The argument for capitalizing the ‘a’ in agile is that it is a proper noun now. A specific system with established frameworks tracing back to the Manifesto. I think it rather pleases us that agile has arrived, deserving its capital ‘A’.
This is where things get tricky.
We’re now implementing Agile, not becoming agile. A noun, not an adjective. It is defined and concrete. A defined set of practices, a target state to reach rather than a quality we embody.
This opens the door for dogma. For the tyranny of practices. Doing Agile becomes the goal, rather than being agile in pursuit of what actually matters.
So write it lowercase. Not because I’m the grammar police, but as a reminder. A reminder that it’s ok for us to pick and choose practices, to evolve what works in context over time.
The adjective gives us permission to adapt. The noun demands compliance. Choose the one that helps teams do work that matters.