chimera-thin-vertical-slice

Trigger

You are starting work that spans layers — a command that reaches an orchestrator that calls a provider that writes to a store — and you are deciding what to build first. The temptation is to build the bottom layer completely, then the next, then wire it up at the end.

It does not apply to work that is genuinely one layer deep: a parser, a formatter, a pure function. There is no vertical there, and inventing one is ceremony. It also does not apply to a change inside a system that already runs end to end; that path exists, and the slice has already been paid for.

Do

1. Name the observable the slice will produce, as a command someone else could type and a single concrete output. Not "the pipeline works" — chimera run "hi" prints one line that came back from a real provider.
2. Write that invocation first, as a script or a test, and watch it fail for the right reason.
3. Implement the shortest path through every layer, degenerate at each one: one provider, no retry, no config file, no cache, no concurrency. Where a policy will eventually live, put a constant.
4. Keep the layer that carries the actual unknown real. If you do not yet know what the provider returns, the provider is the one thing in slice one that must not be a stub.
5. Commit at the moment it runs, before generalising. That commit is the evidence that the layers fit together.
6. Widen one axis at a time — second provider, then configuration, then caching — and keep the command from step 1 working after each. If it breaks, that widening is the suspect.

Avoid

Designing an interface against zero callers. The horizontal order produces a base class, six adapters, a registry and a retry policy on day one, and discovers at wiring time that the shape is wrong — because nothing had yet asked it a real question.

# Day 1, horizontal: none of this has ever run.
class BaseProvider(ABC): ...
class ProviderRegistry: ...
class RetryPolicy: ...

# Day 1, vertical: ugly, hardcoded, and it answers the question.
def complete(prompt: str) -> str:
    return _post(URL, {"model": MODEL, "prompt": prompt})["choices"][0]["text"]