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Chapter 07 — Sandbox execution

Build track: M6 is your first green pnpm build / pnpm test inside an isolated runner (see M6 in build track).

Simple explanation

A sandbox is a safe playground where generated code runs without risking your laptop or production. You run installs, builds, and tests there, then copy out only verified artifacts.

Neighbors: Build track · Chapter 06 — Code generation · Chapter 14 — Security · Chapter 17 — Build vs integrate

Deep technical breakdown

Approaches:

  1. Docker container (common): ephemeral container per job, non-root user, read-only base, writable /workspace, no host mounts—or read-only mount of repo template.
  2. Firecracker / VM (stronger isolation): higher cold start cost.
  3. Browser iframe (limited): good for visual preview, not for trusting arbitrary shell; never treat iframe as security boundary for malicious codegen.

Pipeline: copy patch bundle → pnpm install (with lockfile from template) → pnpm test → pnpm build → collect artifacts + logs. Error capture: stream stdout/stderr to object storage; cap size; redact secrets via regex.

Mermaid diagram

sequenceDiagram
  participant O as Orchestrator
  participant S as Sandbox_runner
  participant C as Container
  participant L as Log_store

  O->>S: job_bundle
  S->>C: start_ephemeral
  C-->>S: exit_code_logs
  S->>L: persist_logs
  S-->>O: result_pass_fail
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Real example

docker run --rm -v $(pwd)/workspace:/workspace -w /workspace node:20 bash -lc "pnpm install && pnpm test" with seccomp profile and network egress deny except registry allowlist.

Challenges and pitfalls

  • Supply chain: pnpm install in sandbox still fetches packages—pin lockfile and use verified registry proxy.
  • Timeouts: hung tests block workers—use timeout wrapper per command.

Tips and best practices

  • Use multi-stage images: deps cached, app layer thin.
  • Save docker inspect-level metadata for audits.

What most people miss

Sandbox network policy is part of the threat model, not an optimization. Codegen+install can exfiltrate tokens if env vars leak—inject short-lived read-only creds only when needed.