JS consumers before merging. Not for generating new API code from a spec (see openapi-to-application-code) or scaffolding new endpoints (see aspnet-minimal-api-openapi)."

API Breaking Change Detector

You are cross-referencing a C# Web API's actual contract (controllers, DTOs, route definitions) against its TypeScript/JavaScript consumers to find contract drift — in both directions — before it reaches production.

When to use this

Trigger when the user asks to:

• Check whether a DTO/controller change will break frontend or client applications
• Verify the client and backend API contract are still in sync
• Audit a specific endpoint, or the whole API surface, for breaking changes before a release

Do not use this for:

• Generating new API code from an OpenAPI spec (see openapi-to-application-code)
• Scaffolding new endpoints with OpenAPI docs (see aspnet-minimal-api-openapi)
• Comparing two OpenAPI spec files directly — this skill reads source code, not exported specs

Process

1. Discover Global JSON & Naming Policies:

   • Check Program.cs or Startup.cs for active JSON options (e.g. JsonNamingPolicy.CamelCase, PropertyNamingPolicy, or Newtonsoft CamelCasePropertyNamesContractResolver).
   • Default to camelCase for TypeScript/JavaScript field mapping if global camelCase is configured, unless overridden by an explicit [JsonPropertyName("...")] attribute on the C# property.
   • Ignore C# properties annotated with [JsonIgnore].

2. Identify the C# contract surface. For each Controller action in scope: