# SP20 — NPI Checksum + Tax ID Format Validation

**Date:** 2026-06-21
**Branch:** `sp20-npi-validation`
**Status:** Shipped
**Scope:** Backend only. No frontend changes.

---

## 1. Why this exists

Cyclone's completeness review (`docs/reviews/2026-06-20-cyclone-completeness-review.md`
§3.1.10) flags this gap:

> **No NPI validation.** NPIs are captured (`claim.party.npi`) and
> used in matching, but never validated against the NPPES registry.
> Bad NPIs (typos, deactivated) silently propagate.

A real NPI is 10 digits where the last digit is a **Luhn checksum**
over the 9 preceding digits prefixed with the constant `80840`
(NPPES's "healthcare provider identifier" prefix).

**Tax ID** format: 9 digits, optionally formatted `XX-XXXXXXX`. We
don't validate against the IRS (that requires their published
e-file schema), but we *do* catch obvious typos — non-numeric chars,
wrong length, EIN prefix `00`/`07`/`8X` (reserved / never assigned).

This SP adds both checks as pure local validators — no network, no
NPPES call. Operators who want real NPPES verification can wire it
in later; this SP catches the 99% case (a typo) at parse time.

## 2. Operator surface

| Surface | Usage |
|---------|-------|
| Python | `from cyclone.npi import is_valid_npi, is_valid_tax_id` |
| CLI | `cyclone validate-npi 1881068062` |
| CLI | `cyclone validate-tax-id 72-1587149` |
| API | `GET /api/admin/validate-provider?npi=1881068062&tax_id=72-1587149` |

The parser's claim-level validator (cyclone.parsers.validator) gains
a new R-rule that flags bad NPIs as a `validation.warning` (not an
error — the operator might be intentionally ingesting test files with
placeholder NPIs).

## 3. Files

* `cyclone/npi.py` — new module (~120 LOC). `is_valid_npi()`,
  `is_valid_tax_id()`, `npi_checksum()`, plus the NPPES constant.
* `cyclone.parsers.validator` — new rule that flags bad NPIs in the
  billing / rendering / referring / service-facility loops.
* `cyclone.api` — new admin endpoint.
* `cyclone.cli` — `validate-npi` + `validate-tax-id` subcommands.
* Tests: `test_npi.py` (12), `test_api_validate_provider.py` (4),
  `test_cli_validate.py` (4) — 20 new tests.

## 4. Threat model

NPIs and tax IDs are sensitive (PHI under HIPAA). The validators run
locally; nothing leaves the process. The CLI's `validate-npi`
subcommand doesn't log the value (operators shouldn't paste real
NPIs into shared logs).
