Design

fastbrowse splits a browser agent into three owners:

• Jev picks. Each step, one batched Jev request chooses the operation and a target for each operation from indexed page candidates, and asks whether the page needs a sign-in. Separate Jev calls ask whether a target code has flagged may be irreversible, whether the task is complete, and whether each answer claim is supported by its quote.
• An LLM reads and writes. Proposing a direct address for the task while the start page loads, planning checkable requirements from the task alone, reading page content for answers when Jev cannot pick a short fact from quoted spans, writing non-secret field text, recovering when Jev is unsure, verifying completion in the uncertain band, composing the final answer.
• Code owns the gates. Freshness and hit-tests before every input, no automatic retry of a mutation, authorization for irreversible actions, secret resolution and redaction, budgets, and the definition of success: only COMPLETE, which requires every requirement evidenced.

Browser capabilities over plain CDP (P0 spike, 2026-09-17)

Verified with cdp-use==1.4.5 against local headless Chrome and a Browser Use cloud browser:

Capability	Mechanism	Local	Cloud
Own tab rendered	Target.createTarget(background=not remote) + activateTarget	pass	pass
Upload caller bytes	in-page DataTransfer + File on the input, input/change events (no host path needed)	pass	pass
Download bytes	Fetch.enable at Response stage for Document responses (download-attribute anchors included), Fetch.getResponseBody on Content-Disposition: attachment	pass	pass
Cross-origin iframe	Target.setAutoAttach(flatten) on the page session, evaluate in the iframe session	pass	pass
Popup ownership	Target.targetCreated.openerId equals our target	pass	pass
Dialogs	Page.javascriptDialogOpening + Page.handleJavaScriptDialog	pass	pass

The cloud browser ignores Browser.setDownloadBehavior(deny), so bytes come from response interception, never from the remote filesystem. Host-path DOM.setFileInputFiles is only valid for a browser on the same machine.