Twitter hosted an invite-only, closed-to-press developer event this week. According to angel investor Dave McClure, founder of business incubator 500 Startups, Twitter is expanding its Card options for developers to help build features for its site and apps. The Cards will allow developers to display photo galleries, media players and deep-links to apps within an individual tweet. Flickr, Path and Foursquare expressed their support at the event.

“One of the most important features in the new Cards is the ability to allow users to download your app (if the user doesn’t already have it installed), or deep-link into your own app (if the app is already installed on the user’s mobile device),” explains the Twitter Developers blog. “The ability to enable app installs and deep-linking is globally available across all Twitter Card types.”

Cards work on both Android and iOS. Deep-linking “allows any card to contain a link not to a website, but directly to the content within the app itself,” reports The Verge. “As McClure notes, Twitter says that it can ‘foresee a world with literally hundreds’ of Card types.”

“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,” suggests the article. “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.”

So while Twitter may be “making nice” with app developers, the courtesy is not necessarily extended to Twitter app developers. Some were hoping that Twitter would be announcing some type of “reversal of its increasingly restrictive APIs for third party apps,” which did not 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,” notes The Verge. “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.”