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Local only

  • The companion binds to 127.0.0.1. There is no cloud component, no telemetry and no account.
  • The extension talks only to that port. The companion refuses any connection or request that carries a web page’s Origin header, so sites you visit cannot reach it.
  • Messages from the extension to the companion are validated; malformed frames are rejected and the socket closed.
Your MCP client may send returned page content to its AI provider. Sharing a tab authorizes that client to read it; local extension communication does not keep the client’s conversation on your machine.

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Access is opt-in per tab

In extension mode, the agent reaches exactly the tabs you share, plus tabs it opened itself in the selected browser. Browser-internal pages, extension pages and protected add-on stores are unavailable. Each Chrome, Brave, Firefox or Zen profile enforces its own sharing. Sharing a tab in one browser grants no access in another. Emergency stop revokes access to that extension profile’s tabs. Other connected profiles and separately launched developer browsers remain available; close a developer context with browser_session when you are finished with it.

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What the agent can see in a shared tab

Everything the page can: DOM, cookies for the page origin, local and session storage, network requests and their bodies while a devtools_session is running, and JavaScript evaluation in the page. Treat a shared tab as fully visible to the agent. Do not share tabs you would not paste into the chat. Firefox extension mode has narrower capabilities: evaluation runs in an isolated world, and network capture and browser cookie APIs are unavailable. Page content is still visible to the agent.

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Restricting what the agent does

  • Disable tools you do not want used.
  • Domain policies limit where a tab, or every tab the agent opens, may navigate and load resources from.
  • Set the developer browser to Never if the agent should not get a second browser.
  • The activity log is off by default and keeps nothing; on, it holds the last 200 commands in memory only, never on disk. See Activity.
  • The connection graph shows agent and browser connection metadata and shared-tab counts, without tab URLs or page contents. It is on by default; turn it off in Settings to hide the page and stop graph updates for that profile.

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Secrets

  • Recorder exports and HAR files can contain form values, cookies and headers. They are written to ~/.browspark/artifacts or the path you give.
  • Developer-context profiles under ~/.browspark/ hold logins like any browser profile. Chrome, Brave, Firefox and Zen developer sessions use separate profiles from your everyday browsers.

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The debugger permission

Chrome grants the extension the debugger permission at install time. The extension attaches only to shared tabs, only when a command arrives, and detaches after 30 seconds of inactivity. The yellow bar Chrome shows is the honest signal that a tab is attached.

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Firefox permissions

The Firefox extension requests access to all websites (<all_urls>), which Firefox requires for its native tab screenshot API, and asks you to enable userScripts in the dashboard. Browspark only automates shared HTTP/HTTPS tabs. Page scripts execute in a separate sandbox without extension APIs, and the background process checks sharing before each command. Firefox does not show Chromium’s debugger bar; use the dashboard to stop or unshare a tab. The manifest declares the browsing activity, website content and website activity sent to your local companion, as described by Mozilla’s data permission guidance.