Yes. claude.ai's prompt box is a ProseMirror editor, and it's on the extension's verified-working list: blanks fill, transforms rewrite, cues dim and cycle, and undo lands as a single history entry. The one honest caveat is text styling, which claude.ai's plain-text composer ignores.
Improving your prompts to Claude, inside the box
This is the natural use case: improve prompt _ re-renders a draft before you spend tokens on it, fix the grammar _ cleans it up, word cues offer sharper vocabulary, and lookups like 4 * 12 = _ or capital of france _ resolve without leaving the prompt. Live-data blanks work in the same box (weather london _, nvda _), and the inline agent can watch spelling while you compose. Everything OpenCues writes stays in your unsent draft; you are always the one who presses Enter.
The undo story, specifically
claude.ai was one of the two sites (with Gmail) where users first reported the blank flash: the first Ctrl+Z after a fill reverted to an empty box. The cause was claude.ai's ProseMirror intercepting plain text insertion with a custom handler that produced two history entries. The fix routes writes through a replacement event that ProseMirror's default handler processes as one transaction, verified atomic on claude.ai, so Ctrl+Z now restores your pre-fill text in one step.
The styling caveat
claude.ai's compose box is configured plain-text-only, so make X bold _ is typically a silent no-op there: the editor refuses the bold mark. You still get clean text rather than clutter, because OpenCues always strips the ** markers before writing; you see wilfred plain, never **wilfred**. If you want rendered styling, that works on generic contenteditables like Gmail.
claude.ai vs Claude Code
Don't confuse this page with the terminal product: claude.ai (the website) is covered by the Chrome extension, while Claude Code (the CLI) has its own deeper integration with a status line, tips packs, and the full keyboard model. If you live in both, the same ~/.cues/ config drives both through the native-messaging host.
Related: The Chrome extension · OpenCues in Claude Code · Make text bold inline · OpenCues on ChatGPT · GitHub repo