Twitter held an invite-only, closed-to-press developer event tonight, and the agenda was helping those developers build features that can appear on Twitter's website and in its own apps. The new cards that Twitter is offering will allow developers to display photo galleries, media players, "deep-link" to apps, and more within a single tweet. The news comes from Dave McClure who, appropriately enough, is tweeting details about the new cards. Flickr, Path, and Foursquare have all taken to the stage to express their support for the new cards on Twitter. All Things D originally reported that Twitter would be expanding its Card options for developers last week.

Twitter has gone into more detail in a blog post about the new features. It says that Cards can now contain a link directly install an app, which works on both Android and iOS. The "Deep-Linking" feature is more interesting. It allows any card to contain a link not to a website, but directly to the content within the app itself. As McClure notes, Twitter says that it can "foresee a world with literally hundreds" of card types.

Improved app discovery is a shot across Facebook's bow

The improved app discovery is a shot across Facebook's bow, which has been making a gigantic push to try to help app developers with discovery. Facebook's goal is to help ensure that every app is Facebook connected, and the carrot it has been offering is improved social discoverability for their apps — all in the effort of turning every phone into a de-facto Facebook phone. Twitter is making the case that it, too, can help app developers gain new users for their app, thanks to these Twitter cards. Essentially, Twitter is making nice with app developers — just not Twitter app developers.

Interestingly, Twitter's cards theoretically cut out the larger web from the loop for mobile users, allowing them to jump from app to app directly rather than jumping out to a web browser. Flickr is reportedly happy to be on board and Path has already detailed how its integration will work

Few were hoping that Twitter would be announcing some kind of reversal of its increasingly restrictive APIs for third party apps, and that definitely didn't happen. CEO Dick Costolo said last year that he prefers that developers "build into Twitter" instead of "build off Twitter," and that walled-garden philosophy is still in full effect. That philosophy cuts both ways: despite the new options for developers, it's unlikely that Instagram will change its mind and take advantage of the new Cards.