Show files sent with SendUserFile in the web UI (inline preview or clickable link) #434

Problem

When Claude sends files with the SendUserFile tool, herdr-web-ui shows only the raw tool call and its text output. The files can't be seen or opened from the web UI. This happens in both modes:

Terminal mode: the caption and plain-text local paths:

⏺ <caption text>

  › [image] /private/tmp/claude-<uid>/<project-slug>/<session-id>/scratchpad/image-a.png (258.6KB)
  › [image] /private/tmp/claude-<uid>/<project-slug>/<session-id>/scratchpad/image-b.png (491.8KB)

Chat mode: a collapsible JSON block plus a text output:

{
  "files": [
    "/private/tmp/claude-<uid>/<project-slug>/<session-id>/scratchpad/image-a.png",
    "/private/tmp/claude-<uid>/<project-slug>/<session-id>/scratchpad/image-b.png"
  ],
  "caption": "<caption text>",
  "status": "normal",
  "display": "render"
}

2 files delivered to user.
  /private/tmp/.../scratchpad/image-a.png → file_uuid: <uuid-a>
  /private/tmp/.../scratchpad/image-b.png → file_uuid: <uuid-b>

SendUserFile exists to put a result (screenshot, rendered preview, report) in front of the user, often when they follow the session from another device. In the web UI the user only gets a local path, which they can't open from a phone or a remote browser.

Proposal

Render SendUserFile calls as a proper message instead of raw JSON:

• Caption shown as normal message text.
• Images (png, jpg, gif, webp, svg) shown as inline previews when "display": "render" is set. Clicking one opens the full-size image.
• Other files (PDF, HTML, text, archives…) shown as a clickable chip with the file name and size, which opens or downloads the file.
• Raw tool input/output kept in a collapsed "details" section.
• Terminal mode: turn the › [image] <path> lines into links (or show a preview on hover/tap).

Implementation notes:

• The tool input already has everything needed: files[], caption, display. The output maps each path to a file_uuid, which could serve as a stable ID for a route such as /files/<file_uuid>.
• herdr-web-ui already stores and shows pasted images (.herdr-web-ui/paste-*.png), so that serving path could probably be reused.
• Security: only serve paths that actually appeared in a SendUserFile call of the current session. No general "serve any local path" endpoint. Resolve symlinks and reject .. before checking.
• Scratchpad files under /private/tmp/... may disappear after the session ends or after a reboot. Show a "file no longer available" placeholder instead of a broken image.

Where

Chat and prompt cards

Workarounds and alternatives

Current workarounds

• Copy the path from the tool output and open it locally (open <path> on macOS). This only works on the machine running the agent, not from another device.
• Ask Claude to publish the content in another form (e.g. a hosted artifact/page) and share the link. This works, but it's an extra step and it's no longer the file itself.
• Have Claude copy the file into a folder synced to the other device. This is manual, slow, and clutters the project.

Alternatives considered

• Links only, no inline preview: simpler and still fixes the core problem. Inline image previews could come later.
• Copy files into .herdr-web-ui/ when the tool is called: keeps them available after the scratchpad is cleaned up, but uses disk space and needs a cleanup policy.
• Generic local file browser in the web UI: much more powerful, but a far bigger security surface. Limiting access to the files Claude explicitly sent is the safer choice.