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The Strava heatmap needs a session cookie from your browser. Follow these steps to grab it.
Go to strava.com/heatmap and make sure you are logged in. The heatmap should be visible on screen.
Press F12 (Windows / Linux) or Cmd+Option+I (Mac).
Click the Network tab at the top of the DevTools panel.
In the filter bar, type content-a.strava.com. A list of image requests will appear. Click on any one of them.
In the right panel, click Headers → scroll to Request Headers → find the line starting with cookie: → right-click it → Copy value.
Paste the copied text into the "Strava cookie string…" textarea and click Save session cookie.
Strava sessions typically expire after a few hours to a few days. If the heatmap stops loading tiles, repeat the steps above to get a fresh cookie.