Why is chrome.runtime undefined?

Question

I am creating a Google Chrome extension (my first one) and I want to send messages from the extension to the current tab.

I am following the documentation:

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Answer 1

or it could just be a Heisenbug which only appears under certain circumstances. in my case, closing the chrome://extensions tab and refreshing my target caused chrome.runtime to be available again. Why is chrome.runtime undefined in the content script?