session.addAssistantMessage("Assistant 2");
        session.addUserMessage("User 3");
        session.addAssistantMessage("Assistant 3");
        assertEquals(6, session.getMessageCount());

        // Trimming to 3 would start at index 3 (assistant), so it should go back to index 2 (user)
        session.trimHistory(3);

        final List<ChatMessage> messages = session.getMessages();
        assertEquals(4, messages.size());

    assertThat(CharMatcher.any().collapseFrom("ab", '_')).isEqualTo("_");
    assertThat(CharMatcher.any().collapseFrom("abcd", '_')).isEqualTo("_");
  }

  public void testTrimFrom() {
    // trimming -
    doTestTrimFrom("-", "");
    doTestTrimFrom("x-", "x");
    doTestTrimFrom("-x", "x");
    doTestTrimFrom("--", "");
    doTestTrimFrom("x--", "x");
    doTestTrimFrom("--x", "x");