Fiddler does not have a "fixed capacity" of any sort; if you are running the 64bit version, Fiddler will log traffic until you run out of memory/disk space.
When you say "an errant program"-- what makes it errant? If the criteria are something that can be computed, you could configure Fiddler to keep only
the errant traffic and discard everything else.
You can configure Fiddler to periodically dump all of its traffic to disk on the schedule of your choosing, but this would require writing some script, and it wouldn't be that different than simply letting Fiddler keep all of the traffic in (virtual) memory.
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