Marginalia turns the margin of any web page into a working notebook with an agent in it. Select a passage and write your note first. Then a Codex agent on GPT-6 Astra does work on that passage: it builds a small model you can adjust, checks what supports a claim, defines a term, or works an example. Each reply stays pinned to the exact words, beneath your note. Park threads for later. Everything joins a local, searchable library with source links and export. You approve what leaves your machine.