diff --git a/lib/openai/internal/util.rb b/lib/openai/internal/util.rb
index e254972b..dff2080d 100644
--- a/lib/openai/internal/util.rb
+++ b/lib/openai/internal/util.rb
@@ -421,18 +421,38 @@ def close
         #
         # @param max_len [Integer, nil]
         #
-        # @return [String]
+        # @return [String, nil]
         private def read_enum(max_len)
           case max_len
           in nil
-            @stream.to_a.join
+            # `loop` rescues StopIteration, but this method handles it below.
+            # rubocop:disable Style/InfiniteLoop
+            @buf << @stream.next.b while true
+            # rubocop:enable Style/InfiniteLoop
+          in Integer if max_len.negative?
+            raise ArgumentError, "negative length #{max_len} given"
           in Integer
-            @buf << @stream.next while @buf.length < max_len
-            @buf.slice!(..max_len)
+            @buf << @stream.next.b while @buf.bytesize < max_len
+            read_buffer(max_len)
           end
         rescue StopIteration
+          return @buf.slice!(0..) if max_len.nil?
+
           @stream = nil
-          @buf.slice!(0..)
+          return nil if @buf.bytesize.zero?
+
+          read_buffer(max_len)
+        end
+
+        # @api private
+        #
+        # @param max_len [Integer]
+        #
+        # @return [String]
+        private def read_buffer(max_len)
+          read = @buf.byteslice(0, max_len)
+          @buf = @buf.byteslice(max_len..) || String.new
+          read
         end
 
         # @api private
@@ -444,14 +464,24 @@ def close
         def read(max_len = nil, out_string = nil)
           case @stream
           in nil
-            nil
+            raise ArgumentError, "negative length #{max_len} given" if max_len&.negative?
+
+            read = max_len.nil? || max_len.zero? ? +"" : nil
+            case out_string
+            in String
+              out_string.replace(read || +"")
+              read.nil? ? nil : out_string
+            in nil
+              read
+            end
           in IO | StringIO
             @stream.read(max_len, out_string)
           in Enumerator
             read = read_enum(max_len)
             case out_string
             in String
-              out_string.replace(read)
+              out_string.replace(read || +"")
+              read.nil? ? nil : out_string
             in nil
               read
             end
diff --git a/rbi/openai/internal/util.rbi b/rbi/openai/internal/util.rbi
index f80846be..5b26f470 100644
--- a/rbi/openai/internal/util.rbi
+++ b/rbi/openai/internal/util.rbi
@@ -268,10 +268,15 @@ module OpenAI
         end
 
         # @api private
-        sig { params(max_len: T.nilable(Integer)).returns(String) }
+        sig { params(max_len: T.nilable(Integer)).returns(T.nilable(String)) }
         private def read_enum(max_len)
         end
 
+        # @api private
+        sig { params(max_len: Integer).returns(String) }
+        private def read_buffer(max_len)
+        end
+
         # @api private
         sig do
           params(
diff --git a/sig/openai/internal/util.rbs b/sig/openai/internal/util.rbs
index 49cc4467..1c9f3f29 100644
--- a/sig/openai/internal/util.rbs
+++ b/sig/openai/internal/util.rbs
@@ -92,7 +92,9 @@ module OpenAI
 
         def close: -> void
 
-        private def read_enum: (Integer? max_len) -> String
+        private def read_enum: (Integer? max_len) -> String?
+
+        private def read_buffer: (Integer max_len) -> String
 
         def read: (?Integer? max_len, ?String? out_string) -> String?
 
diff --git a/test/openai/internal/util_test.rb b/test/openai/internal/util_test.rb
index 560d786d..c989aa1f 100644
--- a/test/openai/internal/util_test.rb
+++ b/test/openai/internal/util_test.rb
@@ -305,6 +305,52 @@ def test_copy_read
     end
   end
 
+  def test_enum_read_respects_max_len
+    # rubocop:disable Lint/EmptyBlock
+    adapter = OpenAI::Internal::Util::ReadIOAdapter.new(%w[abc def].to_enum) {}
+    # rubocop:enable Lint/EmptyBlock
+
+    assert_equal("", adapter.read(0))
+    assert_equal("a", adapter.read(1))
+    assert_equal("bcd", adapter.read(3))
+    assert_equal("ef", adapter.read(99))
+    assert_nil(adapter.read(1))
+  end
+
+  def test_enum_read_respects_byte_lengths
+    input = ["\xC3\xA9b".dup.force_encoding(Encoding::UTF_8)]
+    # rubocop:disable Lint/EmptyBlock
+    adapter = OpenAI::Internal::Util::ReadIOAdapter.new(input.to_enum) {}
+    # rubocop:enable Lint/EmptyBlock
+
+    assert_equal("\xC3".b, adapter.read(1))
+    assert_equal("\xA9b".b, adapter.read(2))
+    assert_nil(adapter.read(1))
+  end
+
+  def test_enum_read_all_includes_buffered_bytes
+    # rubocop:disable Lint/EmptyBlock
+    adapter = OpenAI::Internal::Util::ReadIOAdapter.new(%w[abc def].to_enum) {}
+    # rubocop:enable Lint/EmptyBlock
+
+    assert_equal("ab", adapter.read(2))
+    assert_equal("cdef", adapter.read)
+    assert_equal("", adapter.read)
+  end
+
+  def test_enum_read_clears_out_string_at_eof
+    out = +"stale"
+    # rubocop:disable Lint/EmptyBlock
+    adapter = OpenAI::Internal::Util::ReadIOAdapter.new(["abc"].to_enum) {}
+    # rubocop:enable Lint/EmptyBlock
+
+    assert_equal("abc", adapter.read(99, out))
+    assert_same(out, adapter.read(0, out))
+    assert_equal("", out)
+    assert_nil(adapter.read(1, out))
+    assert_equal("", out)
+  end
+
   def test_copy_write
     cases = {
       StringIO.new => "",