*Update* apparently data tag is not JSON (why use your own almost-json implementation? You could use lenient gson).

Valid JSON does not work with the /summon command.

• Keys are not accepted unless they are un-quoted.
• Escaping does not work (except for double-quotes)

Not working:

/summon Sheep ~ ~ ~ {"CustomName":"foobar"}

(This works now)

Not working:

/summon Sheep ~ ~ ~ {"CustomName":"foobar\\"}

It will take the first backslash as-is and then escape the double-quote, which of course causes an "Unbalanced quotation"

My two examples are 100% valid JSON, see: (they are, but apparently data tags aren't JSON)

• http://jsonlint.com/?json=%7B%22CustomName%22:%22foobar%22%7D
• http://jsonlint.com/?json=%7B%22CustomName%22:%22foobar%5C%5C%22%7D