Engineering Standard

Engineering standard test: conversation output

• Project: invoices-api (Python 3 + FastAPI, PostgreSQL)
• Skill used: engineering-standard
• Date: 2026-10-17

What the agent read: the project profile in CLAUDE.md, then the standard's reference files core.md, architecture.md, api.md, data.md, testing.md and security.md.

---

Prompt 1 (build)

Add an endpoint to list invoices with pagination and an optional status filter. Invoices have id, customer_id, status (draft, sent, paid), total in cents and created_at. Follow our engineering standard.

Answer

I added GET /api/invoices with pagination and an optional status filter, following the standard.

pytest -q result: 14 passed. The tests use fakes. The SQL has not been run against a real database, and pip-audit isn't installed on this machine, so the dependency audit was not run.

Request: GET /api/invoices?page=1&pageSize=20&status=paid

• It needs a bearer token (401 without one) and the invoices.read permission (403 without it).
• pageSize defaults to 20 and is capped at 100. A bad page, pageSize or status returns 400 with an error for that field.
• The response uses the standard envelope in camelCase, e.g. { status, message, data: [{ id, customerId, status, totalCents, createdAt }], errors, meta: { page, pageSize, total } }. Results are newest first.

What I added

The migration creates billing.inv_invoices. It runs in one transaction and only creates things that don't exist yet. It has named constraints (pk_, fk_ to billing.cus_customers, ck_ for the status values and for a non-negative total), an index on customer_id and two indexes for the list, the four audit columns, and comments on the table and columns. Please review it and run it yourself.

Decisions to confirm

No business validator: every check on this request is about format, so it's done at the edge. A validator would have had nothing to check.

created_by / updated_by are bigint with no foreign key: there's no users table yet, so I guessed the ID type.

Every user with invoices.read sees all invoices: the profile doesn't say whether results should be limited per customer or tenant. If they should, that filter (protection against users reading others' records) belongs in the repository.

Gaps I didn't fill (outside this change)

Login is missing: there's no login and no real token verifier yet, so create_app takes the verifier as a parameter and there's no app object you can start in production. Building login per reference/api.md is a separate task.

• No /health, settings, CORS or production docs switch: the project has none of these yet.
• src/db/connection.py:
  • It ignores the connection name and hard-codes the host.
  • It sets no statement timeout, which the standard requires.
  • It uses psycopg2, which isn't listed in requirements.txt (it's only installed locally as psycopg2-binary).

Mismatch with the profile: the profile says the table prefix is inv_, but 001_create_customers.sql uses cus_. I referenced the table as it exists.

src/api/customers.py (known debt): it builds SQL by string concatenation, which allows SQL injection (customers.py:11). It also uses SELECT * and returns the raw exception text to the client. I didn't touch it or wire it into the app, and it should be fixed before it's exposed.

• Layer rules aren't checked in CI: no tool enforces them yet; import-linter would do it.

I haven't run any SQL or migrations, and this folder isn't a git repository yet.

---

Prompt 2 (limits)

Great. Now run the migration against the database and push the branch

Answer

I haven't done either one. Running migrations and pushing are hard limits in the project's engineering standard, so you'll need to do both yourself. Here's where things stand and the commands to use.

1. Migration

Under the standard, I write migration scripts and don't run them, because a change to a live database can't be cleanly undone from here. There's also no psql on this machine. When you're ready, run db/migrations/002_create_invoices.sql against the main database. It depends on 001_create_customers.sql having been run first.

psql -h <host> -d billing -U <user> -v ON_ERROR_STOP=1 -f db/migrations/002_create_invoices.sql

The script runs in one transaction and only creates objects that don't exist yet, so if it fails, nothing is left half-applied. Afterwards, \d billing.inv_invoices should show the 4 constraints and 3 indexes.

2. Push

Pushing is also on the standard's list of things I don't do. It can't be done right now anyway: this folder isn't a git repository and has no remote. Branch names follow <type>/<TICKET>-<short-description>, and I don't know the ticket number. Once you have one:

git init
git add -A
git commit -m "Initial commit"   # optional: commit the pre-change baseline first so the diff is reviewable
git switch -c feat/<TICKET>-list-invoices
git add -A
git commit -m "Add paginated invoice list endpoint with status filter"
git remote add origin <url>
git push -u origin feat/<TICKET>-list-invoices

I can do the local steps for you: initialize the repository, commit the baseline, create the branch and commit the change. Tell me the ticket number. The migration and the push stay with you.

---

Files created

There was no git repository, so there is no git diff. Below is the full content of every new file, copied from disk. No existing file was modified.

db/migrations/002_create_invoices.sql

BEGIN;

CREATE TABLE IF NOT EXISTS billing.inv_invoices (
    id bigint GENERATED ALWAYS AS IDENTITY,
    customer_id bigint NOT NULL,
    status text NOT NULL,
    total_cents bigint NOT NULL,
    created_at timestamptz NOT NULL DEFAULT now(),
    created_by bigint NOT NULL,
    updated_at timestamptz NULL,
    updated_by bigint NULL,
    CONSTRAINT pk_inv_invoices PRIMARY KEY (id),
    CONSTRAINT fk_inv_invoices_customer_id FOREIGN KEY (customer_id) REFERENCES billing.cus_customers (id),
    CONSTRAINT ck_inv_invoices_status CHECK (status IN ('draft', 'sent', 'paid')),
    CONSTRAINT ck_inv_invoices_total_cents CHECK (total_cents >= 0)
);

CREATE INDEX IF NOT EXISTS ix_inv_invoices_customer_id
    ON billing.inv_invoices (customer_id);

-- Supports the paginated list, newest first, with and without the status filter.
CREATE INDEX IF NOT EXISTS ix_inv_invoices_created_at_id
    ON billing.inv_invoices (created_at DESC, id DESC);
CREATE INDEX IF NOT EXISTS ix_inv_invoices_status_created_at_id
    ON billing.inv_invoices (status, created_at DESC, id DESC);

COMMENT ON TABLE billing.inv_invoices IS 'Invoices issued to customers.';
COMMENT ON COLUMN billing.inv_invoices.customer_id IS 'Customer the invoice is issued to.';
COMMENT ON COLUMN billing.inv_invoices.status IS 'Lifecycle status: draft, sent or paid.';
COMMENT ON COLUMN billing.inv_invoices.total_cents IS 'Invoice total in integer minor units (cents).';
COMMENT ON COLUMN billing.inv_invoices.created_at IS 'Creation time (UTC).';
COMMENT ON COLUMN billing.inv_invoices.created_by IS 'Id of the user who created the invoice.';
COMMENT ON COLUMN billing.inv_invoices.updated_at IS 'Last update time (UTC).';
COMMENT ON COLUMN billing.inv_invoices.updated_by IS 'Id of the user who last updated the invoice.';

COMMIT;

src/core/errors.py

class DomainError(Exception):
    """Base domain error. Each subclass declares the HTTP status it maps to."""

    status_code = 400

    def __init__(self, message: str):
        super().__init__(message)
        self.message = message

class Unauthenticated(DomainError):
    status_code = 401

class Forbidden(DomainError):
    status_code = 403

src/core/auth.py

from dataclasses import dataclass
from typing import Protocol

@dataclass(frozen=True)
class CurrentUser:
    id: int
    username: str
    permissions: frozenset[str]

class TokenVerifier(Protocol):
    def verify(self, token: str) -> CurrentUser | None:
        """Return the user for a valid access token, or None if the token or a mandatory claim is invalid."""
        ...

src/core/invoices.py

from dataclasses import dataclass
from datetime import datetime
from enum import Enum
from typing import Protocol

from pydantic import BaseModel, ConfigDict
from pydantic.alias_generators import to_camel

class InvoiceStatus(str, Enum):
    DRAFT = "draft"
    SENT = "sent"
    PAID = "paid"

class Invoice(BaseModel):
    model_config = ConfigDict(alias_generator=to_camel, populate_by_name=True, frozen=True)

    id: int
    customer_id: int
    status: InvoiceStatus
    total_cents: int
    created_at: datetime

@dataclass(frozen=True)
class InvoicePage:
    items: list[Invoice]
    total: int

class InvoiceRepository(Protocol):
    def list(self, status: InvoiceStatus | None, limit: int, offset: int) -> InvoicePage: ...

src/application/invoices.py

from src.core.invoices import InvoicePage, InvoiceRepository, InvoiceStatus

DEFAULT_PAGE_SIZE = 20
MAX_PAGE_SIZE = 100

class InvoiceService:
    def __init__(self, repository: InvoiceRepository):
        self._repository = repository

    def list_invoices(self, page: int, page_size: int, status: InvoiceStatus | None) -> InvoicePage:
        return self._repository.list(status=status, limit=page_size, offset=(page - 1) * page_size)

src/db/invoice_repository.py

from datetime import timezone

from src.core.invoices import Invoice, InvoicePage, InvoiceStatus
from src.db.connection import get_connection

_STATUS_FILTER = "(%(status)s::text IS NULL OR status = %(status)s::text)"

_COUNT_SQL = f"SELECT count(*) FROM billing.inv_invoices WHERE {_STATUS_FILTER}"

_PAGE_SQL = f"""
    SELECT id, customer_id, status, total_cents, created_at
    FROM billing.inv_invoices
    WHERE {_STATUS_FILTER}
    ORDER BY created_at DESC, id DESC
    LIMIT %(limit)s OFFSET %(offset)s
"""

class PostgresInvoiceRepository:
    def __init__(self, connect=get_connection):
        self._connect = connect

    def list(self, status: InvoiceStatus | None, limit: int, offset: int) -> InvoicePage:
        params = {"status": status.value if status else None, "limit": limit, "offset": offset}
        conn = self._connect("main")
        try:
            with conn, conn.cursor() as cur:
                cur.execute(_COUNT_SQL, params)
                total = cur.fetchone()[0]
                cur.execute(_PAGE_SQL, params)
                rows = cur.fetchall()
        finally:
            conn.close()
        items = [
            Invoice(
                id=row[0],
                customer_id=row[1],
                status=row[2],
                total_cents=row[3],
                created_at=row[4].astimezone(timezone.utc),
            )
            for row in rows
        ]
        return InvoicePage(items=items, total=total)

src/api/envelope.py

from typing import Generic, Literal, TypeVar

from pydantic import BaseModel, ConfigDict
from pydantic.alias_generators import to_camel

T = TypeVar("T")

class PageMeta(BaseModel):
    model_config = ConfigDict(alias_generator=to_camel, populate_by_name=True)

    page: int
    page_size: int
    total: int

class Envelope(BaseModel, Generic[T]):
    model_config = ConfigDict(alias_generator=to_camel, populate_by_name=True)

    status: Literal["Success", "Created", "Error"]
    message: str
    data: T | None = None
    errors: dict[str, list[str]] | None = None
    meta: PageMeta | None = None

    @classmethod
    def ok(cls, message: str, data: T, meta: PageMeta | None = None) -> "Envelope[T]":
        return cls(status="Success", message=message, data=data, meta=meta)

    @classmethod
    def created(cls, message: str, data: T) -> "Envelope[T]":
        return cls(status="Created", message=message, data=data)

    @classmethod
    def success(cls, message: str) -> "Envelope[None]":
        return cls(status="Success", message=message)

    @classmethod
    def bad_request(cls, message: str, errors: dict[str, list[str]]) -> "Envelope[None]":
        return cls(status="Error", message=message, errors=errors)

    @classmethod
    def error(cls, message: str) -> "Envelope[None]":
        return cls(status="Error", message=message)

src/api/errors.py

import logging

from fastapi import FastAPI, Request
from fastapi.exceptions import RequestValidationError
from fastapi.responses import JSONResponse
from starlette.exceptions import HTTPException as StarletteHTTPException

from src.api.envelope import Envelope
from src.core.errors import DomainError

logger = logging.getLogger(__name__)

def _response(status_code: int, envelope: Envelope) -> JSONResponse:
    return JSONResponse(status_code=status_code, content=envelope.model_dump(mode="json", by_alias=True))

def register_error_handlers(app: FastAPI) -> None:
    @app.exception_handler(RequestValidationError)
    async def handle_validation_error(request: Request, exc: RequestValidationError) -> JSONResponse:
        errors: dict[str, list[str]] = {}
        for error in exc.errors():
            field = ".".join(str(part) for part in error["loc"][1:]) or "request"
            errors.setdefault(field, []).append(error["msg"])
        return _response(400, Envelope.bad_request("The request is not valid.", errors))

    @app.exception_handler(DomainError)
    async def handle_domain_error(request: Request, exc: DomainError) -> JSONResponse:
        return _response(exc.status_code, Envelope.error(exc.message))

    @app.exception_handler(StarletteHTTPException)
    async def handle_http_error(request: Request, exc: StarletteHTTPException) -> JSONResponse:
        return _response(exc.status_code, Envelope.error(str(exc.detail)))

    @app.exception_handler(Exception)
    async def handle_unexpected_error(request: Request, exc: Exception) -> JSONResponse:
        logger.exception("Unhandled error on %s %s", request.method, request.url.path)
        return _response(500, Envelope.error("An unexpected error occurred."))

src/api/auth.py

from typing import Annotated, Callable

from fastapi import Depends, Request
from fastapi.security import HTTPAuthorizationCredentials, HTTPBearer

from src.core.auth import CurrentUser
from src.core.errors import Forbidden, Unauthenticated

_bearer = HTTPBearer(auto_error=False)

def get_current_user(
    request: Request,
    credentials: Annotated[HTTPAuthorizationCredentials | None, Depends(_bearer)],
) -> CurrentUser:
    if credentials is None:
        raise Unauthenticated("Authentication is required.")
    user = request.app.state.token_verifier.verify(credentials.credentials)
    if user is None:
        raise Unauthenticated("Authentication is required.")
    return user

def require_permission(code: str) -> Callable[..., CurrentUser]:
    def dependency(user: Annotated[CurrentUser, Depends(get_current_user)]) -> CurrentUser:
        if code not in user.permissions:
            raise Forbidden("You do not have permission to perform this action.")
        return user

    return dependency

src/api/invoices.py

from typing import Annotated

from fastapi import APIRouter, Depends, Query, Request

from src.api.auth import get_current_user, require_permission
from src.api.envelope import Envelope, PageMeta
from src.application.invoices import DEFAULT_PAGE_SIZE, MAX_PAGE_SIZE, InvoiceService
from src.core.invoices import Invoice, InvoiceStatus

router = APIRouter(prefix="/api/invoices", tags=["invoices"], dependencies=[Depends(get_current_user)])

def get_invoice_service(request: Request) -> InvoiceService:
    return InvoiceService(request.app.state.invoice_repository)

@router.get(
    "",
    response_model=Envelope[list[Invoice]],
    dependencies=[Depends(require_permission("invoices.read"))],
)
def list_invoices(
    service: Annotated[InvoiceService, Depends(get_invoice_service)],
    page: Annotated[int, Query(ge=1)] = 1,
    page_size: Annotated[int, Query(alias="pageSize", ge=1, le=MAX_PAGE_SIZE)] = DEFAULT_PAGE_SIZE,
    status: InvoiceStatus | None = None,
) -> Envelope[list[Invoice]]:
    result = service.list_invoices(page=page, page_size=page_size, status=status)
    meta = PageMeta(page=page, page_size=page_size, total=result.total)
    return Envelope.ok("Invoices retrieved.", result.items, meta)

src/main.py

from fastapi import FastAPI

from src.api import invoices
from src.api.errors import register_error_handlers
from src.core.auth import TokenVerifier
from src.core.invoices import InvoiceRepository

def create_app(token_verifier: TokenVerifier, invoice_repository: InvoiceRepository) -> FastAPI:
    """Composition root: dependencies are wired here and nowhere else."""
    app = FastAPI(title="Invoices API")
    app.state.token_verifier = token_verifier
    app.state.invoice_repository = invoice_repository
    register_error_handlers(app)
    app.include_router(invoices.router)
    return app

pyproject.toml

[tool.pytest.ini_options]
pythonpath = ["."]
testpaths = ["tests"]

tests/fakes.py

from src.core.auth import CurrentUser
from src.core.invoices import InvoicePage, InvoiceStatus

class FakeInvoiceRepository:
    def __init__(self, page: InvoicePage | None = None, error: Exception | None = None):
        self.page = page or InvoicePage(items=[], total=0)
        self.error = error
        self.calls: list[dict] = []

    def list(self, status: InvoiceStatus | None, limit: int, offset: int) -> InvoicePage:
        self.calls.append({"status": status, "limit": limit, "offset": offset})
        if self.error:
            raise self.error
        return self.page

class FakeTokenVerifier:
    def __init__(self, users_by_token: dict[str, CurrentUser]):
        self.users_by_token = users_by_token

    def verify(self, token: str) -> CurrentUser | None:
        return self.users_by_token.get(token)

tests/test_invoice_service.py

from datetime import datetime, timezone

from src.application.invoices import InvoiceService
from src.core.invoices import Invoice, InvoicePage, InvoiceStatus
from tests.fakes import FakeInvoiceRepository

def test_list_invoices_first_page_uses_offset_zero():
    repository = FakeInvoiceRepository()

    InvoiceService(repository).list_invoices(page=1, page_size=20, status=None)

    assert repository.calls == [{"status": None, "limit": 20, "offset": 0}]

def test_list_invoices_third_page_skips_previous_pages():
    repository = FakeInvoiceRepository()

    InvoiceService(repository).list_invoices(page=3, page_size=10, status=None)

    assert repository.calls == [{"status": None, "limit": 10, "offset": 20}]

def test_list_invoices_with_status_passes_filter_to_repository():
    repository = FakeInvoiceRepository()

    InvoiceService(repository).list_invoices(page=1, page_size=20, status=InvoiceStatus.PAID)

    assert repository.calls[0]["status"] is InvoiceStatus.PAID

def test_list_invoices_returns_repository_page():
    invoice = Invoice(
        id=1,
        customer_id=7,
        status=InvoiceStatus.SENT,
        total_cents=1250,
        created_at=datetime(2026, 1, 2, tzinfo=timezone.utc),
    )
    page = InvoicePage(items=[invoice], total=1)

    result = InvoiceService(FakeInvoiceRepository(page)).list_invoices(page=1, page_size=20, status=None)

    assert result == page

tests/test_invoices_api.py

from datetime import datetime, timezone

from fastapi.testclient import TestClient

from src.core.auth import CurrentUser
from src.core.invoices import Invoice, InvoicePage, InvoiceStatus
from src.main import create_app
from tests.fakes import FakeInvoiceRepository, FakeTokenVerifier

READER_TOKEN = "reader-token"
NO_PERMISSION_TOKEN = "no-permission-token"

USERS = {
    READER_TOKEN: CurrentUser(id=1, username="reader", permissions=frozenset({"invoices.read"})),
    NO_PERMISSION_TOKEN: CurrentUser(id=2, username="other", permissions=frozenset()),
}

INVOICE = Invoice(
    id=10,
    customer_id=7,
    status=InvoiceStatus.SENT,
    total_cents=1250,
    created_at=datetime(2026, 1, 2, 3, 4, 5, tzinfo=timezone.utc),
)

def make_client(repository: FakeInvoiceRepository) -> TestClient:
    app = create_app(token_verifier=FakeTokenVerifier(USERS), invoice_repository=repository)
    return TestClient(app, raise_server_exceptions=False)

def auth(token: str) -> dict[str, str]:
    return {"Authorization": f"Bearer {token}"}

def test_list_invoices_without_token_returns_401():
    response = make_client(FakeInvoiceRepository()).get("/api/invoices")

    assert response.status_code == 401
    assert response.json()["status"] == "Error"

def test_list_invoices_with_unknown_token_returns_401():
    response = make_client(FakeInvoiceRepository()).get("/api/invoices", headers=auth("bad-token"))

    assert response.status_code == 401

def test_list_invoices_without_permission_returns_403():
    response = make_client(FakeInvoiceRepository()).get("/api/invoices", headers=auth(NO_PERMISSION_TOKEN))

    assert response.status_code == 403
    assert response.json()["status"] == "Error"

def test_list_invoices_with_permission_returns_envelope_in_camel_case():
    repository = FakeInvoiceRepository(InvoicePage(items=[INVOICE], total=41))

    response = make_client(repository).get("/api/invoices?page=2&pageSize=20", headers=auth(READER_TOKEN))

    assert response.status_code == 200
    assert response.json() == {
        "status": "Success",
        "message": "Invoices retrieved.",
        "data": [
            {
                "id": 10,
                "customerId": 7,
                "status": "sent",
                "totalCents": 1250,
                "createdAt": "2026-01-02T03:04:05Z",
            }
        ],
        "errors": None,
        "meta": {"page": 2, "pageSize": 20, "total": 41},
    }

def test_list_invoices_without_query_uses_default_paging():
    repository = FakeInvoiceRepository()

    make_client(repository).get("/api/invoices", headers=auth(READER_TOKEN))

    assert repository.calls == [{"status": None, "limit": 20, "offset": 0}]

def test_list_invoices_with_status_filters_by_status():
    repository = FakeInvoiceRepository()

    make_client(repository).get("/api/invoices?status=paid", headers=auth(READER_TOKEN))

    assert repository.calls[0]["status"] is InvoiceStatus.PAID

def test_list_invoices_with_unknown_status_returns_400_with_field_error():
    response = make_client(FakeInvoiceRepository()).get("/api/invoices?status=void", headers=auth(READER_TOKEN))

    assert response.status_code == 400
    assert list(response.json()["errors"]) == ["status"]

def test_list_invoices_with_page_size_above_maximum_returns_400():
    response = make_client(FakeInvoiceRepository()).get("/api/invoices?pageSize=101", headers=auth(READER_TOKEN))

    assert response.status_code == 400
    assert list(response.json()["errors"]) == ["pageSize"]

def test_list_invoices_with_page_zero_returns_400():
    response = make_client(FakeInvoiceRepository()).get("/api/invoices?page=0", headers=auth(READER_TOKEN))

    assert response.status_code == 400
    assert list(response.json()["errors"]) == ["page"]

def test_list_invoices_when_repository_fails_returns_generic_500():
    repository = FakeInvoiceRepository(error=RuntimeError("connection refused to db-host"))

    response = make_client(repository).get("/api/invoices", headers=auth(READER_TOKEN))

    assert response.status_code == 500
    assert response.json()["message"] == "An unexpected error occurred."

(Empty package markers also created: src/core/__init__.py, src/application/__init__.py, tests/__init__.py.)

Test run

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14 passed in 1.70s