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Debugging an experiment from experimenter locally

Trying to iterate on an experiment in Preview on experimenter.services.mozilla.com can be painful, because even after you change something and post to preview, you have a wait a while for the updated recipe to be propagated to the CDN.

Enable Nimbus debugging​

  • In about:config, set:
    • nimbus.debug to true

Enable the Browser Toolbox​

  • In about:config, set:
    • devtools.chrome.enabled to true
    • devtools.debugger.remote-enabled to true

Prepare a few things:​

  • Load about:studies and unenroll this profile from anything that might interfere
  • On the Experimenter page for your experiment, select the contents of the Recipe JSON field from the Details tab, and copy it into your Copy/Paste buffer

Do the Enrollment​

  • Open Tools > Browser Tools > Browser Toolbox
  • Switch to the browser console
  • In the input area, do the following:
    • const branchSlug = 'treatment-a'; // or whatever branch you want
    • let recipe = paste_recipe_json_here ;
    • const { ExperimentManager: em } = ChromeUtils.importESModule("resource://nimbus/lib/ExperimentManager.sys.mjs");
    • let branch = recipe.branches.find(b => b.slug == branchSlug);
    • em.forceEnroll(recipe, branch);

Reload about:studies, and you should see the experiment​