Architecture Agent Skill

Architecture

Three-Layer Architecture

• @cloudflare/sandbox (packages/sandbox/) — Public SDK published to npm
• Sandbox class: Durable Object that manages the container lifecycle
• Modular HTTP clients per capability (CommandClient, FileClient, ProcessClient, …)
• CodeInterpreter: high-level API for Python/JS with structured outputs
• proxyToSandbox(): request handler for preview URL routing

• @repo/shared (packages/shared/) — Internal shared utilities
• Type definitions used by both SDK and container runtime
• Centralized error classes (packages/shared/src/errors/) and logging
• Not published to npm

• @repo/sandbox-container (packages/sandbox-container/) — Container runtime
• Bun-based HTTP server running inside the Docker container
• Dependency-injection container in core/container.ts
• Route handlers for command execution, file operations, process management
• Not published to npm (bundled into the Docker image)

Request Flow

Primary control path:

Worker
  → Sandbox DO (packages/sandbox)
    → ContainerControlClient (packages/sandbox/src/container-control/)
      → capnweb over /rpc WebSocket
        → SandboxControlAPI (packages/sandbox-container/src/control-plane/)
          → container services
            → Shell commands / filesystem

Route-based compatibility path:

Worker
  → Sandbox DO (packages/sandbox)
    → SandboxClient / clients/transport
      → Container HTTP API on port 3000 (packages/sandbox-container)
        → Router / handlers
          → container services
            → Shell commands / filesystem

Errors flow back the same path: container → Sandbox DO → Worker, using the custom error classes in packages/shared/src/errors/ keyed by the ErrorCode enum.

Primary Control Path

The primary Sandbox Durable Object to container control path is the container-control/control-plane path:

• SDK side: packages/sandbox/src/container-control/
• Container side: packages/sandbox-container/src/control-plane/
• Current wire implementation: capnweb RPC over the /rpc WebSocket route

Control-channel/transport-layer capabilities belong in this path. Treat capnweb/RPC as the current implementation detail, not the architectural boundary.

The shared @repo/shared SandboxAPI interface remains named SandboxAPI because it defines the current control API contract used by both sides.

Route-Based Compatibility Path (packages/sandbox/src/clients/)

packages/sandbox/src/clients/ and packages/sandbox/src/clients/transport/ implement the HTTP and custom WebSocket route-based compatibility API. Maintain these for compatibility, debugging, local development, fallback behavior, and bug fixes, but do not add new control-plane capabilities there by default.

The route-based client pattern is:

• BaseHttpClient — abstract route-based HTTP/WebSocket client with shared request/response handling
• SandboxClient — compatibility aggregator that exposes all specialized route-based clients
• Specialized clients — one per domain:
• CommandClient — exec / execStream
• FileClient — read, write, list, delete
• ProcessClient — start, stop, list, signal
• PortClient — port readiness streams
• GitClient — clone, checkout, status
• UtilityClient — ping, metadata
• InterpreterClient — code interpreter sessions

When maintaining route-based compatibility, add or extend specialized clients under packages/sandbox/src/clients/. DO-to-container control capabilities belong in packages/sandbox/src/container-control/ and packages/sandbox-container/src/control-plane/.

Container Runtime (packages/sandbox-container/src/)

• DI container (core/container.ts) — manages service lifecycle and wiring
• Router — simple HTTP router with middleware
• Control plane (control-plane/) — primary container-side API called by the Sandbox DO
• Handlers (handlers/) — route-based compatibility handlers, thin layer that parses requests
• Services (services/) — business logic (CommandService, FileService, ProcessService, …)
• Managers (managers/) — stateful coordinators such as ProcessManager

Entry point: packages/sandbox-container/src/index.ts starts a Bun HTTP server on port 3000.

When adding a new container control operation:

• Add/extend a service in services/ for the business logic.
• Add the control-plane method in packages/sandbox-container/src/control-plane/.
• Mirror the call in packages/sandbox/src/container-control/.
• Add unit tests on both sides; add an E2E test if it touches real shell/filesystem behavior.

Only add a route handler in handlers/ and a route-based SDK client in packages/sandbox/src/clients/ when maintaining HTTP/WebSocket compatibility.

Monorepo Structure

Uses npm workspaces + Turbo:

• packages/sandbox — main SDK package (published)
• packages/shared — shared types and utilities (internal)
• packages/sandbox-container — container runtime (internal, bundled into image)
• examples/ — working example projects
• tooling/ — shared TypeScript configs

turbo.json orchestrates dependency-aware builds.

Cross-Cutting Patterns

• Sessions — isolate execution contexts (cwd, env vars). Default session is auto-created; multiple sessions per sandbox are supported.
• Ports — expose internal services via preview URLs with token auth. Preview URL authorization is Durable Object-owned, while forwarding is active only after exposePort() activates the port for the current runtime. Production preview URLs require a custom domain with wildcard DNS (*.yourdomain.com); .workers.dev does not support the required subdomain patterns.
• Container isolation — handled at the Cloudflare platform level (VMs), not by SDK code.

Container Base Image

The container runtime uses Ubuntu 22.04 with:

• Python 3.11 (matplotlib, numpy, pandas, ipython)
• Node.js 20 LTS
• Bun 1.x (powers the container HTTP server)
• Git, curl, wget, jq, and other common utilities

When modifying packages/sandbox/Dockerfile:

• Keep images lean — every MB affects cold start
• Pin versions for reproducibility
• Clean up package manager caches to reduce image size