A box is a sandboxed Linux container in the cloud with a shell, a filesystem,
git, an optional headless Chromium, and public URLs for its ports. Everything
here goes through the remote Upstash MCP server. There is no SDK to install
and no API key in the environment (recording videos is the exception, see below): the server forwards the session's OAuth
token to the Box API, and screenshot bytes travel from the box to Blob
without passing through the server.
When to take work into a box
- The user asks for it: remote, in a sandbox, in the cloud, in a box, not on
my machine.
- The deliverable is a pull request, a public preview URL, or a
screenshot of the running result. A box has GitHub credentials, public
ports and a browser; the local machine often has none of the three.
- The work needs isolation: untrusted or generated code, a heavy dependency
install, a clean checkout, branches the local tree should not carry.
- The work scales out: several independent tasks, one box each, in parallel.
Once a task is in a box, do all of it there. The box's filesystem is not the
local one, so an edit made locally and a build run in the box act on two
different checkouts, and neither side reports the mismatch.
Connect
The plugin already registers https://mcp.upstash.com/mcp, and the box and
blob tools are part of its default tool set. To add the server by hand:
claude mcp add --scope user --transport http upstash "https://mcp.upstash.com/mcp"
On first use the client opens the Upstash consent page. Pick the account or
team the boxes and buckets should live in, and turn the read-only switch
off: every step below except listing is refused with 403 on a read-only
grant.
Tools
| Tool | Actions / purpose |
|---|
box_manage | create, list, get, delete, pause, resume, fork |
box_exec | run a shell command in the box (command is an argv array, folder is the working directory) |
box_git | clone, status, diff, commit, checkout, push, create_pr |
box_preview | create, list, delete public URLs for ports in the box |
box_browser | goto, content, screenshot, tabs, tab_new, tab_close, live_view |
box_snapshots | create, list, list_all, delete, restore (a new box from a snapshot) |
box_logs, box_runs | what happened inside a box, and its run history |
box_apikey | list, create, delete Box API keys for a deployed app or CI (the key outlives the OAuth grant, so tell the user to revoke it when done) |
blob_bucket | list, create (create defaults to visibility: public) |
blob_upload_url | presigned PUT URLs for paths in a bucket, plus public_url on public buckets |
The flow: one task, one box
- Create.
box_manage create. Set browser: true if you will take
screenshots or check pages. Use ephemeral: true with a ttl for
throwaway work (no paid plan needed); use keep_alive: true when a preview
URL must outlive the session (paid plan). Note the returned id.
- Clone with
box_git clone, never with git in box_exec. The clone
is what writes the account's GitHub credentials into the box; without it
push and create_pr fail with a bare 500. The checkout lands at
/workspace/home/<repo name>. Pass that as folder on every later
box_exec and box_git call: the default is the workspace root, which is
not a repository.
- Work.
box_exec for install, build, tests, and the app itself. Each
call waits for the command, so detach servers:
["sh", "-c", "( pnpm preview --host 0.0.0.0 --port 4321 > /workspace/home/app.log 2>&1 & )"],
then poll the port with curl in a second call. Edit files with shell
commands or a short script in box_exec, not with local file tools.
- Preview URL.
box_preview create with the port. The app must
listen on 0.0.0.0; a server bound to 127.0.0.1 answers curl inside the
box and still gives 502 through the preview. The URL has the shape
https://<box-id>-<port>.preview.box.upstash.com. Add basic_auth or
bearer_token when the page should not be open to anyone with the link;
the credential is returned once. Say in the reply how long the URL lives:
an ephemeral box takes it down at expires_at.
- Screenshots.
box_browser goto the page on http://localhost:<port>,
then screenshot. With no path the PNG comes back as an image you can
look at; with a path it is written inside the box and only {saved, bytes} returns. Look while you work, save the ones that count, and pass
full_page: true for long pages.
- Publish screenshots. GitHub's attachment endpoint rejects the box's
token, so images go through Blob.
blob_bucket list, and create a
public one if none fits (private buckets only serve short-lived
signed URLs). blob_upload_url with bucket_id and
files: [{path: "<repo>/<branch>/after.png", content_type: "image/png", size: <bytes>}],
minted right before use (a URL lives at most 10 minutes). Then box_exec
the curl_example from the result, sending the returned headers verbatim
(they are part of the signature). Bytes go straight from the box to storage.
- Pull request.
box_git checkout a branch, commit, push with the
branch name, then create_pr with base, title, and a body that
carries the preview URL and  for each screenshot.
create_pr pushes nothing itself. Reply with the PR URL, the preview URL,
and the screenshot URLs.
- Clean up.
box_manage delete (or pause) unless the user wants the
box or its preview kept. Ephemeral boxes expire on their own.
Scaling out
- One box per independent task. Create them with a shared
labels entry,
drive them in parallel, and box_manage list with label to find and
delete them at the end. Never run two tasks in one box at once.
- When the client has subagents, give each one its own box and run them in
parallel; agents sharing a box overwrite each other's files and processes.
- When every task needs the same expensive setup (clone, dependency install,
build cache), do it once,
box_snapshots create, then restore one new
box per task from the snapshot. fork does the same for an idle or paused
non-ephemeral box.
size is small, medium or large; pick it per task rather than
oversizing all of them.- A
live_view URL lets a person watch a box's browser tab as it works
(frames out, no input in); hand it over for long runs.
Recording videos in a box
Recording is not an MCP tool yet, so this is the one flow that needs the SDK.
Create a key with box_apikey create, keep it in a file inside the box,
drive the box from there with @upstash/box (box.browser.recordings, and
box.browser.cdpUrl() for Playwright), and delete the key when done.
What gets recorded. The box browser: every tab, following the foreground
one, at a fixed resolution. Anything else has to be put into the browser. A
terminal program goes through ttyd (static binary from its GitHub
releases; Debian has no package) attached to a fixed-size tmux session;
send input with tmux send-keys -l in small chunks, since TUIs collapse
bracketed pastes; tmux set -g status off, or the status bar is in every
frame. Stay in one tab where you can: a newly opened tab is not reliably
followed.
Running an agent or CLI in the box. A CLI agent started by hand needs its
own model key; the box's managed key only reaches prompts the Box runner
starts. Get a key into the box without pasting it in chat: serve a one-shot
form from the box (a small HTTP server that accepts one POST), expose it with box_preview,
send the user the link, and it appends the value to a file and exits. A login
that waits on a localhost OAuth callback cannot be tunneled (Box SSH refuses
-L). If the CLI lets you set the redirect URI (OpenCode: oauth.redirect_uri
in opencode.json), point it at a Box preview URL: the callback server still
binds 127.0.0.1 on the URI's port (443 for https, allowed unprivileged), and
previews reach only 0.0.0.0, so relay the preview port to it for the duration
of the grant. Otherwise run the login in the box, let the user approve in
their own browser, and have them paste back the failed
http://127.0.0.1:<port>/... URL to curl inside the box before the CLI
stops waiting. The consent screen picks the account, so the resources the
recording relies on must exist in that account. For unattended agent runs,
workspace instructions that rule out clarifying questions keep a take from
stalling.
Limits. A recording lasts at most 600 s and stops by itself after 3
minutes without a pixel change. A box holds one active recording; a leftover
one makes the next start return 409, so stop it in a crash handler and
before every start. Record everything you intend to show, including waits you
plan to speed up; a gap cannot be edited back in.
Timing. Cut from timestamps, not by eye. Log the wall-clock time of every
action your script takes, take the recorded program's own timeline from
structured output (a session export, logs, API responses) rather than from
the screen, and convert both to video time. Anchor on the end, not the start:
capture begins about 2 s after startedAt, so the first frame is
endedAt - <file duration> (ffprobe), and startedAt puts every cut early.
Text scraped from a terminal breaks at line wraps. Redirect large CLI output
to a file; pipes can truncate it.
Editing (ffmpeg):
- Speed up long waits instead of cutting them out; a hard cut reads as a
glitch. Use round factors (5, 10, 20, 50x), let the segment's length follow
from the factor, and show the factor on screen as a solid green pill with
dark text in a corner (
box=1:boxcolor=0x00d48a), not a translucent badge:
it has to read at a glance over any background.
- Say what is happening in on-screen text: a short caption per step, held
long enough to read (about 0.7 s or more), thinned out when steps come fast.
- A headless recording has no pointer and no address bar. Draw a cursor and a
click marker where input happens, and draw the URL when the address
matters. Take positions from Playwright bounding boxes (scroll the element
into view first), or for a terminal from ttyd's
window.term: cell size is
the .xterm-screen rect divided by cols and rows, and the row and
column come from searching term.buffer.active.
- Make the cursor look hand-moved. Use a small, thin, anti-aliased arrow
(about 25 px tall at 1280x800) with a soft shadow. Ease each move in and
out (smootherstep) along a slight curve (a quadratic Bezier with the control
point pushed sideways), let it arrive a beat before the click, and add a
faint drift while it rests. Mark a click with a short press (a slightly
smaller sprite for about 0.15 s) and a thin ripple that grows and fades
over about 0.4 s, not a solid ring. Hold the cursor at each segment
boundary so it does not glide toward the next segment's position.
- Render the ripple as a PNG sequence and delay one copy per click with
tpad. Put the filtergraph in -filter_complex_script; commas inside a
quoted expression still need \, (the sketch below leaves that out).
[0:v]trim=T0:T1,setpts=(PTS-STARTPTS)/20,fps=30,
drawtext=text='20x':fontcolor=0x062b1f:box=1:boxcolor=0x00d48a:boxborderw=10:x=w-tw-30:y=26,
drawtext=text='<step>':enable='between(t,2.0,2.8)'[fast]
[ripple]format=rgba,tpad=start_duration=<click t>:color=0x00000000[r]
[v][r]overlay=x=<cx-36>:y=<cy-36>:eof_action=pass[v2]
[v2][cursor]overlay=eval=frame:enable='between(t,A,B)':
x='st(0,clip((t-T0)/D,0,1));st(1,ld(0)^3*(ld(0)*(6*ld(0)-15)+10));
(1-ld(1))^2*X0+2*(1-ld(1))*ld(1)*XC+ld(1)^2*X1':y='...'
Review cuts on an ffmpeg ... tile=3x3 contact sheet served with
python3 -m http.server and opened with box_browser, never while a
recording is running (it is the same browser). Publish the video with
blob_upload_url.
Between takes. An agent recorded with box tools lists what exists before
it creates anything: a box left by the previous take makes it skip the clone
and reuse that box. Delete leftovers first, or say "a fresh box" in the
prompt. Boxes the recorded agent creates live in the account it consented
to, which is not necessarily the one your own session can see.
Several recordings. One box per recording, driven in parallel by
subagents when the client has them. Never share a recording box between
agents: the tmux server, the browser and the recording slot are per box, and
two agents silently kill each other's sessions and overwrite each other's
files.
Gotchas
- Read-only grant → 403 on create, exec, screenshot-to-path, git writes and
upload URLs. Re-consent with read-only off.
box_git create_pr fails if the account has no GitHub installation
covering the repo; ask the user to connect GitHub in the Upstash console
under Box settings.- Prefer
box_git clone over clone_repo on box_manage create: on an
ephemeral box the option can be ignored, and on a persistent one it runs in
the background with no completion signal beyond box_logs.
- Paths inside the box are relative to
/workspace/home unless absolute.
ls /workspace itself is denied (root-owned, mode 711).
- The
node image has no corepack and boxuser cannot npm i -g, so a
packageManager-pinned pnpm fails to self-install. Use
npx -y pnpm@<version> or export npm_config_manage_package_manager_versions=false
(the preinstalled pnpm); sudo npm i -g pnpm@<version> also works, sudo is
passwordless.
blob_upload_url headers are signed. A missing or changed content-type
or cache-control → 403 from storage (SignatureDoesNotMatch). An expired
URL → mint again.- Bucket names are account-wide. Reuse one bucket such as
agent-proof with
per-repo prefixes rather than creating one per run.
box_browser fails with "browser is not enabled for this box" unless the
box was created with browser: true; there is no way to add it later.- A public Blob host caches objects for an hour (
cache-control on the
upload). Re-uploading to the same path keeps serving the old bytes: publish
a replacement under a new name, and add a query string when you read back
something you just overwrote.
- A
box_exec request times out after 60 s. Detach long jobs with
setsid ... &, poll in later calls, and check the process is still alive.
pkill -f <pattern> in box_exec also matches the calling shell's own
command line and kills the call (exit 143). Write pkill -f 'patter[n]'.tmux kill-server followed at once by tmux new-session can fail with
"server exited unexpectedly"; wait for the old server to exit first.- The
node image has no PIL or ImageMagick. sudo apt-get install what you
need, or write small PNGs with Python's zlib.