Trying out autonomous agents

The setup in one paragraph

Sandcastle is Matt Pocock's loop for running a coding agent unattended. It puts Claude Code in a Docker container, points it at a git worktree, gives it one GitHub issue, then merges the commits back to your branch. The init command scaffolds the whole thing in about a minute. Issues go in, branches come out.

Wayfinder charts the map

The scaffold gives you a loop. It does not give you anything for the loop to eat. That is the part I underestimated.

Wayfinder is a planning skill that takes an idea too big for one session and turns it into a map issue with child decision tickets. Each ticket holds one question whose answer is a decision, not a slice of the build. You work them one at a time until nothing is left to decide.

That output is exactly what Sandcastle wants: small, self-contained, labelled issues. I did not plan the pairing, it just fell out. My first attempt without a map was three vague issues, and I got three vague pull requests back.

Grill me before you commit

Before anything went in the queue I ran the grilling skill against the plan. It asks hard questions and does not accept the first answer. Ten uncomfortable minutes.

Then consider the alternative. A question you did not answer becomes a guess the agent makes at 3am, and you find out over coffee that it guessed wrong. A bad assumption caught at the keyboard costs one sentence. Caught in the morning it costs the whole run.

Varlock so nothing lands on disk

The default is a plaintext .env file next to the config. I did not want credentials sitting on my laptop, so I used varlock instead.

You commit a .env.schema holding the shape of each variable and where to find it, never the value. Mine resolves through Doppler. The loop then runs behind varlock, which puts the resolved values in process.env for the length of the run and nowhere else.

Declaring a secret in the schema does not forward it into the container. That still needs its own line in main.mts. I learned this twice.

Would I do it again

Yes, with the same caveat every time. The loop is only as good as the tickets. Sandcastle is solid infrastructure, and the thinking is still mine. It just moved earlier, into the map and the grilling, before any container starts.

I read every diff. So far that has been easy, because I only gave it small, tightly scoped tasks and sat and watched the runs while they happened. Unattended overnight is the pitch, and I am not there yet. But it did the work I handed it, and I did not have to type any of it.