What Is RFC 9457 and How Should APIs Return Errors?

TL;DR RFC 9457 (Problem Details for HTTP APIs) is the standard for API error responses. It establishes a consistent structure— type , title , status , detail , and instance —to replace custom error formats. Modern PetstoreAPI implements RFC 9457 for all error responses with correct content negotiation and validation detail support. Try Apidog today Introduction When your API returns an error, what does the response look like? If you're like most APIs, you likely have a custom format: { "error" : "Invalid email" } { "message" : "Not found" , "code" : 404 } { "success" : false , "errors" : [ "Email required" ]} Every API invents its own error structure, making client-side error handling inconsistent and error-prone. There’s no standardized way to parse, display, or log errors. RFC 9457 fixes this by defining a standard, machine-readable error response format that clients can rely on. The old Swagger Petstore used inconsistent error formats. Modern PetstoreAPI now implements RFC 9457 thro