File viewer: find relative paths in an agent's scratch folder outside the pane's folder #803

Problem

Agents sometimes write files outside the pane's working directory and then refer to them by a path relative to that place. Claude Code keeps a per-session scratchpad at /tmp/claude-<uid>/<project-slug>/<session-id>/scratchpad/ and says things like "the draft is in scratchpad/upstream-pr.md". Opening that link requests /api/fs/file?path=scratchpad%2Fupstream-pr.md&pane_id=…. locateFile resolves it against the pane's cwd, and filesNamed only searches under the cwd, so the viewer answers not_found even though the server can read the file.

Related to #434 / #730, which made SendUserFile paths openable. Those are absolute, so they already work. This is the relative form agents use in prose.

Proposal

An opt-in list of extra roots, tried only after the pane's folder found nothing:

• HERDR_WEB_FILE_ROOTS: :-separated absolute folder globs.
• {cwd-slug} in an entry is the pane's cwd with every non-alphanumeric character replaced by - (how Claude Code names <project-slug>), e.g. /tmp/claude-1000/{cwd-slug}/*.
• One match opens directly. Several come back as the existing ambiguous_path candidates, newest first. Unset means today's behaviour exactly.
• Globbing starts from the longest literal prefix of each entry, never from /.

I'm running this as a ~40-line patch to server/file-view.ts (a new filesInRoots(), called at the end of locateFile) with a test in server/file-view.test.ts, and I'm happy to open it as a PR if the shape works for you. A built-in default for Claude Code's layout could be a follow-up, but opt-in avoids reading other users' /tmp layouts by surprise.

Where

Other (file viewer)

Workarounds and alternatives

Pasting the absolute path into the viewer works, as does asking the agent to always print absolute paths, but the links agents print in chat stay broken.