diff --git a/errorf.go b/errorf.go
index 9be66cc..5f371e5 100644
--- a/errorf.go
+++ b/errorf.go
@@ -1,6 +1,7 @@
 package errors
 
 import (
+	"encoding/json"
 	"fmt"
 	"net/http"
 )
@@ -16,13 +17,14 @@ import (
 //
 //	err := errors.Errorf(http.StatusNotFound, "%w", sqlError).SetMsg("user not found %d", userId)
 func Errorf(status int, format string, parts ...any) SettableError {
+	meta := make(map[string]any)
 	if len(parts) == 0 {
-		return &erf{status, format, ""}
+		return &erf{status, format, "", meta}
 	}
 
 	err := fmt.Errorf(format, parts...)
 	msg := err.Error()
-	er := &erf{status, msg, ""}
+	er := &erf{status, msg, "", meta}
 	if we, ok := err.(wrappedError); ok {
 		return &wrapErr{er, we.Unwrap()}
 	}
@@ -34,11 +36,13 @@ func Errorf(status int, format string, parts ...any) SettableError {
 
 var _ ResponsableError = new(erf)
 var _ SettableError = new(erf)
+var _ json.Marshaler = new(erf)
 
 type erf struct {
 	stat int
 	err  string
 	msg  string
+	meta map[string]any
 }
 
 func (e *erf) Status() int {
@@ -76,6 +80,21 @@ func (e *erf) SetMsg(msg string, parts ...any) SettableError {
 	return e
 }
 
+func (e *erf) SetField(field string, value any) SettableError {
+	e.meta[field] = value
+	return e
+}
+
+func (e *erf) JSON() any {
+	m := e.meta
+	m["error"] = e.Msg()
+	return m
+}
+
+func (e *erf) MarshalJSON() ([]byte, error) {
+	return json.Marshal(e.JSON())
+}
+
 var _ UnwrappableError = new(wrapErr)
 
 type wrapErr struct {
diff --git a/errorf_test.go b/errorf_test.go
index 5abc136..3847ca2 100644
--- a/errorf_test.go
+++ b/errorf_test.go
@@ -2,6 +2,7 @@ package errors
 
 import (
 	"errors"
+	"encoding/json"
 	"net/http"
 	"testing"
 
@@ -102,3 +103,24 @@ func (s *ErrorfTestSuite) TestSetStatusOutsideRange() {
 	err.SetStatus(600)
 	s.Assert().Equal(http.StatusPaymentRequired, err.Status())
 }
+
+func (s *ErrorfTestSuite) TestJSON() {
+	var err SettableError = Errorf(http.StatusPaymentRequired, "Hello")
+	s.Assert().NotNil(err)
+
+	m := err.JSON()
+	ma, _ := m.(map[string]any)
+	s.Assert().Equal("Hello", ma["error"])
+	s.Assert().Nil(ma["number"])
+
+	j, _ := json.Marshal(err)
+	s.Assert().Equal(`{"error":"Hello"}`, string(j))
+
+	err.SetField("number",42)
+	m = err.JSON()
+	ma, _ = m.(map[string]any)
+	s.Assert().Equal(42, ma["number"])
+
+	j, _ = json.Marshal(err)
+	s.Assert().Equal(`{"error":"Hello","number":42}`, string(j))
+}
diff --git a/interface.go b/interface.go
index 38c0ec2..5701b3b 100644
--- a/interface.go
+++ b/interface.go
@@ -1,5 +1,7 @@
 package errors
 
+import "encoding/json"
+
 // ResponsableError is an error that has information useful in an HTTP response.
 // The string returned by Error should be suitable for logging useful information
 // to assist debugging the error.
@@ -8,10 +10,16 @@ package errors
 //
 // Msg should return a message suitable to display to the end user. If the message
 // returned by Error is safe for the end user, it may simply call that.
+//
+// JSON should return something that can be marshalled to JSON for the response body.
+// It can be something as simple as map[string]string{"error":err.Msg()}.
+// JSON's return value should be used by MarshalJSON()
 type ResponsableError interface {
+	json.Marshaler
 	Error() string
 	Status() int
 	Msg() string
+	JSON() any
 }
 
 // SettableError is a ResponsableError which can be modified after initial creation.
@@ -22,10 +30,13 @@ type ResponsableError interface {
 // Methods are chainable.
 //
 // SetStatus should not use a value outside of 100-599. It may either ignore such values, or panic.
+//
+// SetField should add some metadata to the error, which will be included in the data returned by JSON()
 type SettableError interface {
 	ResponsableError
 	SetStatus(int) SettableError
 	SetMsg(string, ...any) SettableError
+	SetField(string, any) SettableError
 }
 
 // UnwrappableError allows a ResponsableError to wrap another error.