Build a durable coding-agent workflow with Modal

This tutorial builds a coding agent in a Modal sandbox and wraps its work in little-durable steps. If a step in the run fails, the journal is saved so the same run continues from the failed step instead of starting over.

Steps are:

• Clone repo
• Install dependencies
• Call Claude Code to make coding agents
• Commit and push a new branch with changes
• Open a PR

Preqreqs

You need:

• Node.js 20 or newer.
• A Modal account and token.
• An Anthropic API key.
• A GitHub personal access token with push access.
• A GitHub repository you can safely create a branch in and push to.

For a classic GitHub token, enable the repo scope. For a fine-grained token, grant Contents: Read and write on the repository. You can create a token in GitHub settings.

Set up the project

Install the dependencies and create your local environment file:

npm install
cp .env.example .env

Open .env and replace each placeholder:

MODAL_TOKEN_ID=...
MODAL_TOKEN_SECRET=...
ANTHROPIC_API_KEY=...
GITHUB_TOKEN=...

Run the tutorial

Pass the repository URL and a short issue description:

npm run modal -- https://github.com/your-user/your-repo "There is a typo in the README"

Resume or restart

The default run ID is run-123. Run the same command again with the same ID to resume: completed steps are read from the journal and are not repeated.

Use a new ID when you want a fresh run:

RUN_ID=run-456 npm run modal -- https://github.com/your-user/your-repo "Add a LICENSE file"

Run IDs share the persistent little-durable-work Modal Volume, so choose a unique ID for each new issue.