Introduction: Gatling is a light weighted load generator. Though it is 'load test as code', sometimes it is hard to debug. Especially when there is lack of documentation and tutorial. I run into an issue when upgrading from 2.3.1 to 3.0.2. The error message is not very helpful. Here I will show this case to illustrate an upgrading issue, as well is tricks to print out some debug information. Some Debug Tricks: According to gatling document , you can print out debug information in 2 ways. change conf/logback.xml to DEBUG/TRACE for logger name="io.gatling.http.engine.response" This generates too much information and my tests times out. Use println in exec. This is more flexible and can prints the exact information I need. How ever I would like to put the debug messages in a separate file. Here is an example: val file = System.getProperty("file", "/tmp/debug_cookie.out") val scn = scenario(myReadScenario) .during (myDur