Reverse-Engineering Claude Web's MicroVM: Uncovering Anthropic's Hidden Antspace

TL;DR Highlight

Poking at a Claude Code web session exposed Anthropic's undisclosed deploy platform, Antspace

Who Should Read

Backend or infra developers designing sandboxes or deployment pipelines for AI coding agents

Core Mechanics

The session runs on a Firecracker MicroVM
- dmesg ACPI tables carry the OEM ID FIRECK.
- Specs were 4 vCPUs, 16GB RAM, 252GB disk, Linux 6.18.5.
- PID 1 is a custom /process_api binary, not systemd.
Sessions are snapshot restores, not boots
- A frozen template VM gets session-specific vda, vdb and vdc block devices swapped in.
- After restore it drops page caches, fixes the clock, reseeds the CRNG and drops CAP_SYS_RESOURCE.
- init_on_free=1 zeroes freed memory pages.
An unstripped Go binary shipped in the VM
- environment-runner is a 27MB Go binary with debug symbols intact.
- go tool objdump and strings were enough to recover package layout and dependencies.
- Build metadata exposed the path into Anthropic's private monorepo.
The deploy package has an Antspace client next to Vercel
- The flow is create deployment, upload dist.tar.gz, then an NDJSON status stream.

Community Reactions

The title was called inaccurate
- One commenter said it is about Claude Code, not Claude web, and that it was already posted 3 times on HN.
A question about how to actually use the platform
- A commenter asked whether you can tell Claude Code web to build and deploy, and whether you get a public URL.
- They also asked if the deployment survives after the session ends, which the article does not answer.
A criticism of padding in the writing
- One commenter said the 'No systemd, no sshd' line can be cut since nobody would expect those here.

How to Apply

Inspect your own sandbox first
- In a Claude Code session today, run dmesg, cat /proc/1/cmdline and ps -ef.
- Confirm the virtualization type and what PID 1 is.
Check whether your Go release binaries are stripped
- Run file on the release binary and look for 'not stripped'.
- If you don't want internal paths exposed, add -ldflags "-s -w" to the build.
Consider the snapshot-restore pattern for agent sandboxes
- If session start latency hurts, prebuild a template VM and attach per-session disks.
- Put cache drop, clock fix and RNG reseed in the post-restore init step.
Add a stop hook to your agent's done criteria
- Write a script that checks git status, dev server logs and tsc --noEmit before exit.
- Block the agent from ending the session if any check fails.

Try It

copy & run

# Paste into a Claude Code session terminal (reproduces steps 1-2 of the analysis)

dmesg | grep -i FIRECK | head -3
cat /proc/1/cmdline | tr '\0' ' '; echo
ps -ef | head

# Check whether your Go binary is stripped (replace {{binary}})
file {{binary}}
go version -m {{binary}} | head

# Next: rebuild with -ldflags "-s -w" and compare the file output and size

Terminology

FirecrackerA lightweight VM technology from AWS. It starts nearly as fast as a container while isolating like a VM.

MicroVM snapshotSaving a running VM's full state and loading it later. Like loading a save file in a game.

Stripped binaryAn executable with debug info like function names removed. If not stripped, its internals are easy to read.

PaaSA service where you upload code and it deploys without you running servers. Heroku and Vercel are examples.

NDJSONNewline-delimited JSON, one object per line. Handy for streaming progress updates.

BYOCBring Your Own Cloud. Execution runs in the customer's infrastructure while the vendor orchestrates.