Some weeks ago a new official Gemma 4 release was posted that corrected some of the chat template problems. So the official release files on hugging face should be the way to go.
Download the original model with the original template, not updated versions of the model or finetuned versions of the model.
Then create your own reasoning tests to verify that it is working correctly. You can set a specific seed value to make sure the generation is the same every time, that way you can identify any tokens that are different.
Afterwards, try making small incremental changes to the template and validate your tests each time in order to try to adopt the improvements from the newer templates. If the reasoning quality degrades, undo your changes and try again or test alternative solutions.
Log all the calls and run on a periodic cadence (cron or ever N turns) a larger model (like Opus) to read samples of the traces and edit the template to fix observed problems. There are some signs that help find interesting things to look at, errors of course, but also overly long responses, prefix cache misses, tool call errors, etc.
