IronRuby has posed a significant challenge for the Microsoft team. Ruby is defined primarily by its premiere interpreter Ruby MRI, also known as CRuby. The ongoing RubySpec project intends to document it, but they are a long way from completion. Most teams trying to implementation a Ruby compiler simply look at the CRuby source code, but this isn’t an option for Microsoft. CRuby is licensed under the GPL, which Microsoft is afraid of for numerous legal reasons. This means they have to reverse engineer it, a daunting feat.

Back in March of 2009, IronRuby was only passing 80% of the documented Ruby Specs. And even then it was running 4.5 times slower than CRuby with a startup time of 10 time slower.

By August, IronRuby had made tremendous advances. It now passes 92.5 % of the specification tests and is running twice as fast as CRuby. The startup time is also greatly improved due to the adaptive compiler. A fully compatible version, labeled IronRuby 1.0, is expected to be ready this fall.