The nunit-agent.exe program is used by other runners when the tests are being run in a separate process. It is not intended for direct execution by users.
NUnit runs tests in a separate process in several situations:
When debugging tests that are run in a separate process, it is not possible to do so by simply running the console or gui runner under the debugger. Rather, it is necessary to attach the debugger to the nunit-agent process after the tests have been loaded.
When running under the Gui, NUnit will continue to use the same process to reload tests so that it is not normally necessary to re-attach to a new process. However, if the settings are changed in a way that requires a different process - for example, by changing the version of the runtime that is being used - the old process will be terminated and a new one created. In that case, it's necessary to re-attach to the new process.