From Valve Developer Community
You don't have to change anything in Hooch's fixed version, link at the bottom of page
Animations: Certain mdl's only have animations in them, but come with a .ani file. Same procedure: Change the .mdl's header to IDST, and the .ani's header to IDAG, NOTE THE COMMA
Some issues with the original Cannonfodder decompiler were fixed by Erix920 but some bugs still remain.
You can download working versions of the tools from the links below.
- Hooch's fixed version of MDL Decompiler (2013) (No need to change "IDST0" to "IDST," and no more AppId error)
- Cannonfodder's original MDLDecompiler (for an updated version use studiocompiler)
- Erik's fixed version of MDLDecompiler (internet archive / wayback machine)
- StudioCompiler (updated 2011) - the official website.
- Crowbar MDL Decompiler