How do I manually input the user agent in the Opera browser?

Where is the configuration for user agent, what file and where?

For version 11, in opera:config I can only pick from some preset browsers, but I want to use something else so I guess I hav开发者_运维百科e to manually change the configuration but I don't know where it is.

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AFAIK you can only add text here:

Opera/9.80 (Windows NT 5.1; U; String; en)

http://www.opera.com/support/usingopera/operaini/#isp

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You can use Opera Dragonfly to replace the User-Agent header while debugging. With a User JavaScript you can also replace navigator.userAgent and friends.