Native fallback

Every neuro.* method has two modes, decided at the call site by the presence of a prompt key on the input object:

prompt field	Behaviour
Missing	Calls the native built-in directly.
Empty string ''	Treated as missing - calls the native built-in directly.
Non-empty string	Routes to the configured LLM.

The native fallback is not a fallback in the “the LLM failed, fall back” sense. It is the default. The LLM is opt-in per call.

import { neuro } from 'neuro-ts';

await neuro.array.map({ array: [1, 2, 3], callbackfn: (n) => n * 2 });
// [2, 4, 6]   <- native call, no client needed

await neuro.string.split({ string: 'a,b,c', separator: ',' });
// ['a', 'b', 'c']   <- native

await neuro.object.keys({ o: { a: 1, b: 2 } });
// ['a', 'b']   <- native

Why both?

The native path keeps neuro-ts a strict superset of the standard library. You can wire neuro into existing code, ship it without breaking anything, then sprinkle prompts in only where they help.

It also means neuro-ts works with no OPENAI_API_KEY configured: as long as the input lacks a prompt field, the library is just a typed Promise wrapper over the original built-ins.

Is it zero-cost?

Yes. The native fallback is a synchronous call to the original built-in wrapped in Promise.resolve(). There is no network round-trip, no client instantiation, and no parsing overhead beyond the argument object lookup.

Does it require configureClient?

No. Native fallback calls never touch the client singleton. You can use neuro.* without calling configureClient() as long as you omit prompt:

import { neuro } from 'neuro-ts';

// Works with no configureClient() call at all.
const sorted = await neuro.array.sort({ array: [3, 1, 2] });
// [1, 2, 3]

Only calls with a non-empty prompt require the client to be configured.

What if the native built-in throws?

Errors from the native built-in propagate as-is. neuro-ts does not catch or wrap them on the fallback path:

await neuro.json.parse({ text: 'not valid json' });
// SyntaxError: Unexpected token 'o', "not valid json" is not valid JSON

This is intentional — the native fallback behaves exactly like calling the original built-in directly. If you want error recovery, use a prompt:

await neuro.json.parse({
  text: 'not valid json',
  prompt: 'fix any syntax errors and parse',
});

Every method supports a prompt

Every one of the 673 wrappers accepts an optional prompt field and routes to the LLM when one is supplied. There are no native-only methods. The only thing that changes is whether the result the LLM produces makes sense for the method’s return type — iterators, live WeakMap handles, and atomic operations that depend on shared memory state cannot be serialised over the wire, so for those methods you almost always want the native path. Pick whichever suits the call.