[BUG] Claude Agent View: localhost URLs returned by tools are not surfaced as previews / no browser handoff

[patrick-yip]

Summary

In Claude Agent View (cloud-hosted background sessions on claude.ai), when an MCP tool
or subprocess invokes webbrowser.open() or surfaces a http://localhost:<port>/ URL,
no browser is opened and the URL is not previewed inline. The URL appears as plain text
in chat, but is unreachable from the user's machine because the server is bound to the
agent sandbox's loopback interface.

This makes any tool that depends on a local rendered page effectively unusable in Agent
View. Examples: streamlit run, matplotlib plt.show(), Jupyter kernel launchers, any
"render and open in browser" affordance.

Repro

1. Run a background session in Claude Agent View.
2. Invoke any tool that opens a localhost URL via webbrowser.open("http://localhost:<port>/").
3. Tool stdout reports the URL; agent surfaces it in chat.

Expected: URL is auto-previewed inline (like the desktop app's Preview pane), or
there is a clickable affordance to open it.

Actual: Nothing visible happens. The URL is plain text. Copy-pasting into a local
browser fails because 127.0.0.1:<port> inside the agent sandbox is not reachable
from the user's machine.

Why it matters

• Many local renderers ship with a "render then open in browser" contract. These are
  silently broken in Agent View today.
• Background sessions are otherwise ideal for long-running render workflows, but
  the missing browser handoff defeats the loop.

Related issues

• PreviewContext blocks legitimate http://localhost URLs in desktop app 1.6608.0 — preview pane and screenshot pipeline both fail #57253 PreviewContext blocks localhost URLs (desktop app) - related but desktop-specific
• Background subagents cannot access OAuth-authenticated MCP servers #46228 Background subagents OAuth limitations - related background-session gap
• [FEATURE] Preview should allow localhost subdomains (e.g. playground.localhost) #47195 Preview should allow localhost subdomains

Suggested directions

• Auto-render localhost URLs in agent message output via the Agent View UI's preview
  pane (parity with desktop app behaviour).
• Or: introduce a "render artifact" tool result type (HTML / image / iframe) so MCP
  tools can return inline-displayable payloads instead of relying on a side-channel
  browser.
• Or: tunnel the agent sandbox's localhost ports back to the user's browser session
  (Cloudflare-tunnel-style relay).

[patrick-yip]

Closing this. Filed prematurely without enough validation that this is a Claude Code surface bug vs. expected sandbox behaviour for cloud-hosted background sessions. May refile with sharper repro and scope if the pattern recurs.

[github-actions]

This issue has been automatically locked since it was closed and has not had any activity for 7 days. If you're experiencing a similar issue, please file a new issue and reference this one if it's relevant.