Issue #21517 has been updated by herwin (Herwin W). Never mind, this was a mistake on my end. The file `spec/core/io/readpartial_spec.rb` has a `# encoding: binary` directive, once I added that to `spec/library/stringio/readpartial_spec.rb` too, the issue was fixed. Please close this issue, I'll add it to the specs. ---------------------------------------- Bug #21517: Behaviour of StringIO#readpartial with multibyte differs from IO#readpartial https://bugs.ruby-lang.org/issues/21517#change-114207 * Author: herwin (Herwin W) * Status: Open * ruby -v: ruby 3.4.5 (2025-07-16 revision 20cda200d3) +PRISM [x86_64-linux] (+ others) * Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN ---------------------------------------- The specs for `IO#readpartial` have the following test (https://github.com/ruby/spec/blob/ed254bae321221f5cd24280beb37e5bddaff6bb6/c...): ```ruby before :each do @rd, @wr = IO.pipe @rd.binmode @wr.binmode end it "reads after ungetc with multibyte characters in the buffer" do @wr.write("∂φ/∂x = gaîté") c = @rd.getc @rd.ungetc(c) @rd.readpartial(3).should == "\xE2\x88\x82" @rd.readpartial(3).should == "\xCF\x86/" end ``` This behaviour is different in `StringIO#readpartial`. If I add this as a spec: ```ruby it "reads after ungetc with multibyte characters in the buffer" do @string = StringIO.new(+"∂φ/∂x = gaîté") c = @string.getc @string.ungetc(c) @string.readpartial(3).should == "\xE2\x88\x82" @string.readpartial(3).should == "\xCF\x86/" end ``` it fails with every supported Ruby version, and even with 3.5-dev of today, with the message `Expected "\xE2\x88\x82" == "∂"` The `getc` + `ungetc` can be removed, but this does not change the behaviour. -- https://bugs.ruby-lang.org/