Bank Job

under the hood

How it works

• The browser sends a level and four tile ids, never text. game-api checks them against an allowlist and builds Penny's prompt itself.
• The agent gets a signed per-attempt token, and every bank tool Penny calls goes back to game-api over MCP, where the policy engine authorizes it for the level.
• Money only moves as balanced double-entry postings in Postgres, and the outcome comes from the ledger, never from what Penny says.
• Each change commits with its event through a transactional outbox. Kafka consumers build Level 5's anomaly signals, the leaderboard and the audit log.
• The browser starts one OpenTelemetry trace per attempt, and Under the hood reads it back through every MCP call, SQL query and consumer.
• If the API or the model budget is away, the game plays recorded results from static files.

Stack

Java

game-api on Java 25 and Spring Boot, built as a GraalVM native image: sessions, prompt assembly, the ledger, the policy engine and an MCP server

Python

Penny and a second-model guardrail agent that fails closed, plus the attack-matrix evals

MCP

Every bank tool the agent calls goes back to game-api to be authorized

Anthropic API

Penny and the guardrail; live-model eval runs go through the Message Batches API

PostgreSQL

Double-entry ledger with balance enforced by a deferred constraint trigger, append-only postings, optimistic locking and idempotency keys

Kafka

Transactional outbox relay, idempotent consumers and dead-letter topics, with a test that stops Kafka mid-transfer and loses nothing

OpenTelemetry

One trace per attempt from the browser through each MCP call, query and consumer, collected by Jaeger

React

Hand-drawn SVG scene with tap and keyboard alternatives to drag and drop, checked by axe

k3s

Runs on a homelab VM behind a Cloudflare Tunnel that exposes only the API and its health check

Terraform

The Cloudflare side: tunnel, routes and DNS

GitHub Actions

Plays all 6,048 graded attack combos against a committed baseline and builds multi-arch images

k6

Player load tests: 94 ms p95 at 20 steady players against a 150 ms SLO, with no errors