Extending a development environment

Concepts and technologies are described herein for extending the behavior of a software development tool. An extension can be accessed and consumed by a software development tool to configure the software development tool to perform an operation in an extended mode. In one example, an extension can extend a compiler based on the input source code. In one configuration, the compiler extension can provide a compiler with one or more runtime semantics of various source code elements for a particular programming language. The compiler can access an extensions list to determine if the compiler is to perform a compilation operation on a particular source code element or logical unit in an extended mode.