jiti 1.0.0

Just In Time Implementation: declare a typed function or method by its interface and let an LLM write, validate, and cache a real implementation.

jiti

Write the signature and the docstring. Let an agent write the body.

from jiti import jiti


@jiti
def slugify(text: str) -> str:
    """Convert text to a URL-safe slug."""
    ...

The first time slugify runs, an agent looks at the real arguments, explores your repo, and writes an implementation into a .jiti/ file beside your source — checked with ruff, ty, and tests before it's saved. Every call after that is plain Python. No model, no API key, no network.

That's the whole point: jiti generates real, committable code once and gets out of the way. It is not an "AI function" that calls a model on every invocation.

Install

Needs Python 3.13+. Not on PyPI yet:

git clone https://github.com/RyanSaxe/jiti && cd jiti
uv pip install -e .

Set ANTHROPIC_API_KEY to generate code. Running already-generated code needs nothing.

Stubs

A stub is a function with a docstring and a placeholder body — ..., pass, or raise NotImplementedError. A real body is an error: @jiti means "write this for me." Add a comment and jiti treats it as a hint:

@jiti
def parse_money(raw: str) -> Decimal:
    """Parse a currency string like '$1,234.56' into a Decimal."""
    # strip the symbols and separators, then Decimal()
    ...

Methods work too — write the class, decorate the methods you want generated, use self freely.

Strict type checkers flag an empty body with a non-None return (empty-body). That's your checker reacting to the stub, not jiti. Disable that rule or use raise NotImplementedError.

Test-driven generation

State a function's definition of done from your test file with @jiti.required_for(target). Tests import the real code, so the reference is type-checked — and running pytest is the loop: generation happens to make your tests pass, red → green.

# tests/test_money.py
from app.money import parse_money
from jiti import jiti


@jiti.required_for(parse_money)        # real body → your gate test, run as-is
def test_parses_symbols():
    assert parse_money("$1,234.56") == Decimal("1234.56")


@jiti.required_for(parse_money)        # empty body → jiti writes this test from the interface
def test_rejects_garbage() -> None:
    """parse_money raises ValueError on '' and 'not money'."""
    ...

An empty-bodied stub is a jiti-test: written before the implementation exists, so it can only see the interface and can't couple to internals. jiti writes it, commits it under .jiti/tests/, and gates the implementation on it. Both are ordinary test_* functions your own pytest run executes.

Gates only register when their test file is imported. pytest does that on collection; for generation outside a test run, jiti imports your test modules first — scanning the tree by default, or Engine(test_paths=("tests",)) to narrow it.

Configuration

Pass your own engine for a custom client, model, store, or thresholds:

from jiti import Engine, jiti

@jiti(engine=Engine(quality_threshold=8))
def slugify(text: str) -> str: ...

Two prose guides shape what the agent writes — a style guide (how code reads) and a test guide (how tests read). Defaults ship with jiti; override either with Engine(style=...) / Engine(test_guide=...), a JITI_STYLE / JITI_TESTS env path, or a jiti.style.md / jiti.tests.md in your project root. All prompt text lives in src/jiti/prompts/.

JITI_LOG=info logs each model call; JITI_LOG=debug adds tool calls. jiti.clear() (or rm -rf .jiti) drops the cache.

A few things worth knowing

• The code is yours. Edit a generated body and jiti runs it as-is — it tracks a hash and won't clobber your edits. Change a stub's signature, docstring, or gates and it regenerates; if you'd hand-edited that section, it surfaces a conflict instead of overwriting.
• git is yours. jiti only writes files into .jiti/. Commit it (so production runs cached code with no key) or gitignore it. jiti never runs git.
• Concurrency. Running generated code is fully safe — it's plain dispatch. Generating does no locking, so warm the cache once single-threaded, then parallelize. Writes are atomic, so a reader never sees a half-written file.

Development

uv sync
uv run pre-commit install

The gate, exactly what CI runs — ruff-format, ruff, ty, then pytest (no API key; uses a fake client):

uv run pre-commit run --all-files
uv run pytest

examples/semver/ is a runnable TDD spec: semver stubs with tests under examples/semver/tests/. Run pytest there to watch jiti build the library.

Status

Early. Today: free functions and methods, lazy agentic generation, in-process validation with cascading generation, test-driven generation via @jiti.required_for with a score-gated refactor pass, the edit/conflict lifecycle, and Anthropic. Scoped to pure functions.

Not yet: a jiti CLI, jiti eject, required_for on methods, a pytest plugin, whole-class generation, multiple providers, and dependency-aware invalidation.