> It appears newer models are better at narive harness tool calls and worse at custom tools that look similar to default tools.

This is true. For example, the other day I was doing some testing with DeepSeek-V4-Flash-0731, and I gave it a tool called "EditFile" with "old_content" argument. It ignored the tool definition and called it with "old_string" instead, presumably because it was trained on a bunch of Claude Code traces.

So either make sure your tools look nothing like the tools in the popular harnesses, or make them similar.