How Meta built Forum with agents
A small Meta team built a new app with AI agents doing most of the building and testing, and wrote down what worked: agents check their own work, and engineers approve the evidence.
Forum is Meta's standalone app for deeper conversations in Facebook Groups. A small team built it from scratch and reached core feature parity with Groups in eight weeks.
The team calls the model human on the loop: the agent plans, builds, tests and corrects in a loop, and the engineer sets the goal, watches, steps in when needed and approves. Every change still needs an engineer's sign-off, backed by build results, test output and recorded sessions of the agent using the app.
Their biggest gains came from changing the code and tools, not the prompts. Agents copied weak patterns they found, so the team fixed those once. Agents guessed which screen to edit until they got tools to read the live view hierarchy. A separate build flavor let designers prototype with AI while keeping that code out of the production build.
8 weeksto core feature parity with Facebook Groups
Technical details
Their operating vocabulary: rules are permanent project instructions (a mistake repeated more than twice is encoded once for everyone); skills are team conventions packaged as loadable steps; agents are automated checks that replace a manual pass; commands chain skills and agents into workflows; hooks are triggers, such as running the linter after every agent turn, checking simulator connectivity before installing, and validating tests before a pull request is created.
Repeatable stress runs (aggressive scrolling, random group browsing, rapid navigation, repeated media loading) surfaced a memory problem. The agent isolated a reproduction, proposed a fix and verified it; the fix roughly halved memory growth and page faults across repeated sessions. Engineers kept the architecture decisions.
After launch, an investigation agent gathers what a specific engineer used to know, posts a write-up with suggested fixes, and an engineer decides whether each finding is a bug or an intended trade-off. Giving it the same code pointers, example queries and design docs a new hire gets improved its answers most.
Why it mattersIt is a full, working description of agent-first development on a real product, and nearly every piece (rules, skills, hooks, a separate prototype build) can be copied by a team of one.
For youTheir vocabulary maps one to one onto your Claude Code setup. The parts to borrow: a rule for any mistake an agent repeats twice, a hook that lints after every turn, and a tool that lets the agent read live game or app state instead of guessing.
- feature parity
- Doing everything the older product does, so users lose nothing by switching.
- human on the loop
- A person supervises and approves an automated loop instead of doing each step themselves.
- view hierarchy
- The tree of on-screen elements an app is drawn from, which a tool can read to know what is really shown.
- build flavor
- A variant of the same app compiled with different code switched on or off.