if one has tools, which are useful but not essential to a task, what's the mechanic to use for them?

I've been treating them as a distinction.

I could see them sometimes being a TN-1.

Or an extra rolled (but not kept) ring die.

But I can't find in the rules any advice on what to use.

TNs should reflect the difficulty of a task. Tools that make a task easier should logically result in a lower difficulty and thus a lower TN. I like the idea of using them as a distinction though, and it will be a situational decision in some cases anyway. Some checks will presume the use of tools anyway (forging a sword without tools for the job is not going to happen, but special tools could call for a distinction) while others will be set up assuming no tools will be used and get adjusted when the PC shows up with a handy doodad for the occasion (climbing that wall is a TN 4 in general, but with the use of that knotted rope with a sturdy hook a TN 1 will suffice).

Is Assistance rules made assist dice cap at 1 extra kept and infinite extra rolled, I would say a good tool= skilled assistance, normal tool = unskilled assistance, bad tools = an adversity

More than anything, this is a callout to the developers that there's no advice on how to adjudicate tools, and there needs to be.

I'd keep it simple and treat tool use as an advantage/disadvantage on the roll. No new mechanics to fuss with and it fits thematically.

There are timers where a job is not possible with out tools...

5 hours ago, Daeglan said:

There are timers where a job is not possible with out tools...

And for those, it's easy. No tools, no roll. It's for the other times that we need a rule.

I would follow the example star wars uses. Something like a boost die

Agreed it needs to be noted in the rules somewhere, but I'd suggest it belongs in page 13, section 4 (Determine Target Number of Successes):

Quote

...the GM selects a target number of successes ("TN") between one and eight. This target number should be based on the innate difficulty of the task, the complexity of the outcome the player described, and the appropriateness of the methods the character is using...

Adding a one-liner about "the availability of equipment" or "the state of the environment" would be the simplest way to cover this, to me; balancing whilst walking along a narrow wall is clearly Fitness (Air), and the absolute difficulty of crossing varies with the distance you've got to go, but also whether you have some decent shoes and something to balance yourself with (tools) and whether or not it's windy or raining (environment).