Just came across this thread, it looks like VI Foundation is required at a minimum to get full R/W access to the VI API (or any other API) even if you're not planning on using vCenter.
ESX vs ESXi comparison
Take a look at the Remote CLI section:
RCLI is limited to read-only access for the free version of VMware ESXi. To enable full functionality of RCLI on a VMware ESXi host, the host must be licensed with VI Foundation, VI Standard, or VI Enterprise.
The VMware Infrastructure toolkit for Windows and the Perl toolkit access ESXi through the same API as RCLI. Similarly, these toolkits are limited to read-only access for the free version of VMware ESXi. When the host is upgraded to VI Foundation, VI Standard, or VI Enterprise, these toolkits have write-access and provide a scriptable method for managing ESXi hosts.
I was hoping there might be just an API license that can be purchased with the use of the free license version of ESXi, but I can see why they would want you to get one of the basic licenses for advanced API access.
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William Lam
VMware vExpert 2009
VMware ESX/ESXi scripts and resources at:
