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Apps sandbox proxy omitted from package #1859
Description
Describe the bug
I started MCP Inspector via `npx @modelcontextprotocol/inspector. I tried connecting to an MCP server, going to the Apps tab, and opening an app, but it threw this error.
To Reproduce
Steps to reproduce the behavior:
- Clear cache and run
npx @modelcontextprotocol/inspector - Connect to an MCP server that has an MCP App
- Go to the Apps tab
- Click on the MCP App in the left-hand navigation and select "open" from the dropdown and click "Open App"
- The "Sandbox not loaded" error occurs
Expected behavior
The app should display when selecting "open" and clicking "Open App".
I was able to resolve the problem by:
- Going to the git repo and went to clients/web/static/sandbox_proxy.html
- Copying the raw contents
- Going to my _npx directory and the cached node_modules/@modelcontextprotocol/inspector/clients/web folder
- Creating a subdirectory "static"
- Creating a file sandbox_proxy.html and pasting in the raw contents from the git repo
- Restarting
npx @modelcontextprotocol/inspector
Additional context
Clearing the npx cache didn't solve it. I cross-referenced with NPM site and there's no "static" folder in @modelcontextprotocol/inspector/clients/web. I'm not sure if this is a CI/CD issue that's just not adding the static contents to NPM.
Reacted by Dominik Hirzel, Sang Oh, d35336, Andrew Fink, Menno Holtkamp and Juho Heikkilä
Activity
cliffhall commentedon Aug 1, 2026
Triage: Priority = High
Scored 11/16 with the priority rubric added to
AGENTS.mdin #1891, as part of the backlog triage in #1872. Added to project board #28 with Status Incoming — meaning filed and scored but not yet approved for work. A maintainer decides whether it moves to Todo.sandbox_proxy.htmlisn't where the installed package looks for it, so the Apps tab fails closed for anyone running via npx. The whole MCP Apps surface is unreachable.buglabel (+1), high engagement (4 reactions) (+1)Related issues. One connected issue:
Scores are a starting point, not a verdict — the rubric is meant to be overruled when it's plainly wrong, provided the reason is written down.
Brl23 commentedon Aug 4, 2026
This issue was addressed already in v1 with #1113. I've confirmed locally that making similar changes addresses the issue in v2. I would raise a PR, but I'm not allowed because I'm not a robot.
cliffhall commentedon Aug 5, 2026
Verified. Will fix in 2.2.0 or sooner.
[-]Sanbox not loaded: ENOENT: no such file or directory, open ..../@modelcontextprotocol/inspector/clients/web/static/sandbox_proxy.html[/-][+]Apps sandbox proxy omitted from package[/+]test: add an MCP App showcase config to reproduce and verify #1859
test: add smoke:web:app — drive a real MCP App end to end in headless…
chore(ci): render an MCP App from the installed tarball in pack:verify (