fix: drop misleading extensions json tag on ProblemDetails

problem_details.go

@@ -10,12 +10,15 @@ const BlankURL = "about:blank"
 
 // ProblemDetails conforms to RFC 9457, providing a standard format for describing errors in HTTP APIs.
 type ProblemDetails struct {
-	Type       string                 `json:"type"`
-	Title      string                 `json:"title"`
-	Status     int                    `json:"status"`
-	Detail     string                 `json:"detail,omitempty"`
-	Instance   string                 `json:"instance,omitempty"`
-	Extensions map[string]interface{} `json:"extensions,omitempty"`
+	Type     string `json:"type"`
+	Title    string `json:"title"`
+	Status   int    `json:"status"`
+	Detail   string `json:"detail,omitempty"`
+	Instance string `json:"instance,omitempty"`
+	// Extensions holds RFC 9457 extension members. MarshalJSON and
+	// UnmarshalJSON flatten them to the top level; there is no nested
+	// "extensions" object on the wire.
+	Extensions map[string]interface{} `json:"-"`
 }
 
 // ValidationErrorDetail provides structured details about a single validation error.

problem_details_test.go

@@ -118,3 +118,28 @@ func TestProblemDetailsMarshalJSONFlattensExtensions(t *testing.T) {
 		t.Fatalf("expected trace_id extension, got %#v", payload["trace_id"])
 	}
 }
+
+func TestProblemDetailsMarshalJSONHasNoNestedExtensions(t *testing.T) {
+	t.Parallel()
+
+	body, err := json.Marshal(ProblemDetails{
+		Type:       BlankURL,
+		Title:      "Bad Request",
+		Status:     400,
+		Extensions: map[string]interface{}{"code": "E1"},
+	})
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+
+	var payload map[string]any
+	if err := json.Unmarshal(body, &payload); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if _, ok := payload["extensions"]; ok {
+		t.Fatalf("unexpected nested extensions key: %s", body)
+	}
+	if payload["code"] != "E1" {
+		t.Fatalf("expected flattened extension, got %s", body)
+	}
+}