With a style like this:

{
  ...
  'circle-fill': 'blue',
  'circle-stroke-color': 'black',
  'circle-opacity': 0.1
}

it's easy to be misled into thinking that the opacity of the whole circle (fill plus stroke) is being set to 0.1. Whereas the opacity setting only applies to the fill, and the stroke has its own opacity setting, circle-stroke-opacity. (I just ran into this minor gotcha).

Possible suggestions:

• rename it to circle-fill-opacity
• document it more clearly (The opacity at which the circle will be drawn. → The opacity of the circle's filled interior., plus maybe some comment about the possible interaction between this property and the alpha channel of the fill color.)
• change the behaviour so that it does affect stroke. (Probably not a good idea...)