What causes "invalid validation error format" in GoogleCloudPlatform/microservices-demo?

Pass the raw error returned by validator.Struct(...) directly, without wrapping

GoogleCloudPlatform/microservices-demo · warning

invalid validation error format

Error message

invalid validation error format

What it means

validator.ValidationErrorResponse converts a validator.ValidationErrors value into a readable message. If the passed err is not actually a validator.ValidationErrors (a failed type assertion), it returns the generic error 'invalid validation error format'. It guards against feeding the wrong error type into the formatting helper.

Solutions

Pass the raw error returned by validator.Struct(...) directly, without wrapping
Check the type before calling: only invoke ValidationErrorResponse when errors.As(err, &validator.ValidationErrors{})
Fix call sites (addToCartHandler, placeOrderHandler, setCurrencyHandler) that wrap the validator error
Log the original error to see what type actually arrived

Example fix

// before
if err := validator.New().Struct(req); err != nil {
    return ValidationErrorResponse(fmt.Errorf("cart error: %w", err))
}
// after
if err := validator.New().Struct(req); err != nil {
    return ValidationErrorResponse(err) // pass unwrapped ValidationErrors
}

Defensive patterns

Strategy: type-guard

Validate before calling

var vErr validator.ValidationErrors
if errors.As(err, &vErr) {
	return ValidationErrorResponse(err) // safe: concrete type present
}
return err // pass through non-validator errors untouched

Type guard

func isValidationErrors(err error) bool {
	var ve validator.ValidationErrors
	return errors.As(err, &ve)
}

Try / catch

if err := validate.Struct(req); err != nil {
	if isValidationErrors(err) {
		return ValidationErrorResponse(err)
	}
	return err
}

Prevention

Never wrap validator errors with fmt.Errorf before passing to ValidationErrorResponse
Keep the raw return value of validator.Struct intact until formatting
Use errors.As to check the concrete type instead of assuming
Update this helper if the validator library is changed or upgraded

When it happens

Trigger: Calling ValidationErrorResponse with an error produced by something other than the validator library's Struct()/Validate() call — e.g. wrapping the error in fmt.Errorf before passing it, or passing a gRPC/transport error.

Common situations: Developers wrap or translate validation errors upstream (losing the concrete type) then pass the result to ValidationErrorResponse in addToCartHandler, placeOrderHandler, or setCurrencyHandler; refactors replacing validator with another library while keeping this helper.

Related errors

AI-assisted analysis of GoogleCloudPlatform/microservices-demo@72ba613a05 (2026-09-02). Data as JSON: /api/errors/06dd00e5396f9261. Report an issue: GitHub.

Appendix: source

Thrown at src/frontend/validator/validator.go:76

// Implementations of the 'Payload' interface.
func (ad *AddToCartPayload) Validate() error {
	return validate.Struct(ad)
}

func (po *PlaceOrderPayload) Validate() error {
	return validate.Struct(po)
}

func (sc *SetCurrencyPayload) Validate() error {
	return validate.Struct(sc)
}

// Reusable error response function.
func ValidationErrorResponse(err error) error {
	validationErrs, ok := err.(validator.ValidationErrors)
	if !ok {
		return errors.New("invalid validation error format")
	}
	var msg string
	for _, err := range validationErrs {
		msg += fmt.Sprintf("Field '%s' is invalid: %s\n", err.Field(), err.Tag())
	}
	return fmt.Errorf("%s", msg)
}

View on GitHub (pinned to 72ba613a05)