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

# AprilNEA - Full Stack Engineer Source: [https://aprilnea.me/en/blog/reverse-engineering-claude-code-antspace](https://aprilnea.me/en/blog/reverse-engineering-claude-code-antspace) *What's inside Claude Code Web: an unstripped Go binary, Anthropic's secret deployment platform, and the architecture of an AI\-native PaaS* --- ## [The Starting Point](https://aprilnea.me/en/blog/reverse-engineering-claude-code-antspace#starting-point) We are building ArcBox, a full\-stack platform from Desktop to Platform, similar to Railway and E2B in positioning\. Our core philosophy is local\-cloud consistency: replacing OrbStack with a fully open\-source ArcBox Desktop that provides Sandbox capabilities locally\. Recently, we noticed more and more Coding Agent platforms launching web\-based entry points, and remarkably, nearly all of them chose Firecracker under the hood\. Claude Code is no exception\. As practitioners in the same space, curiosity about its runtime environment led to some digging\. What began as a casual`strace \-p 1`turned into a full reverse\-engineering session that uncovered unreleased Anthropic infrastructure, including an entirely undocumented application hosting platform\. Everything described here was discovered through standard Linux tooling \(`strace`,`strings`,`objdump`,`go tool objdump`\) running inside a Claude Code session\. No exploits, no privilege escalation, no network attacks\. The binary was sitting right there, unstripped, with full debug symbols\. --- ## [Layer 1: It's a Firecracker MicroVM](https://aprilnea.me/en/blog/reverse-engineering-claude-code-antspace#layer-1-firecracker-microvm) The first question: what exactly is this environment?  ``` $ dmesg | grep FIRECK ACPI: RSDP 0x00000000000E0000 000024 (v02 FIRECK) ACPI: XSDT ... (v01 FIRECK FCMVXSDT ... FCAT 20240119) ACPI: FACP ... (v06 FIRECK FCVMFADT ... FCAT 20240119) ACPI: DSDT ... (v02 FIRECK FCVMDSDT ... FCAT 20240119) ``` The ACPI tables are signed with OEM ID`FIRECK`and creator ID`FCAT`, both hardcoded in[Firecracker's source code](https://github.com/firecracker-microvm/firecracker)\. This is the same MicroVM technology that powers AWS Lambda and Fargate\.   The specs: 4 vCPUs \(Intel Xeon Cascade Lake @ 2\.80GHz\), 16GB RAM, 252GB disk, Linux 6\.18\.5\. No nested virtualization since Firecracker intentionally strips`vmx`/`svm`flags from guests\. The process tree is absurdly minimal: ``` PID 1: /process_api --firecracker-init --addr 0.0.0.0:2024 ... └─ PID 517: /usr/local/bin/environment-manager task-run --session cse_... └─ PID 532: claude (the CLI itself) ``` No systemd\. No sshd\. No cron\. No logging daemon\. PID 1 is a custom binary that acts as both init and a WebSocket API gateway\. The kernel command line confirms it: ``` rdinit=/process_api init_on_free=1 -- --firecracker-init reboot=k panic=1 nomodule ``` `strace`on PID 1 shows it running an epoll event loop, periodically checking`/proc/\*/children`and`/proc/\*/status`to monitor child processes\. Essentially a minimal init supervisor, listening on port 2024 \(WebSocket API\) and port 2025 \(secondary endpoint\)\. --- ## [The Snapshot Architecture](https://aprilnea.me/en/blog/reverse-engineering-claude-code-antspace#snapshot-architecture) Sessions don't boot from scratch — they're restored from frozen VM snapshots\. The dmesg output reveals a 48\.5\-hour gap between template creation and session restore: ``` [ 30.731516] Run /process_api as init process ← Template: 2026-03-16 13:53 UTC ~~~ 48.5 HOUR GAP — VM WAS FROZEN AS SNAPSHOT ~~~ [174695.927758] virtio_blk: [vdc] new size: ... ← Restored: 2026-03-18 14:24 UTC [174695.953952] random: crng reseeded due to virtual machine fork [174695.980760] tokio-runtime-w: drop_caches: 3 [174695.993628] EXT4-fs (vda): mounted filesystem r/w without journal ``` During restore, the Firecracker host**hot\-swaps block devices**: DeviceTemplateAfter RestoreContentvdaplaceholder256 GiB ext4Session rootfs \(Ubuntu 24\.04\)vdbplaceholder63\.7 MB squashfs`/opt/claude\-code`vdcplaceholder12\.1 MB squashfs`/opt/env\-runner` The initramfs is deliberately minimal: a 3\.1MB cpio archive containing only`/process\_api`\. The actual Ubuntu rootfs is on the ext4 block device \(vda\), injected at restore time\. The ext4 has mount count=11, indicating the image has been reused across 11 sessions\. ### [Snapstart: The Deferred Mount Pattern](https://aprilnea.me/en/blog/reverse-engineering-claude-code-antspace#snapstart) **Template creation phase:** 1. Firecracker boots: kernel \+ 3\.1MB initramfs 2. `process\_api`performs minimal init: mount`/proc`,`/sys`,`/dev`, cgroups; configure networking \(IP=192\.0\.2\.2/24, GW=192\.0\.2\.1, MTU=1400\) 3. Signals`SNAPSTART\_READY`to the host 4. Host calls`PUT /snapshot/create`→ saves entire VM state **Session restore phase:** 1. Host prepares session\-specific block devices \(vda/vdb/vdc\) 2. Host calls`PUT /snapshot/load`with new device backends 3. VM resumes — kernel detects device changes, reseeds CRNG 4. `process\_api`detects restore and runs:- **Drop page caches**— stale template cache would return garbage - **Remount devtmpfs**— refresh device nodes - **Mount ext4**→`pivot\_root`to new rootfs - **Mount squashfs**overlays \(claude\-code, env\-runner\) - **Fix wall clock**via`clock\_settime\(\)`— otherwise stuck at template epoch - **Drop CAP\_SYS\_RESOURCE**— security hardening - **Accept connections**— WebSocket server ready ### [Security Measures](https://aprilnea.me/en/blog/reverse-engineering-claude-code-antspace#security-measures) MeasurePurpose`init\_on\_free=1`Zero freed pages between sessionsCAP\_SYS\_RESOURCE dropLimit PID 1's capabilities post\-initCRNG reseedPrevent crypto predictability across snapshot forks`\-\-block\-local\-connections`Block localhost WebSocket accessJWT authWebSocket connection verificationToken scrubbingRemove secrets from configs after use --- ## [process\_api: The Wire Protocol](https://aprilnea.me/en/blog/reverse-engineering-claude-code-antspace#process-api) PID 1 exposes two network interfaces — a WebSocket API for process management and an HTTP API for container control\. Unlike typical init systems,`process\_api`is a Rust/tokio binary that implements a full remote process supervisor\. ### [WebSocket API \(port 2024\)](https://aprilnea.me/en/blog/reverse-engineering-claude-code-antspace#websocket-api) Connection handshake: optional JWT →`ProcessConnection`JSON → process creation or reattach\. Process creation accepts a`CreateProcess`struct: ``` { "cmd": "/bin/bash", "args": ["-l"], "env": {"KEY": "VALUE"}, "cwd": "/home/user", "rows": 24, "cols": 80, "timeout": 300, "memory_limit_bytes": 1073741824, "uid": 1000, "gid": 1000, "allow_process_id_reuse": false } ``` I/O uses a two\-phase binary protocol: - **Stdin**:`ExpectStdIn`\(text\) → binary frame - **Stdout/Stderr**:`ExpectStdOut`/`ExpectStdErr`\(text\) → binary frame →`StdOutEOF`/`StdErrEOF` ``` Client Server |--- WS Connect ------------------->| |--- JWT (optional) --------------->| |--- ProcessConnection JSON ------->| |<-- ProcessCreated ----------------| | | |<-- ExpectStdOut ------------------| |<-- [binary: stdout data] ---------| |--- ExpectStdIn ------------------>| |--- [binary: stdin data] --------->| | | |--- SendSignal ------------------->| SIGTERM, etc. |--- Resize ----------------------->| PTY resize |--- Detach ----------------------->| process keeps running |--- KeepAlive -------------------->| heartbeat | | |<-- ProcessExited -----------------| |<-- StdOutEOF --------------------| ``` Process termination reasons include: normal exit, signal, per\-process OOM, container\-level OOM, timeout, and server shutdown\. Internally,`process\_api`tracks per\-process cgroups \(v1 at`/sys/fs/cgroup/memory/process\_api/`, v2 at`/sys/fs/cgroup/process\_api/`\), implements orphan adoption \(reparenting to PID 1\), and runs a configurable OOM polling loop\. ### [HTTP Control API \(port 2025\)](https://aprilnea.me/en/blog/reverse-engineering-claude-code-antspace#http-control-api) Six endpoints manage container lifecycle: EndpointPurpose`GET /status`Health check`POST /fs\_sync`Flush filesystem buffers`POST /shutdown`Graceful shutdown with page cache drop`POST /auth\_public\_key`Set JWT verification key`POST /mount\_root`Mount rootfs \(snapstart restore\)`POST /container\_name`Set container identity The`/mount\_root`endpoint accepts a`MountRootConfig`with network config \(`etc\_hosts`,`resolv\_conf`\), CA certs, squashfs mounts, FUSE mounts \(with VFS cache config\), and the wall clock timestamp — everything needed to initialize a session from a blank snapshot\. During mount, root is frozen via`FIFREEZE`/`FITHAW`ioctls\. --- ## [Layer 2: The Unstripped Go Binary](https://aprilnea.me/en/blog/reverse-engineering-claude-code-antspace#layer-2-go-binary) The real discovery was`/usr/local/bin/environment\-runner`\(symlinked as`environment\-manager`\):  ``` $ file /usr/local/bin/environment-runner ELF 64-bit LSB executable, x86-64, dynamically linked, Go BuildID=..., with debug_info, not stripped $ go version -m /usr/local/bin/environment-runner go1.25.7 path github.com/anthropics/anthropic/api-go/environment-manager mod github.com/anthropics/anthropic/api-go (devel) build -ldflags=-X main.Version=staging-68f0dff496 ``` A 27MB Go binary\.**Not stripped\. Full debug info\. Full symbol table\.**Built from Anthropic's private monorepo at`github\.com/anthropics/anthropic/api\-go/environment\-manager/`\. Using`go tool objdump`and`strings`, the complete internal package structure can be extracted: ``` internal/ ├── api/ # API client (session ingress, work polling, retry) ├── auth/ # GitHub app token provider ├── claude/ # Claude Code install, upgrade, execution ├── config/ # Session modes (new/resume/resume-cached/setup-only) ├── envtype/ │ ├── anthropic/ # Anthropic-hosted environment │ └── byoc/ # Bring Your Own Cloud environment ├── gitproxy/ # Git credential proxy server ├── input/ # Stdin parser + secret handling ├── manager/ # Session manager, MCP config, skill extraction ├── mcp/ │ └── servers/ │ ├── codesign/ # Code signing MCP server │ └── supabase/ # Supabase integration MCP server ├── orchestrator/ # Poll loop, hooks, whoami ├── podmonitor/ # Kubernetes lease manager ├── process/ # Process exec + script runner ├── sandbox/ # Sandbox runtime config ├── session/ # Activity recorder ├── sources/ # Git clone + source classification ├── tunnel/ # WebSocket tunnel + action handlers │ └── actions/ │ ├── deploy/ # ← THIS IS WHERE IT GETS INTERESTING │ ├── snapshot/ # File snapshots │ └── status/ # Status reporting └── util/ # Git helpers, retry, stream tailer ``` Key dependencies extracted from the binary: DependencyPurpose`github\.com/anthropics/anthropic/api\-go`Internal Anthropic Go SDK`github\.com/gorilla/websocket`WebSocket tunnel to API`github\.com/mark3labs/mcp\-go v0\.37\.0`Model Context Protocol`github\.com/DataDog/datadog\-go v5`Metrics reporting`go\.opentelemetry\.io/otel v1\.39\.0`Distributed tracing`google\.golang\.org/grpc v1\.79\.0`gRPC \(session routing\)`github\.com/spf13/cobra`CLI framework --- ## [Layer 3: Antspace, Anthropic's Hidden PaaS](https://aprilnea.me/en/blog/reverse-engineering-claude-code-antspace#layer-3-antspace) Inside the`tunnel/actions/deploy/`package, there are function symbols for two deployment clients:  **VercelClient**, the expected one: - `CreateDeployment`→ POST`/v13/deployments` - `UploadFile`→ PUT`/v2/files`with`x\-vercel\-digest`header - `WaitForReady`→ Poll until`readyState == "READY"` And then,**AntspaceClient**, the unexpected one: ``` deploy.(*AntspaceClient).Deploy deploy.(*AntspaceClient).createDeployment deploy.(*AntspaceClient).uploadTarball deploy.(*AntspaceClient).streamStatus ``` Extracting the associated strings from the binary revealed a complete deployment protocol:  **Phase 1: Create Deployment** ``` POST to antspaceControlPlaneURL Content-Type: application/json Authorization: Bearer {antspaceAuthToken} Body: { app name, metadata } ``` **Phase 2: Upload Build Artifact** ``` POST multipart/form-data File: dist.tar.gz (the built application) Size limit enforced: "project exceeds %dMB limit" ``` **Phase 3: Stream Deployment Status** ``` Response: application/x-ndjson (streaming) Status progression: packaging → uploading → building → deploying → deployed Error: "Streaming unsupported" if client can't handle NDJSON ``` A search for "Antspace" across the entire public internet turned up nothing: Anthropic's website, GitHub, blog, documentation, LinkedIn, job postings, conference talks, patent filings\.**Zero results\.**This platform has never been publicly mentioned anywhere\. The name likely derives from "Ant" \(reportedly an internal nickname for Anthropic employees\) \+ "Space" \(hosting space\), following the same naming pattern as platforms like Heroku or Vercel\. ### [Antspace vs\. Vercel: Architectural Differences](https://aprilnea.me/en/blog/reverse-engineering-claude-code-antspace#antspace-vs-vercel) AspectVercelAntspaceFile uploadSHA\-based dedup, per\-fileSingle tar\.gz archiveBuildRemote \(Vercel builds it\)Local`npm run build`, upload outputStatusPolling\-basedStreaming NDJSONAuthVercel API token \+ Team IDBearer token \+ dynamic control plane URLPublic APIYes, documentedNo, completely internal The fact that Anthropic built a full deployment protocol from scratch, rather than just wrapping Vercel's API, signals this is a strategic platform investment, not a quick integration\. --- ## [Layer 4: Baku, The Web App Builder](https://aprilnea.me/en/blog/reverse-engineering-claude-code-antspace#layer-4-baku) "Baku" is the internal codename for the web app builder experience on claude\.ai\. When you ask Claude on the web to build you a web application, it launches a Baku environment\. From the embedded resources extracted from the binary: **Project Template:** - Source:`/opt/baku\-templates/vite\-template` - Stack: Vite \+ React \+ TypeScript - Auto\-managed dev server via supervisord, logs to`/tmp/vite\-dev\.log` **Supabase Auto\-Provisioning:** Six MCP tools are automatically available: 1. `provision\_database`: create a Supabase project on demand 2. `execute\_query`: run SQL queries 3. `apply\_migration`: versioned schema changes with auto type generation 4. `list\_migrations`: list applied migrations 5. `generate\_types`: regenerate TypeScript types from DB schema 6. `deploy\_function`: deploy Supabase Edge Functions Environment variables auto\-written to`\.env\.local`: ``` SUPABASE_URL, SUPABASE_ANON_KEY, VITE_SUPABASE_URL, VITE_SUPABASE_ANON_KEY ``` **Stop Hooks \(embedded shell scripts\):** The Baku environment has a pre\-stop hook that prevents the session from ending if: - There are uncommitted or unpushed git changes - The Vite dev server log contains errors - `tsc \-\-noEmit`reports TypeScript type errors **Default Deploy Target: Antspace**, not Vercel\. Vercel exists as an alternative, but Baku's native deployment path goes through Anthropic's own platform\. **Internal Organization:** - Drafts stored in`\.baku/drafts/` - Explorations in`\.baku/explorations/` - Git commits use`\[email protected\]`as the author - No git remote configured \(local\-only version control\) --- ## [Layer 5: BYOC \(Bring Your Own Cloud\)](https://aprilnea.me/en/blog/reverse-engineering-claude-code-antspace#layer-5-byoc) The`envtype/`package contains two environment implementations: 1. **`anthropic`**: Anthropic\-hosted \(Firecracker MicroVMs\) 2. **`byoc`**: Bring Your Own Cloud BYOC allows enterprise customers to run`environment\-runner`in their own infrastructure while sessions are orchestrated by Anthropic's API\. Key characteristics: - Default session mode:`resume\-cached`\(fastest restarts, reuses existing state\) - Custom auth:`containProvideAuthRoundTripper`injects container\-level credentials - Smart git handling: checks if task branch exists on remote before fetch - Sub\-types:`antspace`\(Anthropic internal\) and`baku`\(Vite project builder\) - Kubernetes integration:`podmonitor`package implements lease management The BYOC API surface includes 7 endpoints: EndpointPurpose`/v1/environments/whoami`Identity discoveryWork polling \+ ackJob queueSession contextConfiguration retrievalCode signingBinary verificationWorker WebSocketReal\-time tunnelSupabase DB query proxyDatabase access relay --- ## [The Strategic Picture](https://aprilnea.me/en/blog/reverse-engineering-claude-code-antspace#strategic-picture) What we're looking at is a**vertically integrated AI application platform**: ``` User describes what they want (natural language) ↓ Claude generates the application (Baku environment) ↓ Supabase database auto-provisioned (MCP tools) ↓ Application deployed to Antspace (Anthropic's PaaS) ↓ Live application, user never left Anthropic's ecosystem ``` This is not just an AI coding assistant\. It's the architecture of an**AI\-native PaaS**where the user's journey from idea to production happens entirely within Anthropic's infrastructure\. The competitive implications are significant\. This positions Anthropic against: - **Vercel / Netlify**in hosting and deployment - **Replit / Lovable / Bolt**in AI app generation - **Supabase / Firebase**in managed backend \(via tight integration\) But with one structural advantage none of these competitors have:**Anthropic owns the entire stack**, from the LLM that understands your intent, to the runtime that builds your code, to the platform that hosts your application\. --- ## [Methodology](https://aprilnea.me/en/blog/reverse-engineering-claude-code-antspace#methodology) All findings were obtained through standard Linux tools running inside my own Claude Code session\. Here's the step\-by\-step approach: **Step 1 — Identify the hypervisor\.**`dmesg \| grep FIRECK`— ACPI tables carry the OEM ID, immediately fingerprinting Firecracker\. **Step 2 — Identify PID 1\.**`cat /proc/1/cmdline`— reveals`/process\_api`as a custom init, not systemd\. The`\-\-firecracker\-init`flag confirms it\. **Step 3 — Extract Go build metadata\.**`go version \-m`on the environment\-runner binary yields the Go version, module path \(`github\.com/anthropics/anthropic/api\-go`\),`\(devel\)`monorepo marker, and full dependency list\. **Step 4 — Recover package structure from symbols\.**Because the binary is not stripped,`objdump \-t`gives fully qualified function names\. Filtering for`environment\-manager/internal/`and extracting unique paths produces the complete architecture tree\. **Step 5 — Targeted string extraction\.**Naive`strings \| grep`on Go binaries is noisy \(the linker concatenates all string literals\)\. Two better approaches: - *Known\-pattern search*: scan for specific byte sequences \(`dist\.tar\.gz`,`application/x\-ndjson`, etc\.\) - *Struct tag extraction*: Go embeds struct field tags as string literals\. Searching for`json:"status"`patterns reveals wire protocol formats **Step 6 — Symbol\-table\-driven feature mapping\.**`objdump \-t binary \| grep 'deploy\\\.'`yields method inventories per component — this is how both Vercel and Antspace clients were discovered with full method signatures\. **Step 7 — Runtime observation\.**`strace \-p 1 \-e trace=epoll\_pwait,read,write \-f`confirms the epoll event loop, child process monitoring, and WebSocket communication patterns\. TechniqueWhat it reveals`dmesg`ACPI OEM IDHypervisor identity`go version \-m`Toolchain, dependencies, monorepo structureSymbol table \(`objdump \-t`\)Package layout, type names, method signaturesStruct tag stringsWire protocol / JSON formatsTargeted byte searchError messages, status strings, flow logic`strace`on PID 1Runtime behavior, IPC patterns **What made this possible:**\(1\) The binary was not stripped — the single biggest factor; a stripped binary would have required Ghidra\. \(2\) Go's build metadata embedding provides a free dependency manifest\. \(3\) Go's string concatenation model preserves struct tags and error strings\. \(4\) Root access inside the VM allowed`strace`on PID 1\. --- ## [Closing Thoughts](https://aprilnea.me/en/blog/reverse-engineering-claude-code-antspace#closing-thoughts) Shipping an unstripped binary with full debug symbols to production is\.\.\. a choice\. It made this analysis trivial\. What would normally require Ghidra and hours of decompilation was accomplished with`go tool objdump`and`grep`\. Antspace is clearly still in an early or internal stage \(the version string is prefixed with`staging\-`\), but the deployment protocol is mature and production\-grade\. Whether Anthropic plans to launch this as a public product or keep it as internal infrastructure for Claude's web experience remains to be seen\. What's clear is that Anthropic's ambitions extend far beyond being just an LLM and AI agent company\. They're building the infrastructure for a world where applications are spoken into existence, and they want to own every layer of that stack\. --- *All analysis was performed on March 18, 2026, inside a Claude Code Web session running on a Firecracker MicroVM with kernel 6\.18\.5, environment\-runner version staging\-68f0dff496\.* *The complete raw analysis files \(wire protocol specs, snapshot architecture details, decompiled code\) are available at[github\.com/AprilNEA/reverse\-engineering\-claude\-code\-antspace](https://github.com/AprilNEA/reverse-engineering-claude-code-antspace)\.*