The core Hard City mechanic is a tag system applied to FitD. Each character has a Past, Present and Perk Trademark. Trademarks are composed of Traits and Flaws. To create a character, you pick 5 Traits as Edges and 2 Flaws out of your Trademarks. For example, out of Past Criminal, Present Finder and Perk Number Man, you could take Forgery, Criminal Contacts, Sneak, Tail Someone and Count Cards as Edges and Known to Police and Heartless as Flaws. For checks, you built up opposed dice pools - start with one Action die and add an Action die for a relevant Trademark, relevant Edge, Scene or Threat Tags you can exploit, good position, equipment with a useful Tag, and help from allies. Add a Danger die for injuries, conditions, Scene or Threat Tags that cause issues, bad position, not having required items and scale. Remove pairs of Action/Danger die with the same number. The remaining highest action die is your result and works just like FitD, except that if all Action die are eliminated, or the highest is a 1, you get a Botch (read: fumble).

The equivalent to Stress in FitD is Moxie. It allows you to use a second Trademark, change any die up or down by one, use an actino remove a Condition or reduce the severity of an Injury, provided you can narratively pause to do it, or do a Voice-Over to add an interesting or helpful detail to a scene.

You get XP for success and failures, which are tracked separately. You get an Advance for every 10 XP, but failures can cause a loss of Moxie or Conditions going into the next case.

A case (read: score) falls into either an Investigation or a Caper. Capers work very much like a FitD score. Investigations are just what they sound like. The book has good information for the GM on running an investigation and is very similar to Justin Alexander's 3-Clue Rule method.

This is definitely in the To-Be-Run pile. I think it would be great for one-shots, or extended out into a campaign.