diff --git a/CHANGES.md b/CHANGES.md
index 079fd0c49..f405db416 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -3,6 +3,7 @@
 ### Unreleased
 
 * Fix `JSON::ParserError#json_path` raising `NoMethodError` when the document was parsed with `symbolize_names: true`.
+* Fix `JSON.generate` not detecting duplicate keys such as `{ "1" => 1, 1 => 2 }`.
 
 ### 2026-10-13 (3.1.0)
 
diff --git a/ext/json/ext/generator/generator.c b/ext/json/ext/generator/generator.c
index 38c88f231..87da10027 100644
--- a/ext/json/ext/generator/generator.c
+++ b/ext/json/ext/generator/generator.c
@@ -1020,6 +1020,9 @@ json_object_i(VALUE key, VALUE val, VALUE _arg)
                     raise_generator_error(key, "%"PRIsVALUE" not allowed as object key in JSON", CLASS_OF(key));
                 }
             }
+
+            json_inspect_hash_with_mixed_keys(arg);
+
             key_to_s = rb_convert_type(key, T_STRING, "String", "to_s");
             break;
     }
diff --git a/lib/json/truffle_ruby/generator.rb b/lib/json/truffle_ruby/generator.rb
index 1cacd76c5..f73708f2d 100644
--- a/lib/json/truffle_ruby/generator.rb
+++ b/lib/json/truffle_ruby/generator.rb
@@ -451,7 +451,11 @@ def generate(obj, anIO = nil)
             end
             obj.each_pair do |k,v|
               if first
-                key_type = k.class
+                if ::String == k.class || ::Symbol == k.class
+                  key_type = k.class
+                else
+                  JSON.send(:on_mixed_keys_hash, obj)
+                end
               else
                 if key_type && !@allow_duplicate_key && key_type != k.class
                   key_type = nil # stop checking
@@ -592,7 +596,11 @@ def json_transform(state)
 
             hash.each { |key, value|
               if first
-                key_type = key.class
+                if ::String == key.class || ::Symbol == key.class
+                  key_type = key.class
+                else
+                  JSON.send(:on_mixed_keys_hash, hash)
+                end
               else
                 if key_type && !state.allow_duplicate_key? && key_type != key.class
                   key_type = nil # stop checking
diff --git a/test/json/json_generator_test.rb b/test/json/json_generator_test.rb
index 1d9318fd5..bfc324104 100755
--- a/test/json/json_generator_test.rb
+++ b/test/json/json_generator_test.rb
@@ -1202,11 +1202,32 @@ def test_generate_duplicate_keys_allowed
   end
 
   def test_generate_duplicate_keys_disallowed
-    hash = { foo: 1, "foo" => 2 }
+    assert_duplicate_key({ foo: 1, "foo" => 2 })
+    assert_duplicate_key({ "1" => 1, 1 => 2 })
+    assert_duplicate_key({ :"1" => 1, 1 => 2 })
+
+    fake_string = Class.new do
+      def initialize(str)
+        @str = str
+      end
+
+      def to_s
+        @str.dup
+      end
+    end
+
+    assert_duplicate_key({ fake_string.new("a") => 1, fake_string.new("a") => 2 })
+  end
+
+  def assert_duplicate_key(hash)
     error = assert_raise JSON::GeneratorError do
       JSON.generate(hash, allow_duplicate_key: false)
     end
-    assert_equal %(detected duplicate key "foo" in #{hash.inspect}), error.message
+    assert_equal %(detected duplicate key "#{hash.keys.first}" in #{hash.inspect}), error.message
+  end
+
+  def test_generate_mixed_keys_without_duplicates
+    assert_equal %({"1":1,"2":2}), JSON.generate({ "1" => 1, 2 => 2 }, allow_duplicate_key: false)
   end
 
   def test_frozen