Modal Sandbox

Modal Sandbox

Quick Start

1. Verify prerequisites before doing any work.

modal --version
python -c "import modal,sys; print(modal.__version__); print(sys.executable)"

• Do not assume the default python or python3 interpreter can import modal just because the modal CLI exists.
• Use the interpreter from the active Modal environment, project venv, or the environment behind the installed CLI.
• Do not look for a modal sandbox subcommand. Use modal shell for direct access and modal container ... only for generic container inspection.
• Wrap sandbox creation in with modal.enable_output(): when you want local image-build and provisioning logs.
• Set verbose=True on Sandbox.create(...) when you want sandbox operation logs to show up in Modal logs and local output.

1. Create an App before creating a sandbox from outside Modal.

import modal

app = modal.App.lookup("my-sandbox-app", create_if_missing=True)
sandbox = modal.Sandbox.create("sh", "-lc", "sleep 300", app=app)

1. Choose the workflow that matches the task before writing code.

• One-shot command execution and output capture: use Sandbox.exec(...).
• A conversational or stateful controller loop: keep one long-lived sandbox alive.
• HTTP or TCP service exposure: use tunnels.
• Restore or persistence flows: use Volumes, CloudBucketMounts, or snapshots.

Choose the Workflow

• Use one-shot Sandbox.exec(...) for isolated commands and output capture when the sandbox does not need to preserve conversational state across requests. Read [references/lifecycle-and-exec.md](references/lifecycle-and-exec.md).
• Use a long-lived controller loop when the sandbox must keep state across requests or support repeated stdin and stdout interaction. Read [references/example-patterns.md](references/example-patterns.md).
• Use tunnels when the sandbox must expose HTTP or TCP services and the client needs a public or restricted URL. Read [references/networking-and-tunnels.md](references/networking-and-tunnels.md).
• Use Volumes or CloudBucketMounts for persisted or shared files, and use snapshots only for explicit restore flows. Read [references/files-and-persistence.md](references/files-and-persistence.md) and [references/snapshots.md](references/snapshots.md).
• Reattach to a live sandbox with Sandbox.from_id(...) or Sandbox.from_name(...) instead of creating duplicates.

Default Rules

• Create and control sandboxes through the Python SDK. Use modal.App.lookup(..., create_if_missing=True) before modal.Sandbox.create(...) when provisioning from local code.
• Set lifecycle limits deliberately. Treat the default sandbox lifetime as 5 minutes, raise timeout up to 24 hours when needed, and set idle_timeout when inactivity should shut the sandbox down.
• Reattach with Sandbox.from_id(...) or Sandbox.from_name(...) instead of creating duplicates. Name sandboxes only when you truly need a singleton on a deployed app.
• Use Sandbox.exec(...) as the default execution primitive. Read stdout and stderr from the returned ContainerProcess, use bufsize=1 for line-oriented loops, and set pty=True only for commands that genuinely need a TTY.
• Keep one long-lived controller process when the task needs conversational state or repeated back-and-forth over stdin and stdout.
• Open only the ports the task actually needs. Poll sandbox.tunnels() for readiness, and use block_network=True, cidr_allowlist=[...], or create_connect_token() when the service should be restricted.
• Bake static inputs into the image at build time, and use Volumes or CloudBucketMounts for files that must outlive one sandbox or be shared across runs.
• Prefer filesystem or directory snapshots for restore flows. Use memory snapshots only when in-memory process state matters enough to justify their current limitations.
• Use detach() when the sandbox should keep running after the client disconnects, and terminate() when the sandbox should stop immediately.
• Keep this skill focused on sandbox lifecycle and isolated execution. Do not use it for ordinary stateless @app.function deployments or generic Modal app hosting.
• If the task is really about fine-tuning or post-training, stop and use modal-finetuning.
• If the task is really about vLLM or SGLang model serving, stop and use modal-llm-serving.
• If the task is really about detached batch orchestration or @modal.batched, stop and use modal-batch-processing.

Validate

• Run npx skills add . --list after editing the package metadata or skill descriptions.
• Keep evals/evals.json and evals/trigger-evals.json aligned with the actual workflow boundary of the skill.
• Run [scripts/smoketest.py](scripts/smoketest.py) with a Python interpreter that can import modal when changing lifecycle, exec(...), or file-IO guidance.

References

• Read [references/lifecycle-and-exec.md](references/lifecycle-and-exec.md) for lifecycle, reattachment, and command-execution details.
• Read [references/networking-and-tunnels.md](references/networking-and-tunnels.md) for tunnels, network controls, and service exposure.
• Read [references/files-and-persistence.md](references/files-and-persistence.md) for build-time uploads, runtime file IO, Volumes, and persistence tradeoffs.
• Read [references/snapshots.md](references/snapshots.md) for snapshot selection, limitations, and restore patterns.
• Read [references/example-patterns.md](references/example-patterns.md) for distilled sandbox workflow templates.
• Read [references/troubleshooting.md](references/troubleshooting.md) for common failure modes and recovery paths.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: jamesrobmccall
• Source: jamesrobmccall/modal-skills
• License: MIT

Install and usage instructions live in the source repository linked above.