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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.

Option A: Enrollment via Nimbus Developer Tools​

Nimbus Developer Tools provides a simple interface to debug experiments. You can download and install the extension from the Nimbus DevTools GitHub release page. Here are some of the ways you can use it to debug experiments locally:

Recipe JSON Enrollment​

  1. Load about:studies and unenroll this profile from anything that might interfere
  2. On the Experimenter page for your experiment, select the contents of the Recipe JSON field from the Summary tab, and copy it into your Copy/Paste buffer
  3. Navigate to the Nimbus Developer Tools JSON Enrollment page
  4. Paste the JSON from the Recipe JSON field in the provided textarea
  5. Click Enroll
  6. Reload about:studies, and you should see the experiment

Feature Configuration Enrollment​

  1. Load about:studies and unenroll this profile from anything that might interfere
  2. Navigate to the Nimbus Developer Tools Feature Configuration page.
  3. Select the feature ID from the dropdown
  4. Set the isRollout option to either true or false as needed
  5. Paste the feature configuration JSON into the textarea.
  6. Click Enroll.
  7. Reload about:studies, and you should see the experiment

For a more comprehensive overview of the Nimbus Developer Tools, including additional features beyond this, check out the Nimbus Developer Tools Guide.

Option B: Manual Enrollment via Browser Console​

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​