The phrase "subtle tool" is correct and usable in written English.
The phrase generally refers to something that is not obviously useful, but that can have useful effects. For example, you could say, "He used subtle tools to influence the outcome of the election."
It's a subtle tool for managing congestion".
From Cervantes on, the novel has become our most subtle tool of ontological inquiry.
At Motorola, enhanced severance is even viewed as a subtle tool for future recruiting, once the economy revives and hiring resumes.
Instead, it becomes a subtle tool to investigate the complexities of race in Jasper, which has a black mayor, and even the divisions depicted in the film.
The data patterns built up by tracking vehicle movements, credit-card use, phone calls and contact matching produce a sophisticated and almost infinitely subtle tool for monitoring and predicting behaviour.
He's also partial to the dog-with-tongue-out filter, a subtle tool Snapchat has invented to allow people to confirm for the rest of the world that they will never be edgy.
He quickly recognized the need for more powerful lenses, more subtle tools.
For this reason, sound is one of the most visceral, subtle tools available to filmmakers.
Using a number of subtle tools to help the social workers process and reframe their experiences, our intense engagement midwifed some ambitious but well-argued visions for change that were more convincing than anything the government had offered from their disconnected perches.
Biomarkers can be subtle tools to aid the diagnosis, prognosis and monitoring of therapy and disease progression.
First, the state is experienced in dealing with protest, with a wide array of subtle and not-so-subtle tools at its disposal.
