tenchi 0.9.0

A contract-first, Python-native framework for building REST APIs around use cases, ports, and explicit wiring.

Tenchi

Tenchi is a small, contract-first Python framework for building typed HTTP APIs. Contracts define the boundary, plain async functions implement use cases, and frozen dataclasses carry explicitly wired dependencies.

Tenchi uses Pydantic for validation, Starlette for ASGI, and httpx for its typed client. It requires Python 3.12 or newer and is currently pre-1.0.

Read the documentation for the quickstart, mental model, complete contract and runtime guides, production workflow, comparisons, and module reference.

Quick start

Create and run a working application:

uvx tenchi new my_app
cd my_app
uv sync
uv run tenchi dev

The generated app includes a todos feature, SQLite persistence, memory-backed unit tests, Swagger UI, health and OpenAPI routes, and a CI compatibility gate. With the server running, open http://127.0.0.1:8000/docs or call it directly:

curl -X POST http://127.0.0.1:8000/todos \
  -H 'content-type: application/json' \
  -d '{"title": "Buy milk"}'

To add Tenchi to an existing project instead:

uv add tenchi

How it works

A contract declares the HTTP boundary:

class CreatedTodoHeaders(BaseModel):
    location: str = Field(alias="Location")


create_todo_contract = contract(
    method="POST",
    path="/todos",
    request=CreateTodo,
    response=Todo,
    response_headers=CreatedTodoHeaders,
    status=201,
)

A use case is an ordinary async function whose dependencies come from the app context:

async def create_todo(request: CreateTodo, context: AppContext) -> Todo:
    return await context.todos.create(title=request.title)

A route binds them together. Tenchi immediately checks that every boundary parameter and the return annotation exactly match the contract, so invalid wiring fails during application composition rather than on a request:

def create_todo_headers(todo: Todo) -> CreatedTodoHeaders:
    return CreatedTodoHeaders(Location=f"/todos/{todo.id}")


routes = route_group(
    route(
        create_todo_contract,
        create_todo,
        response_headers=create_todo_headers,
    ),
)

The synchronous response-header projector keeps HTTP metadata at the route boundary while the use case continues to return only domain data. Tenchi validates and serializes those headers before the request scope commits. The typed client validates them on every call; use call_with_response() when you also want the typed headers and underlying httpx response.

Applications use this structure:

app/
  features/<feature>/   # contracts, schemas, ports, routes, use cases
  shared/               # shared errors and domain concepts
  infra/                # concrete port implementations
  server/               # context, hooks, route composition, ASGI app
tests/                  # HTTP integration tests

The main pieces are:

• Pydantic validation for request bodies, path parameters, query parameters, request and successful response headers, and response bodies; field aliases are the names used on the wire and in OpenAPI, and nullable request types can send JSON null explicitly. Declared media types are enforced against wire Content-Type: mismatched requests receive a framework-owned 415 and the typed client rejects mismatched responses. Charset-qualified text contracts are encoded and decoded strictly in both directions; unsupported declared charsets fail when the contract is built.
• typing.Protocol ports and explicit dependency wiring instead of a DI container.
• Declared application errors with a stable JSON envelope.
• A named exception hierarchy that distinguishes configuration mistakes from runtime application and transport failures.
• A contract-driven async client, OpenAPI 3.1 generation, and optional Swagger UI route.
• Lifespan resources, request-scoped contexts, authentication hooks, middleware, request deadlines, outcome observers, pagination, health checks, and in-process testing helpers.

public defaults to False. Set public=True for operations that an authentication hook should exempt, then inspect the metadata in the hook:

health = contract(method="GET", path="/health", response=Health, public=True)


def authenticate(info: RequestInfo, context: AppContext) -> AppContext | None:
    if info.contract.public:
        return None
    # Authenticate and return an enriched context, or raise AppError.

When OpenAPI security schemes are configured, public operations receive an empty per-operation security requirement. health_route() and openapi_route() and swagger_ui_route() are public by default; pass public=False to protect them. The metadata itself does not authenticate requests—application hooks remain in control.

For endpoints with more than one successful status, declare response definitions and select one in a synchronous presenter. Their body and header types become the contract's aggregate typed-client result, so they are the only source of truth. The same mechanism is Tenchi's controlled HTTP escape hatch: a passthrough definition may return a Starlette StreamingResponse, FileResponse, or redirect while its status, media type, media-type parameters, and headers remain contract-owned. No-body definitions accept only a concrete response with an empty materialized body; streaming definitions must declare their body type. The typed client reports the selected definition on ClientResponse.definition and validates its body and headers.

from tenchi.responses import PresentedResponse, present, response

created = response(Todo, status=201)
existing = response(Todo, status=200)

put_todo_contract = contract(
    method="PUT",
    path="/todos",
    request=CreateTodo,
    responses=(created, existing),
    timeout=5.0,
)

def present_put(result: PutTodoResult) -> PresentedResponse:
    outcome = created if result.created else existing
    return present(outcome, result.todo)

route(put_todo_contract, put_todo, present=present_put)

When one status permits alternative top-level body schemas, pass them as separate positional alternatives so Pyright preserves the exact union:

flexible = response(Todo, str, status=200)  # ResponseDef[Todo | str, None]

Nested unions retain their ordinary spelling, such as response(list[Todo | str], status=200). Each response definition has one fixed object-shaped header schema; differing header shapes belong in separate definitions.

timeout= cooperatively cancels overdue work, lets request-scope cleanup and rollback finish, then returns the framework's 504 even if application code catches the injected cancellation. create_app(observers=...) delivers an immutable RequestOutcome, including a read-only header mapping, after each matched route has finalized. Observer failures are logged and never change the response.

See examples/todos for the small teaching app and examples/taskboard for a larger application with authentication, authorization, SQLite transactions, optimistic concurrency through ETag / If-Match, idempotent task creation, multiple successful outcomes, request observation, deadlines, and background work.

CLI

tenchi new my_app
tenchi make feature notes
tenchi make use-case notes create_note
tenchi routes
tenchi openapi
tenchi openapi --diff openapi.json
tenchi openapi --diff-ref origin/main --snapshot openapi.json
tenchi openapi --check openapi.json
tenchi openapi --write openapi.json
tenchi doctor
tenchi dev

openapi --write stores canonical, key-sorted JSON. Before accepting a changed snapshot, run openapi --diff to classify changes as breaking, additive, metadata-only, or unknown. Breaking and unknown changes return a non-zero status; additive and metadata-only changes pass. Use --diff-format json for machine-readable output. --diff-ref reads the snapshot at a Git commit, which keeps a pull-request gate historical even when the branch updates its snapshot. openapi --check remains the exact drift check for tests and CI. Pass the same --routes, --title, --version, --description, and --security options in every command when your document uses them. Run --diff before replacing the baseline with --write. --output and -o remain aliases for --write. For programmatic checks, import analyze_openapi_compatibility from tenchi.compatibility.

Run tenchi <command> --help for command options.

Development

uv sync
uv run pytest
uv run ruff format --check .
uv run ruff check .
uv run pyright

The documentation is a separate Bun and Next.js application:

cd docs
bun install
bun run dev

Run bun run check in docs/ to lint, type-check, test, and build the static GitHub Pages export. Search data and llms.txt files are generated during the build from the registered MDX pages.

Tenchi is licensed under the MIT License.