Chrome's Local Network Access prompt makes running localhost helpers look uninstalled

"This week I found a bug in my own code by reading another company's support page," wrote the developer behind the post, who builds the same kind of local agent for receipt printers ^[18][15]. He checked his own agent against the symptom Dynamsoft documents, and it failed the same way ^[15].

His health check had the common shape. It sends GET /v1/health with a 1.5 second timeout and returns false on any exception, and the app then tells the user to install the helper ^[10]. Under Local Network Access, the pending prompt holds that request open with no error and no event ^[9]. The timer fires first. A 1.5 second limit assumes a person who answers browser prompts faster than that, and the user in the post's example is a cashier looking at the till, not the address bar ^[19]. "The helper is running. The browser is waiting for a click nobody knows they need to make," he wrote ^[16].

He tested on Chrome 155 on Windows, with the agent on 127.0.0.1:17777 and a public https origin that had never asked for permission before ^[7]. Chrome accepted all three names, loopback-network, local-network and local-network-access, and each returned "prompt" ^[7]. The test covers one build on one operating system. For the result to transfer, Chrome would have to behave the same on macOS and Linux, and the post does not test them ^[7].

A Block click leaves less room. The post says this part comes from the author's reading, not his own testing: Chrome does not ask that site again, and every later request fails ^[21]. Dynamsoft's page shows the console line "Permission was denied for this request to access the unknown address space." ^[21] Page script cannot read that string. It gets the same TypeError a missing agent would produce ^[11].

His replacement, agentStatus, keeps the 1.5 second AbortController and passes targetAddressSpace: 'loopback' to fetch. When the request fails, it asks the Permissions API for the state ^[13]. Denied returns permission_denied; prompt plus an AbortError returns permission_prompt; anything else returns not_running ^[13]. The middle branch is good engineering. Chrome prompts only when something is listening, so a timeout while the state reads "prompt" means a live helper is waiting on a click ^[8][20]. The state lookup tries loopback-network first and falls back to local-network-access for Chrome 142 to 144. It reports "unsupported" for Safari and older browsers ^[14].

We think three states is the minimum for anyone shipping a localhost agent, and the message text matters as much as the branch logic. Only the user can fix a denied state, in the site setting Chrome labels "Apps on device", and the denied message should send them there ^[12][3]. Support desks have a check of their own to run. He wrote that when a customer says they fixed it with a policy, support should ask which policy ^[17].