Returning RFC 9457 Problem Details from Go Validation Errors

How to turn struct-tag validation failures into a standard "application/problem+json" response — the RFC 9457 format — with one method call, no hand-rolled error envelope. Most Go APIs invent their own validation error shape. One team returns {"errors": [...]} , another {"field_errors": {...}} , a third just a flat {"error": "message"} and hopes the client parses it. Every one of those is a private contract the client has to learn from your docs, because there's no shared shape for "here's what's wrong with your request." RFC 9457 — Problem Details for HTTP APIs — is the IETF standard that fixes this: a application/problem+json body with type , title , status , and room for problem-specific extensions. Checker now builds one of these directly from a failed struct validation, via CheckErrors.ProblemDetails() . The shape RFC 9457 defines four base members — type , title , status , detail , instance — and lets a specific problem type add its own. For validation errors, RFC 9457 §3.1 sketches exactly this extension: an invalid-params array listing which fields failed and why. That's what Checker produces. From a failed struct to a problem+json body Take a struct with a missing required field: type Person struct { Name string `checkers:"required"` } person := & Person {} errs , ok := checker . CheckStruct ( person ) if ! ok { data , _ := json . Marshal ( errs . ProblemDetails ()) fmt . Println ( string ( data )) } { "type" : "about:blank" , "title" : "Your request parameters failed validation." , "status" : 400 , "invalid-params" : [ { "name" : "Name" , "reason" : "Required value is missing." , "code" : "REQUIRED" } ] } One method call — errs.ProblemDetails() — turns the same CheckErrors you'd otherwise call .JSON() on into a *ProblemDetails value, ready to marshal. type defaults to "about:blank" (RFC 9457's own default for "no more specific problem type registered"), status defaults to 400 , and each invalid-params entry carries the field name , a localized human-readab