diff --git a/README.md b/README.md
index 9b1cb49..a1d6acd 100644
--- a/README.md
+++ b/README.md
@@ -370,6 +370,65 @@ fingerprints and the post-rotation table count. The old key is
 retained in the `cyclone.db.key.previous` Keychain account for a
 grace period so a botched rotation can be rolled back by hand.
 
+## NPI checksum + Tax ID format validation (SP20)
+
+Two pure local validators — no NPPES round-trip, no IRS e-file
+lookup. Catches the 99% case (a typo at the end of an NPI, a letter in
+an EIN, an extra digit, the reserved `00`/`07`/`8X` EIN prefix).
+
+| Check | Algorithm | Surface |
+|-------|-----------|---------|
+| NPI  | 10 digits where the last is a Luhn checksum over `80840 + body`. CMS-published example: body `123456789` → check `3` → valid NPI `1234567893`. | `cyclone.npi.is_valid_npi`, CLI `cyclone validate-npi <npi>`, API `GET /api/admin/validate-provider?npi=...`, validator rule `R021_npi_checksum` (warning) |
+| Tax ID (EIN) | 9 digits, optional `XX-XXXXXXX` formatting. Rejects reserved prefixes `00`, `07`, `80`–`89` (IRS Pension Plan Branch). | `cyclone.npi.is_valid_tax_id`, CLI `cyclone validate-tax-id <ein>`, API `GET /api/admin/validate-provider?tax_id=...` |
+
+### CLI
+
+```bash
+$ cyclone validate-npi 1234567893
+OK: 10-digit NPI passes Luhn checksum
+$ cyclone validate-npi 1234567890
+INVALID: '1234567890' fails NPI Luhn checksum   # exit 1
+
+$ cyclone validate-tax-id 72-1587149
+OK: 9-digit EIN (normalized=721587149)
+$ cyclone validate-tax-id 00-1234567
+INVALID: 9-digit EIN has reserved prefix (00); EIN is not assignable by IRS   # exit 1
+```
+
+### API
+
+```bash
+curl 'http://localhost:8000/api/admin/validate-provider?npi=1234567893&tax_id=72-1587149'
+# {
+#   "npi":     {"valid": true,  "skipped": false},
+#   "tax_id":  {"valid": true,  "skipped": false, "normalized": "721587149"}
+# }
+```
+
+Both query params are optional; omitted fields return
+`{"valid": null, "skipped": true}` so the caller can render "no check
+performed" rather than treating absent input as a hard fail.
+
+### Parser integration
+
+The `R021_npi_checksum` rule runs alongside the existing `R020_npi_format`
+in `cyclone.parsers.validator`. A billing-provider NPI that passes
+R020 (right shape) but fails R021 (bad Luhn) is yielded as a
+**warning**, not an error — operators sometimes ingest test fixtures
+with placeholder NPIs (e.g. all-same-digit) and we don't want to block
+that path. In strict mode (`--strict` / `?strict=true`) warnings are
+promoted to errors.
+
+### Files
+
+* `cyclone/npi.py` — new module (~155 LOC).
+* `cyclone.parsers.validator` — new `R021_npi_checksum` rule.
+* `cyclone.api_routers.admin` — new `validate-provider` endpoint.
+* `cyclone.cli` — `validate-npi` + `validate-tax-id` subcommands.
+* Tests: `test_npi.py` (27), `test_api_validate_provider.py` (4),
+  `test_cli_validate.py` (8), `test_validator.py::test_r021_*` (4) —
+  **43 new tests**.
+
 ## Security hardening (SP19)
 
 Three pure-ASGI middlewares sit in front of every FastAPI request.
@@ -674,6 +733,17 @@ Shipped sub-projects (most recent first):
   MFT scheduler state, backup scheduler state, live pubsub
   subscriber counts, last batch id + timestamp. See
   [Security hardening (SP19)](#security-hardening-sp19) below.
+- **Sub-project 20 (shipped) — NPI checksum + Tax ID format validation.**
+  Pure local validators (`cyclone.npi`) — no NPPES round-trip, no IRS
+  e-file lookup. Catches the 99% typo case at parse time. NPI uses
+  CMS-published Luhn over `80840 + body` (example: `1234567893` is
+  valid). EIN rejects reserved prefixes (`00`, `07`, `80`–`89`).
+  Surface: `cyclone validate-npi` / `validate-tax-id` CLI subcommands,
+  `GET /api/admin/validate-provider`, new `R021_npi_checksum`
+  validator rule (warning, not error — placeholder NPIs in test
+  fixtures shouldn't block ingest). See
+  [NPI checksum + Tax ID format validation (SP20)](#npi-checksum--tax-id-format-validation-sp20)
+  below.
 - **Sub-project 18 (shipped) — Structured JSON logging.** All logs
   emitted by the API, CLI, scheduler tick loop, and backup service
   flow through a `JsonFormatter` (newline-delimited JSON, ISO-8601 ms
diff --git a/backend/src/cyclone/api.py b/backend/src/cyclone/api.py
index bc871ae..9e1a3a5 100644
--- a/backend/src/cyclone/api.py
+++ b/backend/src/cyclone/api.py
@@ -248,11 +248,12 @@ app.add_middleware(SecurityHeadersMiddleware)
 # and are wired in here. (Kept as a top-level package rather than nested
 # under `cyclone.api` so the existing ``cyclone.api`` module path keeps
 # working — Python prefers packages over same-named modules.)
-from cyclone.api_routers import acks, health, ta1_acks  # noqa: E402
+from cyclone.api_routers import acks, admin, health, ta1_acks  # noqa: E402
 
 app.include_router(health.router)
 app.include_router(acks.router)
 app.include_router(ta1_acks.router)
+app.include_router(admin.router)
 
 
 def _resolve_payer(name: str) -> PayerConfig:
diff --git a/backend/src/cyclone/api_routers/admin.py b/backend/src/cyclone/api_routers/admin.py
new file mode 100644
index 0000000..de0164a
--- /dev/null
+++ b/backend/src/cyclone/api_routers/admin.py
@@ -0,0 +1,60 @@
+"""``/api/admin/validate-provider`` — NPI + Tax ID liveness probe (SP20).
+
+Pure read-only endpoint that runs the local NPI Luhn + EIN format checks
+without touching the DB. Useful for:
+- operators vetting a new provider before adding them to the registry,
+- the dashboard's "validate" button on a Provider row,
+- smoke-testing the SP20 checks after a deploy.
+
+Both query params are optional; omitting one just skips that check.
+Returns the per-check result dict so the caller can distinguish "bad
+format" from "bad checksum".
+"""
+from __future__ import annotations
+
+from fastapi import APIRouter, Query
+
+from cyclone.npi import is_valid_npi, is_valid_tax_id, normalize_tax_id
+
+
+router = APIRouter()
+
+
+@router.get("/api/admin/validate-provider")
+def validate_provider(
+    npi: str | None = Query(None, description="10-digit NPI to validate (Luhn checksum)"),
+    tax_id: str | None = Query(None, description="9-digit EIN to validate (format + reserved-prefix check)"),
+) -> dict:
+    """Return per-field validation results for ``npi`` and ``tax_id``.
+
+    Each field's payload is the same shape:
+
+    * ``valid`` — bool, the operator's "yes/no" answer
+    * ``normalized`` — for ``tax_id``: the 9-digit plain form, or null
+      if the input is unparseable
+
+    An empty/unset query param returns ``{"valid": None, "skipped": true}``
+    so the caller can render "no check performed" rather than treating
+    ``None`` as a hard fail.
+    """
+    result: dict = {}
+
+    if npi is None or npi == "":
+        result["npi"] = {"valid": None, "skipped": True}
+    else:
+        result["npi"] = {
+            "valid": is_valid_npi(npi),
+            "skipped": False,
+        }
+
+    if tax_id is None or tax_id == "":
+        result["tax_id"] = {"valid": None, "skipped": True, "normalized": None}
+    else:
+        normalized = normalize_tax_id(tax_id)
+        result["tax_id"] = {
+            "valid": is_valid_tax_id(tax_id),
+            "skipped": False,
+            "normalized": normalized,
+        }
+
+    return result
\ No newline at end of file
diff --git a/backend/src/cyclone/cli.py b/backend/src/cyclone/cli.py
index 8ffe13a..ab6385d 100644
--- a/backend/src/cyclone/cli.py
+++ b/backend/src/cyclone/cli.py
@@ -228,6 +228,63 @@ def _count_issues(report) -> dict[str, int]:
     return counts
 
 
+# ---------------------------------------------------------------------------
+# SP20: `cyclone validate-npi` + `cyclone validate-tax-id`
+#
+# Pure local validators. No DB, no Keychain, no network — operators can
+# run them on a developer laptop without standing up the full Cyclone
+# stack. Exit code is 0 (valid) or 1 (invalid) so they compose with
+# shell scripting / CI gates.
+# ---------------------------------------------------------------------------
+
+
+@main.command("validate-npi")
+@click.argument("npi")
+@click.option("--log-level", default="WARNING", show_default=True, type=click.Choice(["DEBUG", "INFO", "WARNING", "ERROR"]))
+def validate_npi_cmd(npi: str, log_level: str) -> None:
+    """Validate a 10-digit NPI's Luhn checksum locally (SP20).
+
+    Exit 0 if valid, 1 if not. No logging of the value itself — NPIs
+    are PHI under HIPAA, so the operator's CLI history is the only
+    audit trail.
+    """
+    # SP18: re-run so --log-level overrides the group default.
+    setup_logging(level=log_level)
+    from cyclone.npi import is_valid_npi
+    if is_valid_npi(npi):
+        click.echo(f"OK: {len(npi)}-digit NPI passes Luhn checksum")
+        return
+    click.echo(f"INVALID: {npi!r} fails NPI Luhn checksum", err=True)
+    sys.exit(1)
+
+
+@main.command("validate-tax-id")
+@click.argument("tax_id")
+@click.option("--log-level", default="WARNING", show_default=True, type=click.Choice(["DEBUG", "INFO", "WARNING", "ERROR"]))
+def validate_tax_id_cmd(tax_id: str, log_level: str) -> None:
+    """Validate a 9-digit EIN's format + prefix locally (SP20).
+
+    Accepts both ``XX-XXXXXXX`` and ``XXXXXXXXX``. Exit 0 if valid,
+    1 if not. EIN is sensitive (PII), so we don't echo the value back
+    on failure — only the validation verdict.
+    """
+    # SP18: re-run so --log-level overrides the group default.
+    setup_logging(level=log_level)
+    from cyclone.npi import is_valid_tax_id, normalize_tax_id
+    plain = normalize_tax_id(tax_id)
+    if plain is None:
+        click.echo("INVALID: input is not a 9-digit EIN (XX-XXXXXXX or XXXXXXXXX)", err=True)
+        sys.exit(1)
+    if is_valid_tax_id(tax_id):
+        click.echo(f"OK: 9-digit EIN (normalized={plain})")
+        return
+    click.echo(
+        f"INVALID: 9-digit EIN has reserved prefix ({plain[:2]}); EIN is not assignable by IRS",
+        err=True,
+    )
+    sys.exit(1)
+
+
 if __name__ == "__main__":
     main()
 
diff --git a/backend/src/cyclone/npi.py b/backend/src/cyclone/npi.py
new file mode 100644
index 0000000..f50fe08
--- /dev/null
+++ b/backend/src/cyclone/npi.py
@@ -0,0 +1,159 @@
+"""SP20 — NPI checksum + Tax ID format validation.
+
+The National Provider Identifier (NPI) is a 10-digit number where
+the last digit is a **Luhn checksum** over the 9 preceding digits
+prefixed with the constant ``80840`` (the NPPES "healthcare
+provider identifier" prefix). See CMS / HHS NPI Standard:
+
+    https://www.cms.gov/medicare/health-care-provider-identifier
+
+The Tax ID (EIN) is a 9-digit number, optionally formatted with a
+hyphen after the second digit (``XX-XXXXXXX``). We don't validate
+against the IRS (that needs their e-file schema), but we *do* catch
+the 99% typo case at parse time.
+
+Everything in this module is local — no NPPES, no network. Operators
+who want real NPPES verification can wire it in later; this module
+catches typos (an off-by-one in a 10-digit NPI, a letter in an EIN,
+an extra digit, the all-zeros EIN prefix ``00`` / ``07``).
+"""
+from __future__ import annotations
+
+import re
+
+# NPPES prefix per the NPI Luhn algorithm. Prepended to the 9-digit
+# NPI body before running the Luhn check.
+_NPPES_PREFIX = "80840"
+
+
+def npi_checksum(npi_body: str) -> int:
+    """Compute the Luhn check digit for a 9-digit NPI body.
+
+    ``npi_body`` must be exactly 9 digits; the caller is responsible
+    for length + character validation. Returns the check digit (0–9).
+    """
+    if not npi_body.isdigit() or len(npi_body) != 9:
+        raise ValueError(f"npi_body must be 9 digits, got {npi_body!r}")
+    digits = _NPPES_PREFIX + npi_body
+    return _luhn_check_digit(digits)
+
+
+def is_valid_npi(npi: str | None) -> bool:
+    """True if ``npi`` is a well-formed 10-digit NPI with valid checksum.
+
+    Returns False for ``None`` / empty string / non-strings / wrong
+    length / non-digit characters / wrong Luhn check digit. Doesn't
+    call NPPES — see module docstring for why.
+
+    >>> is_valid_npi("1234567893")  # CMS-published example NPI
+    True
+    >>> is_valid_npi("1234567894")  # last digit off by one
+    False
+    >>> is_valid_npi("1234567890")  # passes digit but fails Luhn
+    False
+    >>> is_valid_npi("")
+    False
+    """
+    if not isinstance(npi, str):
+        return False
+    if len(npi) != 10 or not npi.isdigit():
+        return False
+    return npi[-1] == str(npi_checksum(npi[:-1]))
+
+
+# ---------------------------------------------------------------------------
+# Tax ID (EIN)
+# ---------------------------------------------------------------------------
+
+
+# EIN prefix table (subset). The IRS publishes a full table; the
+# common "this is obviously a typo" prefixes we reject are:
+#   00 — reserved / never assigned
+#   07 — campus prefixes reserved for future use
+#   8X — formerly used by the IRS Pension Plan Branch
+# Other 00-prefixed EINs (e.g., 000000000) are technically not
+# assigned but we don't reject them here — the operator might have
+# a deliberate placeholder.
+_EIN_FORBIDDEN_PREFIXES = {"00", "07"}
+_EIN_RESERVED_PREFIX_8X = re.compile(r"^8\d$")
+
+# 9 digits, optionally formatted as XX-XXXXXXX.
+_EIN_FORMATTED = re.compile(r"^\d{2}-\d{7}$")
+_EIN_PLAIN = re.compile(r"^\d{9}$")
+
+
+def normalize_tax_id(tax_id: str | None) -> str | None:
+    """Return ``tax_id`` in 9-digit plain form, or None if it's malformed.
+
+    >>> normalize_tax_id("72-1587149")
+    '721587149'
+    >>> normalize_tax_id("721587149")
+    '721587149'
+    >>> normalize_tax_id("not-an-ein")
+    None
+    """
+    if not isinstance(tax_id, str):
+        return None
+    s = tax_id.strip()
+    if _EIN_FORMATTED.match(s):
+        return s.replace("-", "")
+    if _EIN_PLAIN.match(s):
+        return s
+    return None
+
+
+def is_valid_tax_id(tax_id: str | None) -> bool:
+    """True if ``tax_id`` is a 9-digit EIN (formatted or plain) with
+    a non-reserved prefix.
+
+    >>> is_valid_tax_id("72-1587149")  # Touch of Care
+    True
+    >>> is_valid_tax_id("00-1234567")  # reserved prefix
+    False
+    >>> is_valid_tax_id("07-1234567")  # reserved prefix
+    False
+    >>> is_valid_tax_id("not-an-ein")
+    False
+    """
+    plain = normalize_tax_id(tax_id)
+    if plain is None:
+        return False
+    prefix = plain[:2]
+    if prefix in _EIN_FORBIDDEN_PREFIXES:
+        return False
+    if _EIN_RESERVED_PREFIX_8X.match(prefix):
+        return False
+    return True
+
+
+# ---------------------------------------------------------------------------
+# Luhn internals
+# ---------------------------------------------------------------------------
+
+
+def _luhn_check_digit(digits: str) -> int:
+    """Return the Luhn check digit for ``digits``.
+
+    The Luhn algorithm doubles every second digit starting from the
+    RIGHTMOST position (i.e., the first digit doubled is the rightmost
+    character of ``digits``). If the doubled value exceeds 9, subtract
+    9. Sum all digits; the check digit is ``(10 - sum % 10) % 10``.
+
+    ``digits`` here is the body WITHOUT the check digit — for the NPI
+    case it's the 14-character ``80840`` + 9-digit NPI body. The
+    CMS-published example ``123456789`` (body) yields check digit
+    ``3`` → full NPI ``1234567893`` (verified against
+    https://www.cms.gov/.../NPIcheckdigit.pdf).
+    """
+    total = 0
+    # The rightmost digit of ``digits`` is the FIRST one doubled (i=0
+    # in the reversed iteration). Per CMS, doubling starts at the
+    # rightmost and alternates leftward.
+    for i, ch in enumerate(reversed(digits)):
+        d = int(ch)
+        if i % 2 == 0:  # rightmost, third-from-right, fifth-from-right, ...
+            d *= 2
+            if d > 9:
+                d -= 9
+        total += d
+    return (10 - total % 10) % 10
diff --git a/backend/src/cyclone/parsers/validator.py b/backend/src/cyclone/parsers/validator.py
index bd764bb..6f51b12 100644
--- a/backend/src/cyclone/parsers/validator.py
+++ b/backend/src/cyclone/parsers/validator.py
@@ -35,6 +35,36 @@ def _r020_npi_format(claim: ClaimOutput, _: PayerConfig) -> Iterable[ValidationI
         yield ValidationIssue(rule="R020_npi_format", severity="error", message=f"Billing provider NPI must be 10 digits, got {claim.billing_provider.npi!r}")
 
 
+def _r021_npi_checksum(claim: ClaimOutput, _: PayerConfig) -> Iterable[ValidationIssue]:
+    """SP20: validate the billing-provider NPI's Luhn check digit.
+
+    A 10-digit NPI whose body passes R020's format check can still have
+    a bad Luhn check digit (a typo at the end). Yielded as a WARNING —
+    not an error — because operators sometimes ingest test fixtures with
+    placeholder NPIs (e.g. all-same-digit) and we don't want to block
+    that path. Local-only check, no NPPES round-trip.
+    """
+    npi = claim.billing_provider.npi
+    if not npi:
+        return
+    # Skip silently if R020 already flagged the format — we don't want to
+    # duplicate the operator's screen with a second issue about the same NPI.
+    if not NPI_RE.match(npi):
+        return
+    # Lazy import keeps the validator module importable even if
+    # ``cyclone.npi`` is unavailable (e.g. in some legacy test setups).
+    try:
+        from cyclone.npi import is_valid_npi
+    except ImportError:  # pragma: no cover — defensive
+        return
+    if not is_valid_npi(npi):
+        yield ValidationIssue(
+            rule="R021_npi_checksum",
+            severity="warning",
+            message=f"Billing provider NPI {npi!r} fails Luhn checksum (likely typo)",
+        )
+
+
 def _r030_frequency_allowed(claim: ClaimOutput, cfg: PayerConfig) -> Iterable[ValidationIssue]:
     if not claim.claim.frequency_code:
         return
@@ -392,6 +422,7 @@ _RULES: list[Rule] = [
     _r010_clm01_present,
     _r011_total_charge_positive,
     _r020_npi_format,
+    _r021_npi_checksum,
     _r030_frequency_allowed,
     _r031_ref_g1_optional,
     _r034_ref_g1_required,
diff --git a/backend/tests/fixtures/minimal_837p.txt b/backend/tests/fixtures/minimal_837p.txt
index 6e19e83..47c8040 100644
--- a/backend/tests/fixtures/minimal_837p.txt
+++ b/backend/tests/fixtures/minimal_837p.txt
@@ -7,7 +7,7 @@ PER*IC*Test Contact*EM*test@example.com~
 NM1*40*2*COLORADO MEDICAL ASSISTANCE PROGRAM*****46*COMEDASSISTPROG~
 HL*1**20*1~
 PRV*BI*PXC*251E00000X~
-NM1*85*2*Test Provider Inc*****XX*1234567890~
+NM1*85*2*Test Provider Inc*****XX*1993999998~
 N3*123 Test St~
 N4*Denver*CO*80202~
 REF*EI*123456789~
diff --git a/backend/tests/test_api_validate_provider.py b/backend/tests/test_api_validate_provider.py
new file mode 100644
index 0000000..8f72f43
--- /dev/null
+++ b/backend/tests/test_api_validate_provider.py
@@ -0,0 +1,69 @@
+"""Tests for ``GET /api/admin/validate-provider`` (SP20).
+
+Pure read-only endpoint — runs the local NPI Luhn + EIN format checks.
+"""
+from __future__ import annotations
+
+import pytest
+from fastapi.testclient import TestClient
+
+from cyclone.api import app
+
+
+@pytest.fixture
+def client() -> TestClient:
+    return TestClient(app)
+
+
+# ---------------------------------------------------------------------------
+# Both fields populated
+# ---------------------------------------------------------------------------
+
+
+def test_validate_provider_both_valid(client: TestClient):
+    resp = client.get("/api/admin/validate-provider", params={
+        "npi": "1234567893",  # CMS-published valid NPI
+        "tax_id": "72-1587149",  # Touch of Care EIN
+    })
+    assert resp.status_code == 200
+    body = resp.json()
+    assert body["npi"]["valid"] is True
+    assert body["npi"]["skipped"] is False
+    assert body["tax_id"]["valid"] is True
+    assert body["tax_id"]["normalized"] == "721587149"
+
+
+def test_validate_provider_both_invalid(client: TestClient):
+    resp = client.get("/api/admin/validate-provider", params={
+        "npi": "1234567890",  # format OK but Luhn fails
+        "tax_id": "00-1234567",  # reserved prefix
+    })
+    assert resp.status_code == 200
+    body = resp.json()
+    assert body["npi"]["valid"] is False
+    assert body["tax_id"]["valid"] is False
+    assert body["tax_id"]["normalized"] == "001234567"
+
+
+# ---------------------------------------------------------------------------
+# Param omission → skipped
+# ---------------------------------------------------------------------------
+
+
+def test_validate_provider_skips_missing_npi(client: TestClient):
+    resp = client.get("/api/admin/validate-provider", params={"tax_id": "721587149"})
+    assert resp.status_code == 200
+    body = resp.json()
+    assert body["npi"]["skipped"] is True
+    assert body["npi"]["valid"] is None
+    assert body["tax_id"]["valid"] is True
+
+
+def test_validate_provider_skips_missing_tax_id(client: TestClient):
+    resp = client.get("/api/admin/validate-provider", params={"npi": "1234567893"})
+    assert resp.status_code == 200
+    body = resp.json()
+    assert body["tax_id"]["skipped"] is True
+    assert body["tax_id"]["valid"] is None
+    assert body["tax_id"]["normalized"] is None
+    assert body["npi"]["valid"] is True
\ No newline at end of file
diff --git a/backend/tests/test_cli_validate.py b/backend/tests/test_cli_validate.py
new file mode 100644
index 0000000..c56ef62
--- /dev/null
+++ b/backend/tests/test_cli_validate.py
@@ -0,0 +1,80 @@
+"""Tests for ``cyclone validate-npi`` + ``cyclone validate-tax-id`` (SP20).
+
+CLI smoke tests — verify exit codes (0 = valid, 1 = invalid) and that
+the help text references the new subcommands. We don't pipe the value
+into shared logs (NPI / EIN are PHI / PII); the CliRunner captures it.
+"""
+from __future__ import annotations
+
+import pytest
+from click.testing import CliRunner
+
+from cyclone.cli import main
+
+
+# ---------------------------------------------------------------------------
+# validate-npi
+# ---------------------------------------------------------------------------
+
+
+def test_cli_validate_npi_valid_exits_zero():
+    runner = CliRunner()
+    result = runner.invoke(main, ["validate-npi", "1234567893"])
+    assert result.exit_code == 0, result.output
+    assert "OK" in result.output
+
+
+def test_cli_validate_npi_bad_checksum_exits_one():
+    runner = CliRunner()
+    result = runner.invoke(main, ["validate-npi", "1234567890"])
+    assert result.exit_code == 1
+    assert "INVALID" in result.output
+
+
+def test_cli_validate_npi_wrong_length_exits_one():
+    runner = CliRunner()
+    result = runner.invoke(main, ["validate-npi", "12345"])
+    assert result.exit_code == 1
+    assert "INVALID" in result.output
+
+
+# ---------------------------------------------------------------------------
+# validate-tax-id
+# ---------------------------------------------------------------------------
+
+
+def test_cli_validate_tax_id_formatted_exits_zero():
+    runner = CliRunner()
+    result = runner.invoke(main, ["validate-tax-id", "72-1587149"])
+    assert result.exit_code == 0, result.output
+    assert "721587149" in result.output  # normalized form echoed
+
+
+def test_cli_validate_tax_id_unformatted_exits_zero():
+    runner = CliRunner()
+    result = runner.invoke(main, ["validate-tax-id", "721587149"])
+    assert result.exit_code == 0
+
+
+def test_cli_validate_tax_id_reserved_prefix_exits_one():
+    runner = CliRunner()
+    result = runner.invoke(main, ["validate-tax-id", "00-1234567"])
+    assert result.exit_code == 1
+    assert "reserved" in result.output.lower()
+
+
+def test_cli_validate_tax_id_malformed_exits_one():
+    runner = CliRunner()
+    result = runner.invoke(main, ["validate-tax-id", "not-an-ein"])
+    assert result.exit_code == 1
+    assert "9-digit" in result.output
+
+
+def test_cli_validate_subcommands_appear_in_help():
+    """The two new subcommands are wired into ``main`` (regression guard
+    against future refactors that drop the imports)."""
+    runner = CliRunner()
+    result = runner.invoke(main, ["--help"])
+    assert result.exit_code == 0
+    assert "validate-npi" in result.output
+    assert "validate-tax-id" in result.output
\ No newline at end of file
diff --git a/backend/tests/test_npi.py b/backend/tests/test_npi.py
new file mode 100644
index 0000000..2e9c7e8
--- /dev/null
+++ b/backend/tests/test_npi.py
@@ -0,0 +1,199 @@
+"""Tests for ``cyclone.npi`` — NPI checksum + Tax ID (EIN) format validation.
+
+Pure local validation, no NPPES calls. Covers the NPPES-published Luhn
+checksum (with prefix ``80840``) and a small set of obvious EIN typo cases.
+"""
+from __future__ import annotations
+
+import pytest
+
+from cyclone.npi import (
+    _luhn_check_digit,
+    is_valid_npi,
+    is_valid_tax_id,
+    npi_checksum,
+    normalize_tax_id,
+)
+
+
+# ---------------------------------------------------------------------------
+# Luhn internals
+# ---------------------------------------------------------------------------
+
+
+def test_luhn_check_digit_known_sequence():
+    """Standard Luhn for the empty body returns 0.
+
+    With no digits the sum is 0, so (10 - 0 % 10) % 10 = 0.
+    """
+    assert _luhn_check_digit("") == 0
+
+
+def test_luhn_check_digit_single_digit():
+    """For a single body digit the check digit is the standard Luhn value.
+
+    With the corrected "double at rightmost" pattern: "0" doubles to 0
+    (total=0, check=0); "1" doubles to 2 (total=2, check=(10-2)%10=8).
+    """
+    assert _luhn_check_digit("0") == 0
+    assert _luhn_check_digit("1") == 8
+
+
+def test_luhn_check_digit_nppes_published():
+    """CMS-published example: body 123456789 → check digit 3.
+
+    Per https://www.cms.gov/.../NPIcheckdigit.pdf the Luhn sum of the
+    prefixed body ``80840123456789`` is 67, so check digit = (10-7)%10 = 3,
+    giving the full NPI ``1234567893``.
+    """
+    assert _luhn_check_digit("80840123456789") == 3
+
+
+# ---------------------------------------------------------------------------
+# npi_checksum
+# ---------------------------------------------------------------------------
+
+
+def test_npi_checksum_cms_published_body():
+    """For NPI body 123456789 the check digit is 3 → NPI 1234567893.
+
+    Confirmed against the CMS-published NPI Luhn example.
+    """
+    assert npi_checksum("123456789") == 3
+
+
+def test_npi_checksum_rejects_non_digits():
+    with pytest.raises(ValueError):
+        npi_checksum("12345abc6")  # letter in body
+
+
+def test_npi_checksum_rejects_wrong_length():
+    with pytest.raises(ValueError):
+        npi_checksum("12345")  # only 5 digits
+    with pytest.raises(ValueError):
+        npi_checksum("1234567890")  # 10 digits — includes the check digit
+
+
+# ---------------------------------------------------------------------------
+# is_valid_npi
+# ---------------------------------------------------------------------------
+
+
+def test_is_valid_npi_nppes_sample_is_true():
+    """The CMS-published example NPI 1234567893 must validate."""
+    assert is_valid_npi("1234567893") is True
+
+
+def test_is_valid_npi_off_by_one_is_false():
+    assert is_valid_npi("1234567894") is False
+
+
+def test_is_valid_npi_all_zeros_is_false():
+    """All zeros fails the Luhn check."""
+    assert is_valid_npi("0000000000") is False
+
+
+def test_is_valid_npi_rejects_empty():
+    assert is_valid_npi("") is False
+
+
+def test_is_valid_npi_rejects_none():
+    assert is_valid_npi(None) is False  # type: ignore[arg-type]
+
+
+def test_is_valid_npi_rejects_non_string():
+    assert is_valid_npi(1234567890) is False  # type: ignore[arg-type]
+    assert is_valid_npi(["1881068062"]) is False  # type: ignore[arg-type]
+
+
+def test_is_valid_npi_rejects_short():
+    assert is_valid_npi("123456789") is False  # 9 digits
+
+
+def test_is_valid_npi_rejects_long():
+    assert is_valid_npi("12345678901") is False  # 11 digits
+
+
+def test_is_valid_npi_rejects_non_digits():
+    assert is_valid_npi("188106806X") is False
+    assert is_valid_npi("18810 68062") is False  # space
+
+
+def test_is_valid_npi_all_ones_fails_luhn():
+    """1111111111 has all-1 sum: alternating double = 1,2,1,2,..., 1+2=3 then
+    collapse. Total for 10 digits body (body=9 of all 1's, sum doubled):
+    Position from right (i=0..8): 1, 1, 1, 1, 1, 1, 1, 1, 1
+    i even (not doubled): 1, 1, 1, 1, 1 → 5
+    i odd (doubled): 1*2=2, 1*2=2, 1*2=2, 1*2=2 → 8
+    Total body = 13. Body alone has check digit (10 - 13 % 10) % 10 = 7.
+    So full NPI 1111111111's body (9 ones) check = 7, and 1 != 7 → invalid.
+    """
+    assert is_valid_npi("1111111111") is False
+
+
+# ---------------------------------------------------------------------------
+# is_valid_tax_id / normalize_tax_id
+# ---------------------------------------------------------------------------
+
+
+def test_is_valid_tax_id_touch_of_care_true():
+    """The operator's reference EIN — Touch of Care Family Practice."""
+    assert is_valid_tax_id("72-1587149") is True
+
+
+def test_is_valid_tax_id_unformatted_true():
+    assert is_valid_tax_id("721587149") is True
+
+
+def test_is_valid_tax_id_00_prefix_rejected():
+    """``00`` is reserved / never assigned by the IRS."""
+    assert is_valid_tax_id("00-1234567") is False
+    assert is_valid_tax_id("001234567") is False
+
+
+def test_is_valid_tax_id_07_prefix_rejected():
+    """``07`` is a campus prefix reserved for future use."""
+    assert is_valid_tax_id("07-1234567") is False
+    assert is_valid_tax_id("071234567") is False
+
+
+def test_is_valid_tax_id_8x_prefix_rejected():
+    """``80``–``89`` is the IRS Pension Plan Branch — never assigned otherwise."""
+    assert is_valid_tax_id("80-1234567") is False
+    assert is_valid_tax_id("89-1234567") is False
+
+
+def test_is_valid_tax_id_rejects_malformed():
+    assert is_valid_tax_id("not-an-ein") is False
+    assert is_valid_tax_id("12345") is False
+    assert is_valid_tax_id("1234567890") is False  # 10 digits
+    assert is_valid_tax_id("12-345678") is False  # 8 digits after hyphen
+
+
+def test_is_valid_tax_id_rejects_none_and_non_string():
+    assert is_valid_tax_id(None) is False
+    assert is_valid_tax_id(721587149) is False  # type: ignore[arg-type]
+
+
+def test_normalize_tax_id_returns_plain_form():
+    assert normalize_tax_id("72-1587149") == "721587149"
+    assert normalize_tax_id("721587149") == "721587149"
+
+
+def test_normalize_tax_id_strips_whitespace():
+    assert normalize_tax_id("  72-1587149  ") == "721587149"
+
+
+def test_normalize_tax_id_returns_none_for_invalid():
+    assert normalize_tax_id("not-an-ein") is None
+    assert normalize_tax_id(None) is None
+    assert normalize_tax_id("") is None
+    assert normalize_tax_id("12-345678") is None  # wrong digit count
+
+
+def test_normalize_tax_id_keeps_reserved_prefix():
+    """normalize_tax_id is a *structural* normalizer — it doesn't reject
+    reserved prefixes. That's ``is_valid_tax_id``'s job. Operators who
+    want to store 00-prefixed EINs as placeholders still get a clean
+    9-digit string."""
+    assert normalize_tax_id("00-1234567") == "001234567"
\ No newline at end of file
diff --git a/backend/tests/test_parse_837.py b/backend/tests/test_parse_837.py
index b5510ec..2e76067 100644
--- a/backend/tests/test_parse_837.py
+++ b/backend/tests/test_parse_837.py
@@ -14,7 +14,7 @@ def test_parse_minimal_fixture_returns_one_claim():
     assert len(result.claims) == 1
     claim = result.claims[0]
     assert claim.claim_id == "CLM001"
-    assert claim.billing_provider.npi == "1234567890"
+    assert claim.billing_provider.npi == "1993999998"
     assert claim.subscriber.last_name == "Doe"
     assert claim.subscriber.first_name == "John"
     assert claim.subscriber.member_id == "ABC123"
diff --git a/backend/tests/test_validator.py b/backend/tests/test_validator.py
index 24d52ca..ec550ca 100644
--- a/backend/tests/test_validator.py
+++ b/backend/tests/test_validator.py
@@ -100,6 +100,53 @@ def test_r020_npi_must_be_ten_digits():
     assert any(i.rule == "R020_npi_format" for i in report.errors)
 
 
+# --------------------------------------------------------------------------- #
+# R021 — NPI Luhn checksum (SP20)
+# --------------------------------------------------------------------------- #
+
+
+def test_r021_npi_checksum_valid_passes_silently():
+    """A valid Luhn NPI (CMS-published 1234567893) yields no R021 issue."""
+    cfg = PayerConfig.co_medicaid()
+    claim = _build_claim()
+    claim.billing_provider.npi = "1234567893"
+    report = validate(claim, cfg)
+    assert not any(i.rule == "R021_npi_checksum" for i in report.errors + report.warnings)
+
+
+def test_r021_npi_checksum_bad_luhn_is_warning():
+    """An NPI that passes format but fails Luhn is a WARNING, not an error."""
+    cfg = PayerConfig.co_medicaid()
+    claim = _build_claim()
+    claim.billing_provider.npi = "1234567890"  # right length, wrong check digit
+    report = validate(claim, cfg)
+    assert report.passed is True  # WARNINGs don't fail the report
+    assert any(i.rule == "R021_npi_checksum" and i.severity == "warning" for i in report.warnings)
+
+
+def test_r021_npi_checksum_skipped_when_format_bad():
+    """When R020 already flagged the format, R021 stays silent
+    (avoids a duplicate 'this NPI is wrong' message)."""
+    cfg = PayerConfig.co_medicaid()
+    claim = _build_claim()
+    claim.billing_provider.npi = "12345"  # wrong length
+    report = validate(claim, cfg)
+    # R020 errors as expected; R021 stays quiet.
+    assert any(i.rule == "R020_npi_format" for i in report.errors)
+    assert not any(i.rule == "R021_npi_checksum" for i in report.errors + report.warnings)
+
+
+def test_r021_npi_checksum_skipped_when_npi_missing():
+    """An empty NPI doesn't trigger R021 (only R020 would, but R020
+    only fires when NPI is *present* and wrong). R021 must stay silent
+    when NPI is empty so we don't double-fire."""
+    cfg = PayerConfig.co_medicaid()
+    claim = _build_claim()
+    claim.billing_provider.npi = ""
+    report = validate(claim, cfg)
+    assert not any(i.rule == "R021_npi_checksum" for i in report.errors + report.warnings)
+
+
 def test_r030_frequency_allowed():
     cfg = PayerConfig.co_medicaid()  # only 1, 7, 8
     claim = _build_claim()
diff --git a/docs/superpowers/specs/2026-06-21-cyclone-npi-validation-design.md b/docs/superpowers/specs/2026-06-21-cyclone-npi-validation-design.md
new file mode 100644
index 0000000..4feb68d
--- /dev/null
+++ b/docs/superpowers/specs/2026-06-21-cyclone-npi-validation-design.md
@@ -0,0 +1,62 @@
+# 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).