Python, FastAPI, React development
Python, FastAPI, React — and refactoring that blows nothing up.
The real problem
Code in a local AI system has an unpleasant property: it is almost entirely asynchronous and almost entirely bound to scarce resources. One model, one GPU, one memory budget. The resulting bugs — leaked reservations, double loads, orphaned requests — do not reproduce under naive unit tests.
How I solve it
- Hexagonal architecture: decision logic is pure, side effects live at the boundary. You test decisions without starting a process.
- Dependency injection through Python Protocols: one extracted package, three heterogeneous consumers, zero cross-imports.
- Serious asyncio: lock plus double-check on load, in-flight request tracking, cleanup guaranteed by
BackgroundTask. - Front end: React and Tauri for a desktop dashboard driving the agent locally over WebSocket. Next.js and React where the context calls for it.
A trap learned in the field
Extracting a shared package out of a live system is the operation where you break everything at once. Method used: strangler fig. Extract behind back-compat shims, migrate one consumer at a time, keep both paths alive until the last caller has moved. No big-bang. Validated by ~1,600 tests at every step.
Proof that the abstraction was right did not come from the tests: it came from the second consumer, written later, which used the package without importing a single line of the original project. As long as there is one consumer, you do not know whether you have an abstraction or just a moved file.
What it delivers, measured
- ~1,600 tests covering the extraction, green at every step
- 3 heterogeneous consumers of the extracted package
- Zero dependencies on some projects — MIDI parser and writer hand-written on the standard library
- Adversarial multi-agent review before the first commit: 10 confirmed issues out of 22 raised
Where it runs in production
- Klody Core — Control plane
- Libretto — Musical structure analysis
Your data cannot leave the building?
Describe the use case, the data involved and the available hardware. I answer personally within 48 business hours.