A popular solo setup is one developer running two agents at once: Claude Code on the backend and Cursor on the frontend. It is fast, until the two halves stop agreeing on the API between them. You rename an endpoint on the backend, the frontend agent keeps calling the old one, and you find out at runtime. The fix is to make the contract between the two agents explicit and shared, so a change on one side warns the other before it writes against the old shape.
Why the split works
Running a backend agent and a frontend agent in parallel mirrors how the work actually divides. The backend agent owns routes, types, and data; the frontend agent owns components, state, and calls to those routes. You, the single human, are the integration point. When both sides hold their assumptions in your head, it flows.
Where it breaks
It breaks at the seam: the API contract. The two agents do not share a context window, so the only thing keeping them aligned is you, remembering the exact shape of every endpoint as it changes. That works until it does not:
- Backend agent renames POST /api/checkout or changes its response fields; the frontend agent keeps sending and reading the old shape.
- Backend agent makes a query parameter required; the frontend agent omits it.
- Frontend agent assumes a field exists that the backend agent never added.
Each side's code is internally correct. The mismatch only shows up where they meet, usually at runtime, usually after you have moved on. This is interface contract drift in its smallest form: two agents, one human, one seam.
Make the seam explicit
Instead of holding the contract in your head, have the backend agent declare it and the frontend agent build against it. When the backend reshapes an endpoint, the frontend agent is told the moment it happens, so it updates the calls instead of guessing. You stay the human in the loop, but you are no longer the only memory of the interface.
Aethereum keeps that one shared contract honest between your two agents over MCP. It is worth being precise about the value here: on a single machine the win is the shared contract between the backend and frontend agents, which do not otherwise see each other. If you also run across a laptop and a desktop, the same room keeps them in sync across machines too.
Setting it up
Run one command in the project; it wires both Claude Code and Cursor, then you work as usual:
npx aethereum initFree room, no signup. The backend agent declares POST /api/checkout; the frontend agent reads that exact shape and is alerted if it changes. The same idea scales up to a team or a project with several agents.
Only the contract and a one-line intent are shared. Your backend and frontend source stay on your machine.