diff --git a/lib/openai/internal/util.rb b/lib/openai/internal/util.rb
index 429fbaf7..3f20340b 100644
--- a/lib/openai/internal/util.rb
+++ b/lib/openai/internal/util.rb
@@ -535,12 +535,13 @@ class << self
         # @param key [Symbol, String]
         # @param val [Object]
         # @param closing [Array<Proc>]
-        private def write_multipart_chunk(y, boundary:, key:, val:, closing:)
+        private def write_multipart_chunk(y, boundary:, key:, val:, closing:, is_array: false)
           y << "--#{boundary}\r\n"
           y << "Content-Disposition: form-data"
 
           unless key.nil?
             name = ERB::Util.url_encode(key.to_s)
+            name = "#{name}[]" if is_array
             y << "; name=\"#{name}\""
           end
 
@@ -577,7 +578,14 @@ class << self
                 case val
                 in Array if val.all? { primitive?(_1) }
                   val.each do |v|
-                    write_multipart_chunk(y, boundary: boundary, key: key, val: v, closing: closing)
+                    write_multipart_chunk(
+                      y,
+                      boundary: boundary,
+                      key: key,
+                      val: v,
+                      closing: closing,
+                      is_array: true
+                    )
                   end
                 else
                   write_multipart_chunk(y, boundary: boundary, key: key, val: val, closing: closing)
diff --git a/sig/openai/internal/util.rbs b/sig/openai/internal/util.rbs
index ec425e9f..98dcf392 100644
--- a/sig/openai/internal/util.rbs
+++ b/sig/openai/internal/util.rbs
@@ -118,7 +118,8 @@ module OpenAI
         boundary: String,
         key: Symbol | String,
         val: top,
-        closing: ::Array[^-> void]
+        closing: ::Array[^-> void],
+        ?is_array: bool
       ) -> void
 
       def self?.encode_multipart_streaming: (
diff --git a/test/openai/internal/util_test.rb b/test/openai/internal/util_test.rb
index 560d786d..289d8922 100644
--- a/test/openai/internal/util_test.rb
+++ b/test/openai/internal/util_test.rb
@@ -264,7 +264,7 @@ def test_hash_encode
     cases = {
       {a: 2, b: 3} => {"a" => "2", "b" => "3"},
       {a: 2, b: nil} => {"a" => "2", "b" => "null"},
-      {a: 2, b: [1, 2, 3]} => {"a" => "2", "b" => "1"},
+      {a: 2, b: [1, 2, 3]} => {"a" => "2", "b[]" => "1"},
       {strio: StringIO.new("a")} => {"strio" => "a"},
       {strio: OpenAI::FilePart.new("a")} => {"strio" => "a"},
       {pathname: Pathname(__FILE__)} => {"pathname" => -> { _1.read in /^class OpenAI/ }},