---
title: "Sandboxes for coding agents: warm repos, ready in milliseconds"
description: "Promigence runs coding agents in sandboxes forked from a verified, already-built snapshot of the repo, so every step starts warm and a twenty-step task finishes in about a minute."
url: "https://www.promigence.ai/for/coding-agents"
site: "Promigence"
---

# Coding agents that start work on a repo that is already built

Promigence gives a coding agent a sandbox forked from a verified snapshot of the repository, with dependencies installed and the build done. The agent starts at a built tree instead of a git clone, so its edit-and-check loop runs at warm speed from the first step.

**Who this is for:** Teams building coding-agent products and software factories that run agents against customer repositories.

## What breaks today
### Every task starts with a clone and an install
Before an agent writes a line, it waits for the repository to download, the dependencies to install and the first build to finish. On a real repository that is minutes, paid again on every task.

### Each check in the loop is a cold build
An agent that edits, type-checks and edits again pays the cold price on every check. A type-check that takes 24 seconds cold sets the pace of the whole loop.

### Your users watch the spinner
The wait is theirs. A task that takes nine minutes loses the person who asked for it.

## What changes with Promigence
### Start from a built snapshot
Build the repository once, with a verify command. Every sandbox forks from that result, so the agent's first command runs against an installed, built tree.

### Warm checks at every step
The type-check that costs 24 seconds on a cold machine costs 1.5 seconds on a warm fork, so a twenty-step task finishes in about a minute instead of nine.

### A sandbox per task, as many as you need
Fork one per task. Each is isolated, and one that sits idle pauses until it is needed again.

## One snapshot per repo, one fork per task
```bash
# Once per repo, or when the lockfile changes
promigence snapshot create --image node:22 \
  --setup "git clone https://github.com/your-org/app /work/repo && git -C /work/repo checkout <40-char sha>" \
  --setup "cd /work/repo && npm ci && npm run build" \
  --verify "cd /work/repo && npm test" --alias app --wait --wait-timeout 10m

# Per task: a warm sandbox, then your agent's commands
promigence fork --snapshot app --count 1 --cap 1
promigence exec --sandbox "$SANDBOX" -- sh -c "cd /work/repo && npm run typecheck"
```