[UM-CLONE-1] Browser-Use — DOM-tree-to-LLM serializer with element indices

[Delqhi]

Source

• Project: Browser-Use
• License: MIT
• Clone mode: code

Feature we want

Serialize the live DOM into a numbered, LLM-friendly tree where each interactable element gets a stable index. The LLM acts by index, not by selector — drastically reduces hallucinated selectors.

Why we want it

Foundation for our observe/extract/act API. Every modern agent framework has this. Without it, our LLM layer is hopelessly behind.

Acceptance criteria

• Module src/serialize/dom_tree.ts exposes serializeForLLM(page) -> {tree, indexMap}
• Includes ARIA role + accessible name + bounding box + data-testid if present
• Filters non-interactable / off-screen / hidden elements
• Round-trip test: index → element → click works on 5 fixture pages
• Token-budget mode: produce <=N tokens by collapsing subtrees
• PR body cites source repo + commit SHA + license
• THIRD_PARTY_NOTICES.md updated
• Parity test added (proves our clone behaves like source for the relevant case)
• CHANGELOG entry: cloned from Browser-Use@<sha>

Reference

See COMPETITIVE_STRATEGY.md for the full doctrine.