Nothing gave an instance back. A flow that ended, or an agent that simply stopped
calling, left its service running and its copy on disk until somebody noticed -
which is how dev-server-instances became the one volume that only ever grows.
Two clocks, because they answer different questions. Idle asks whether anybody
is still looking: a preview request or an MCP call resets it, and running out
stops the process while keeping the copy, so an evaluator who stepped away for a
meeting pays a restart and not a reinstall. The absolute one asks whether this
has been here long enough to be forgotten, and discards everything - the only
thing in the whole stack that hands disk back on its own.
Asking after an instance counts as using it, not only preview traffic. The
browser reaches a running service straight on its port, which the worker never
sees, so an agent mid-loop would otherwise look idle to the clock watching it.
What the sweep reclaimed is logged. A preview that vanishes without a word is
indistinguishable from one that broke, and the person it vanished under has no
way to tell the difference.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Until now a running instance was reachable only from inside the Docker network:
the browser-mcp could see it, a human could not. That was the hole under the
whole human-evaluation node - it asks somebody to judge software, with no
software to judge.
The route rides the gateway that already exists, on 443 with the certificate
already issued, rather than publishing a port per execution. Twenty open ports
on an institutional VM is the kind of thing a firewall refuses; one path does
not. The cost is that the previewed application lives under a prefix, which is
acceptable here because the agent writes it under our own instructions.
Authorisation is the part that needed care. A browser attaches no bearer token,
and the execution key is no secret - rootExecutionId is on screen in the editor
and in every event the flow logs - so each preview carries a token of its own,
issued when the service starts, swapped on first visit for an HttpOnly cookie
scoped to that one preview's path, and gone from the address bar afterwards so
it cannot travel onward in Referer. A restart invalidates the previous link,
because a port handed back to the allocator could be serving someone else.
One defect found by walking the whole path through the real gateway rather than
testing the proxy alone: the redirect pointed at the path the proxy receives,
which the gateway has already stripped, so the browser was sent out of the mount
into the gateway's own 404. The cookie had assumed the prefix and the redirect
had not. The mount is now one parameter feeding both, with a test on the
Location header that the first round had no reason to have.
Verified end to end against the stack: link opened, cookie set, navigation
inside the application arriving at the path it expects, and 403 for anyone
holding the key but not the link.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
npm ci wants a lockfile, so an agent starting from an empty directory cannot use
the example configuration until it has written a whole npm project. This one
runs a server.js on node's own http module with no install step at all, which
makes the shortest loop that exercises all three servers: write a file, start it,
open it in the browser.
It reads the port from PORT as well as from the argument. An agent writing a
server from scratch reaches for process.env.PORT far more readily than for argv,
and a preview listening on the wrong port fails its health check with nothing in
the log to explain why.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
A compose .env file is read literally, so MCP_UID=$(id -u) written there arrives
as those eight characters. What came back was "groupadd: invalid group ID", from
a line four steps into a build - true, but it names neither the variable nor the
file that holds it. The build now refuses with both, and says that the file wants
the output of the command rather than the command.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
A named volume whose mount point is absent from the image is created owned by
root, so the worker - which drops root - could not have written a single copy
into /instances. The failure would have surfaced at the first start_service, as
a refused execution far from the line that caused it. Creating the two
directories in the image and giving them to the runtime user is what makes the
volume inherit that ownership.
The user id follows as a build argument, because one number has to satisfy three
things at once: who the process is, who owns the volumes, and who owns the
workspace directory on the host. Left hardcoded, changing MCP_UID to match a
deployment's own user would have brought the same fault back. The guard around
useradd is for the ordinary case of pointing it at an id the base image already
uses - 1000 is the node user - which is otherwise a build failure.
Verified by building both ways and writing into a fresh volume as 10001 and as
1000, and by checking the image carries node 24, Python 3.11 and OpenJDK 25.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
They were three folders on one laptop: a coding agent with a git history of its
own, and two servers - a development server and a browser - with none at all.
What makes them a stack is what sits between them, and that lived nowhere: the
compose file, the gateway, the egress proxy and the networks that keep the worker
off the internet. So the whole thing is one repository, and the coding agent's
separate history is folded into it rather than kept alongside.
What is deliberately absent: .env, node_modules, and the live service
definitions. The .example files next to them say what belongs there.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Lucio Lelii