Initial extraction from monorepo

hmrc_sync/__main__.py

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+import logging
+import os
+import subprocess
+import sys
+
+import click
+import uvicorn
+
+
+@click.group()
+def cli() -> None:
+    logging.basicConfig(level=os.environ.get("LOG_LEVEL", "INFO"))
+
+
+@cli.command()
+def serve() -> None:
+    """Run the FastAPI server (K8s entrypoint)."""
+    uvicorn.run("hmrc_sync.app:app", host="0.0.0.0", port=8080)
+
+
+@cli.command()
+@click.option("--tax-year", default="current", help="Tax year to fetch, e.g. 2024-25 or 'current'.")
+def sync(tax_year: str) -> None:
+    """One-shot sync of HMRC figures — used by the CronJob."""
+    raise click.ClickException("Sync stub — implement after HMRC prod approval lands")
+
+
+@cli.command()
+def migrate() -> None:
+    """Run `alembic upgrade head`."""
+    result = subprocess.run(["alembic", "upgrade", "head"], check=False)
+    sys.exit(result.returncode)
+
+
+if __name__ == "__main__":
+    cli()

hmrc_sync/app.py

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+"""FastAPI entrypoint for hmrc-sync.
+
+Endpoints:
+- GET  /authorize           — redirect to HMRC OAuth, primes refresh_token
+- GET  /callback            — OAuth callback; exchange code, persist token
+- POST /callback-metadata   — browser-side session attributes (fraud headers)
+- POST /sync                — pull latest HMRC figures for a given tax year
+- GET  /healthz             — readiness + queue depth
+"""
+from __future__ import annotations
+
+import logging
+import os
+import secrets
+import urllib.parse
+from contextlib import asynccontextmanager
+from typing import Any
+
+from fastapi import FastAPI, HTTPException, Request
+from fastapi.responses import HTMLResponse, RedirectResponse
+from prometheus_fastapi_instrumentator import Instrumentator
+
+from hmrc_sync import oauth
+from hmrc_sync.fraud_headers import SessionContext
+
+log = logging.getLogger(__name__)
+
+REQUIRED_ENV = [
+    "HMRC_PROD_CLIENT_ID",
+    "HMRC_PROD_CLIENT_SECRET",
+    "HMRC_PROD_REDIRECT_URI",
+    "DB_CONNECTION_STRING",
+]
+
+
+def _verify_env() -> None:
+    missing = [k for k in REQUIRED_ENV if not os.environ.get(k)]
+    if missing:
+        raise RuntimeError(f"Missing required env vars: {', '.join(missing)}")
+
+
+@asynccontextmanager
+async def lifespan(app: FastAPI):  # type: ignore[no-untyped-def]
+    _verify_env()
+    app.state.session_context = SessionContext(
+        device_id=os.environ.get("HMRC_DEVICE_ID", ""),
+        public_ip=os.environ.get("HMRC_VENDOR_PUBLIC_IP", ""),
+    )
+    app.state.oauth_states = {}  # anti-CSRF state → expires_at
+    yield
+
+
+app = FastAPI(title="HMRC Sync", lifespan=lifespan)
+Instrumentator().instrument(app).expose(app, endpoint="/metrics")
+
+
+@app.get("/healthz")
+async def healthz() -> dict[str, Any]:
+    return {"status": "ok"}
+
+
+@app.get("/authorize")
+async def authorize() -> RedirectResponse:
+    creds = oauth.load_creds_from_env()
+    state = secrets.token_urlsafe(24)
+    app.state.oauth_states[state] = True
+    params = urllib.parse.urlencode({
+        "response_type": "code",
+        "client_id": creds.client_id,
+        "scope": "read:self-assessment",
+        "redirect_uri": creds.redirect_uri,
+        "state": state,
+    })
+    return RedirectResponse(f"{oauth.PROD_BASE}/oauth/authorize?{params}")
+
+
+@app.get("/callback", response_class=HTMLResponse)
+async def callback(code: str, state: str) -> HTMLResponse:
+    if state not in app.state.oauth_states:
+        raise HTTPException(status_code=400, detail="unknown state (CSRF)")
+    del app.state.oauth_states[state]
+    creds = oauth.load_creds_from_env()
+    token = await oauth.exchange_code(creds, code)
+    oauth.persist_to_vault(token)
+    # Serve a 1-page form that POSTs browser attributes to /callback-metadata
+    # so we capture the per-session values HMRC wants in fraud headers.
+    return HTMLResponse(_metadata_capture_html())
+
+
+@app.post("/callback-metadata")
+async def callback_metadata(request: Request) -> dict[str, str]:
+    body = await request.json()
+    session: SessionContext = app.state.session_context
+    session.user_agent = str(body.get("user_agent", "") or "")
+    session.screen_width = int(body.get("screen_width", 0) or 0)
+    session.screen_height = int(body.get("screen_height", 0) or 0)
+    session.screen_colour_depth = int(body.get("screen_colour_depth", 0) or 0)
+    session.window_width = int(body.get("window_width", 0) or 0)
+    session.window_height = int(body.get("window_height", 0) or 0)
+    session.timezone_offset = int(body.get("timezone_offset", 0) or 0)
+    return {"status": "captured"}
+
+
+@app.post("/sync")
+async def sync(tax_year: str | None = None) -> dict[str, Any]:
+    """Pull latest HMRC figures for `tax_year` (default: current fiscal year)."""
+    raise HTTPException(status_code=501, detail="Sync not yet implemented — awaiting HMRC prod approval")
+
+
+def _metadata_capture_html() -> str:
+    return """<!doctype html>
+<html><head><title>hmrc-sync — capturing session</title></head><body>
+<h2>Capturing session attributes for HMRC fraud headers...</h2>
+<script>
+fetch('/callback-metadata', {
+  method: 'POST',
+  headers: {'Content-Type': 'application/json'},
+  body: JSON.stringify({
+    user_agent: navigator.userAgent,
+    screen_width: screen.width,
+    screen_height: screen.height,
+    screen_colour_depth: screen.colorDepth,
+    window_width: window.innerWidth,
+    window_height: window.innerHeight,
+    timezone_offset: -new Date().getTimezoneOffset()
+  })
+}).then(() => document.body.innerHTML = '<h2>Done. You can close this tab.</h2>');
+</script>
+</body></html>"""

hmrc_sync/client.py

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+"""HMRC Individual Tax API v1.1 wrapper.
+
+One method per endpoint we consume. Every request attaches the full fraud-
+prevention header set built by `fraud_headers.build_headers()`.
+
+Individual Tax API v1.1 returns tax-paid + income-breakdown figures per
+employment per tax year — exactly the ground-truth data we reconcile
+against the payslip-ingest monthly aggregate.
+"""
+from __future__ import annotations
+
+import logging
+import time
+from dataclasses import dataclass
+from typing import Any
+
+import httpx
+
+from hmrc_sync.fraud_headers import SessionContext, build_headers
+
+log = logging.getLogger(__name__)
+
+PROD_BASE = "https://api.service.hmrc.gov.uk"
+INDIVIDUAL_TAX_VERSION = "application/vnd.hmrc.1.1+json"
+
+
+@dataclass
+class HmrcResponse:
+    status_code: int
+    body: dict[str, Any]
+    duration_ms: int
+    request_id: str | None
+    correlation_id: str | None
+    fraud_headers_sent: dict[str, str]
+
+
+class HmrcClient:
+
+    def __init__(self,
+                 access_token: str,
+                 session: SessionContext,
+                 connection_method: str = "BATCH_PROCESS_DIRECT",
+                 base_url: str = PROD_BASE):
+        self._access_token = access_token
+        self._session = session
+        self._connection_method = connection_method
+        self._base_url = base_url.rstrip("/")
+
+    async def individual_tax_summary(self, utr: str, tax_year: str) -> HmrcResponse:
+        """GET /individuals/tax/sa/{utr}/summary/{taxYear}
+
+        `utr` is the 10-digit Self Assessment reference; tax_year format
+        is `YYYY-YY` (e.g. `2024-25`).
+        """
+        path = f"/individuals/tax/sa/{utr}/summary/{tax_year}"
+        return await self._get(path)
+
+    async def _get(self, path: str) -> HmrcResponse:
+        fraud = build_headers(self._session, self._connection_method)
+        headers = {
+            "Accept": INDIVIDUAL_TAX_VERSION,
+            "Authorization": f"Bearer {self._access_token}",
+        }
+        headers.update(fraud)
+        started = time.perf_counter()
+        async with httpx.AsyncClient(timeout=30.0) as client:
+            resp = await client.get(f"{self._base_url}{path}", headers=headers)
+        duration_ms = int((time.perf_counter() - started) * 1000)
+        body: dict[str, Any]
+        try:
+            body = resp.json() if resp.content else {}
+        except ValueError:
+            body = {"raw": resp.text[:2000]}
+        log.info("hmrc %s status=%s duration=%dms", path, resp.status_code, duration_ms)
+        return HmrcResponse(
+            status_code=resp.status_code,
+            body=body,
+            duration_ms=duration_ms,
+            request_id=resp.headers.get("x-request-id"),
+            correlation_id=resp.headers.get("x-correlation-id"),
+            fraud_headers_sent=fraud,
+        )

hmrc_sync/db.py

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+import os
+from datetime import datetime
+from decimal import Decimal
+from typing import Any
+
+from sqlalchemy import JSON, TIMESTAMP, Integer, Numeric, String, text
+from sqlalchemy.dialects.postgresql import JSONB
+from sqlalchemy.ext.asyncio import AsyncEngine, async_sessionmaker, create_async_engine
+from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column
+
+SCHEMA_NAME = "hmrc_sync"
+
+
+class Base(DeclarativeBase):
+    pass
+
+
+JSON_TYPE = JSONB().with_variant(JSON(), "sqlite")
+
+
+class TaxYearSnapshot(Base):
+    """One row per (tax_year, employer_paye_ref, snapshot_date).
+
+    HMRC returns the `hmrc-held` view of annual PAYE/NI for a given
+    employment. Taking a daily snapshot lets us see HMRC's figures evolve
+    as late RTI filings land, and lets the dashboard always show the
+    latest value by snapshot_date.
+    """
+    __tablename__ = "tax_year_snapshot"
+    __table_args__ = {"schema": SCHEMA_NAME}  # noqa: RUF012
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    tax_year: Mapped[str] = mapped_column(String, nullable=False, index=True)
+    employer_paye_ref: Mapped[str] = mapped_column(String, nullable=False)
+    snapshot_date: Mapped[datetime] = mapped_column(TIMESTAMP(timezone=True), nullable=False)
+    gross_pay: Mapped[Decimal] = mapped_column(Numeric(12, 2), nullable=False)
+    income_tax: Mapped[Decimal] = mapped_column(Numeric(12, 2), nullable=False)
+    ni_contributions: Mapped[Decimal] = mapped_column(Numeric(12, 2), nullable=False)
+    source: Mapped[str] = mapped_column(String, nullable=False, server_default="hmrc-held")
+    raw_response: Mapped[dict[str, Any]] = mapped_column(JSON_TYPE, nullable=False)
+    fetched_at: Mapped[datetime] = mapped_column(TIMESTAMP(timezone=True),
+                                                 nullable=False,
+                                                 server_default=text("now()"))
+
+
+class FetchLog(Base):
+    """Audit trail of every HMRC API call — for fraud-header compliance review."""
+    __tablename__ = "fetch_log"
+    __table_args__ = {"schema": SCHEMA_NAME}  # noqa: RUF012
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    endpoint: Mapped[str] = mapped_column(String, nullable=False)
+    status_code: Mapped[int] = mapped_column(Integer, nullable=False)
+    request_id: Mapped[str | None] = mapped_column(String, nullable=True)
+    correlation_id: Mapped[str | None] = mapped_column(String, nullable=True)
+    fraud_headers_sent: Mapped[dict[str, Any]] = mapped_column(JSON_TYPE, nullable=False)
+    response_snippet: Mapped[str | None] = mapped_column(String, nullable=True)
+    duration_ms: Mapped[int] = mapped_column(Integer, nullable=False)
+    fetched_at: Mapped[datetime] = mapped_column(TIMESTAMP(timezone=True),
+                                                 nullable=False,
+                                                 server_default=text("now()"))
+
+
+def create_engine_from_env() -> AsyncEngine:
+    url = os.environ["DB_CONNECTION_STRING"]
+    return create_async_engine(url, pool_pre_ping=True)
+
+
+def make_session_factory(engine: AsyncEngine) -> async_sessionmaker[Any]:
+    return async_sessionmaker(engine, expire_on_commit=False)

hmrc_sync/fraud_headers.py

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+"""Build HMRC MTD fraud-prevention headers (Gov-Client-* / Gov-Vendor-*).
+
+HMRC's BATCH_PROCESS_DIRECT connection method (what our CronJob uses)
+mandates 11 headers on every MTD API call; WEB_APP_VIA_SERVER adds a
+handful of browser-derived fields. Shipping without these risks fines
+and API-access revocation per the HMRC fraud-prevention guide.
+
+Layout:
+
+- **Static** — vendor-constant across runs (product name/version,
+  hashed license id).
+- **Runtime** — collected at module load from the pod's own network
+  stack + OS: MAC addresses, local IPs, OS family/version, device model.
+- **Per-request** — built at call time (timestamps, request ids).
+- **Per-session** — captured from the browser on `/callback-metadata`
+  (screen dimensions, public IP, MFA timestamp). Only WEB_APP_VIA_SERVER.
+
+The public entry point is `build_headers(session, connection_method)`.
+Run `tests/test_fraud_headers.py::test_headers_pass_hmrc_validator`
+with `HMRC_VALIDATOR=1` to verify against the HMRC sandbox validator.
+
+Spec references:
+  https://developer.service.hmrc.gov.uk/guides/fraud-prevention/
+  https://developer.service.hmrc.gov.uk/guides/fraud-prevention/connection-method/batch-process-direct/
+  https://developer.service.hmrc.gov.uk/api-documentation/docs/api/service/txm-fph-validator-api/1.0
+"""
+from __future__ import annotations
+
+import getpass
+import hashlib
+import logging
+import os
+import platform
+import secrets
+import socket
+import time
+import urllib.parse
+import uuid
+from dataclasses import dataclass, field
+from pathlib import Path
+from typing import Any
+
+log = logging.getLogger(__name__)
+
+VENDOR_PRODUCT_NAME = "hmrc-sync"
+VENDOR_PRODUCT_VERSION = "0.1.0"
+# Self-assigned for a personal single-user tool. HMRC permits arbitrary
+# vendor strings; the header value is then SHA-256-hashed per spec
+# (`Gov-Vendor-License-IDs: <name>=<hashed-value>`).
+VENDOR_LICENSE_ID = os.environ.get("HMRC_VENDOR_LICENSE_ID",
+                                   "hmrc-sync-private-single-user")
+VENDOR_PUBLIC_IP = os.environ.get("HMRC_VENDOR_PUBLIC_IP", "")
+
+# Valid HMRC connection-method enum values.
+CONNECTION_METHOD_BATCH = "BATCH_PROCESS_DIRECT"
+CONNECTION_METHOD_WEB_APP = "WEB_APP_VIA_SERVER"
+CONNECTION_METHOD_MFA = "AUTH_USING_MFA"
+
+_NET_CLASS = Path("/sys/class/net")
+_EMPTY_MAC = "00:00:00:00:00:00"
+
+
+@dataclass
+class SessionContext:
+    """Browser-side attributes captured on the `/callback-metadata` POST.
+
+    Only relevant for WEB_APP_VIA_SERVER flows (browser-initiated OAuth
+    + server-side API calls). BATCH_PROCESS_DIRECT flows derive their
+    context from `RuntimeContext` (see below) without touching these.
+    """
+    user_agent: str = ""
+    screen_width: int = 0
+    screen_height: int = 0
+    screen_colour_depth: int = 0
+    window_width: int = 0
+    window_height: int = 0
+    timezone_offset: int = 0
+    device_id: str = ""
+    mfa_timestamp: str = ""
+    public_ip: str = ""
+    public_port: int = 0
+
+
+@dataclass
+class RuntimeContext:
+    """Pod-side environment values required on every API call.
+
+    Collected once at module load (cheap — all local syscalls). If any
+    field is empty, the header emitter falls back to safe defaults so
+    the call never goes out with an empty mandatory header.
+    """
+    mac_addresses: list[str] = field(default_factory=list)
+    local_ips: list[str] = field(default_factory=list)
+    os_family: str = ""
+    os_version: str = ""
+    device_manufacturer: str = "Kubernetes"
+    device_model: str = ""
+    os_user: str = ""
+
+
+def _collect_mac_addresses() -> list[str]:
+    """Read every non-loopback interface MAC from `/sys/class/net/*/address`.
+
+    Colons are kept raw; `_format_mac_list` percent-encodes on output per spec.
+    """
+    out: list[str] = []
+    if not _NET_CLASS.exists():
+        return out
+    for iface in sorted(_NET_CLASS.iterdir()):
+        if iface.name == "lo":
+            continue
+        addr_file = iface / "address"
+        if not addr_file.exists():
+            continue
+        try:
+            mac = addr_file.read_text().strip()
+        except OSError:
+            continue
+        if mac and mac != _EMPTY_MAC:
+            out.append(mac)
+    return out
+
+
+def _collect_local_ips() -> list[str]:
+    """Every IP bound to this host — IPv4 + IPv6, loopback excluded."""
+    ips: set[str] = set()
+    try:
+        hostname = socket.gethostname()
+        for family, _, _, _, sockaddr in socket.getaddrinfo(hostname, None):
+            raw = sockaddr[0]
+            if not isinstance(raw, str):
+                continue
+            if family == socket.AF_INET and not raw.startswith("127."):
+                ips.add(raw)
+            elif family == socket.AF_INET6 and not raw.startswith("::1"):
+                ips.add(raw.split("%")[0])  # strip zone id
+    except (socket.gaierror, OSError):
+        pass
+    # Also grab the primary outbound IP — `getaddrinfo(hostname)` can miss
+    # it inside containers whose hostname has no DNS entry.
+    try:
+        with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as s:
+            s.connect(("10.255.255.255", 1))
+            ips.add(s.getsockname()[0])
+    except OSError:
+        pass
+    return sorted(ips)
+
+
+def _detect_runtime_context() -> RuntimeContext:
+    uname = platform.uname()
+    return RuntimeContext(
+        mac_addresses=_collect_mac_addresses(),
+        local_ips=_collect_local_ips(),
+        os_family=uname.system or "Linux",
+        os_version=uname.release or "unknown",
+        device_manufacturer="Kubernetes",
+        device_model=uname.node or socket.gethostname() or "pod",
+        os_user=_safe_getuser(),
+    )
+
+
+def _safe_getuser() -> str:
+    try:
+        return getpass.getuser()
+    except (KeyError, OSError):
+        return os.environ.get("USER", "unknown")
+
+
+RUNTIME_CONTEXT: RuntimeContext = _detect_runtime_context()
+
+
+def build_headers(session: SessionContext | None = None,
+                  connection_method: str = CONNECTION_METHOD_BATCH,
+                  runtime: RuntimeContext | None = None) -> dict[str, str]:
+    """Return the full header dict to attach to every HMRC API call.
+
+    Defaults to BATCH_PROCESS_DIRECT — the mode the CronJob uses. Pass
+    a populated `SessionContext` + `connection_method=WEB_APP_VIA_SERVER`
+    for browser-initiated flows; the browser-only fields layer on top.
+    """
+    session = session or SessionContext()
+    rt = runtime or RUNTIME_CONTEXT
+    headers: dict[str, str] = {}
+    headers.update(_static_headers())
+    headers.update(_per_request_headers())
+    headers.update(_mandatory_runtime_headers(rt, session, connection_method))
+    if connection_method == CONNECTION_METHOD_WEB_APP:
+        headers.update(_web_app_session_headers(session))
+    if connection_method == CONNECTION_METHOD_MFA and session.mfa_timestamp:
+        headers["Gov-Client-MFA-Timestamp"] = session.mfa_timestamp
+    return headers
+
+
+def _static_headers() -> dict[str, str]:
+    """Vendor-constant identity headers that apply to every connection method.
+
+    Product-Name is percent-encoded per spec; License-IDs value is SHA-256-
+    hashed per spec; Version is a key-value pair of `<software-name>=<version>`.
+
+    Gov-Vendor-Public-IP and Gov-Vendor-Forwarded are NOT emitted here — the
+    HMRC validator rejects them for BATCH_PROCESS_DIRECT (where no vendor
+    server sits between the client and the HMRC API). They're added in
+    `_web_app_session_headers` for the WEB_APP_VIA_SERVER path only.
+    """
+    license_hash = hashlib.sha256(VENDOR_LICENSE_ID.encode()).hexdigest()
+    return {
+        "Gov-Vendor-Product-Name": _pct(VENDOR_PRODUCT_NAME),
+        "Gov-Vendor-Version": f"{VENDOR_PRODUCT_NAME}={VENDOR_PRODUCT_VERSION}",
+        "Gov-Vendor-License-IDs": f"{VENDOR_PRODUCT_NAME}={license_hash}",
+    }
+
+
+def _per_request_headers() -> dict[str, str]:
+    """Per-call trace + timestamp headers. Local-IPs-Timestamp uses HMRC's
+    exact format `yyyy-MM-ddThh:mm:ss.sssZ` — always UTC, always millis."""
+    now_ms = int(time.time() * 1000)
+    iso_ms = time.strftime("%Y-%m-%dT%H:%M:%S", time.gmtime(now_ms / 1000))
+    now_iso = f"{iso_ms}.{now_ms % 1000:03d}Z"
+    return {
+        "Gov-Client-Timezone": "UTC+00:00",
+        "Gov-Client-Local-IPs-Timestamp": now_iso,
+        "x-correlation-id": str(uuid.uuid4()),
+        "x-request-id": secrets.token_hex(16),
+    }
+
+
+def _mandatory_runtime_headers(rt: RuntimeContext, session: SessionContext,
+                               connection_method: str) -> dict[str, str]:
+    """The 8 headers mandatory for BATCH_PROCESS_DIRECT that come from the
+    host — Connection-Method, Device-ID, User-IDs, User-Agent, Local-IPs,
+    MAC-Addresses (+ Timezone and Local-IPs-Timestamp live in
+    `_per_request_headers`)."""
+    return {
+        "Gov-Client-Connection-Method": connection_method,
+        "Gov-Client-Device-ID": session.device_id or _fallback_device_id(),
+        "Gov-Client-User-IDs": _user_ids(rt, session),
+        "Gov-Client-User-Agent": _user_agent(rt, session),
+        "Gov-Client-Local-IPs": _format_ip_list(rt.local_ips),
+        "Gov-Client-MAC-Addresses": _format_mac_list(rt.mac_addresses),
+    }
+
+
+def _web_app_session_headers(session: SessionContext) -> dict[str, str]:
+    """WEB_APP_VIA_SERVER-only headers — browser context + vendor hop trail.
+
+    Gov-Vendor-Public-IP and Gov-Vendor-Forwarded describe the vendor server
+    that sits between the user's browser and HMRC — only meaningful for
+    WEB_APP_VIA_SERVER. BATCH_PROCESS_DIRECT must omit them (validator
+    rejects them there).
+    """
+    out: dict[str, str] = {}
+    if session.screen_width and session.screen_height:
+        out["Gov-Client-Screens"] = (
+            f"width={session.screen_width}&height={session.screen_height}"
+            f"&scaling-factor=1&colour-depth={session.screen_colour_depth}")
+    if session.window_width and session.window_height:
+        out["Gov-Client-Window-Size"] = (f"width={session.window_width}&"
+                                         f"height={session.window_height}")
+    if session.public_ip:
+        out["Gov-Client-Public-IP"] = session.public_ip
+    if session.public_port:
+        out["Gov-Client-Public-Port"] = str(session.public_port)
+    vendor_ip = VENDOR_PUBLIC_IP or (RUNTIME_CONTEXT.local_ips[0] if RUNTIME_CONTEXT.local_ips
+                                     else "")
+    if vendor_ip:
+        out["Gov-Vendor-Public-IP"] = vendor_ip
+        out["Gov-Vendor-Forwarded"] = f"by={vendor_ip}&for={vendor_ip}"
+    return out
+
+
+def _user_ids(rt: RuntimeContext, session: SessionContext) -> str:
+    """Per spec: `os=<device-user>&<app>=<app-user>`. The `os=` field is
+    mandatory; we additionally tag our application with the OAuth user
+    so HMRC can correlate activity in a breach investigation.
+    """
+    os_user = rt.os_user or "unknown"
+    pairs = [f"os={_pct(os_user)}"]
+    oauth_user = os.environ.get("HMRC_OAUTH_USER", "viktor")
+    pairs.append(f"hmrc-sync={_pct(oauth_user)}")
+    _ = session  # reserved for future per-session identity extension
+    return "&".join(pairs)
+
+
+def _user_agent(rt: RuntimeContext, session: SessionContext) -> str:
+    """Per spec: `os-family=…&os-version=…&device-manufacturer=…&device-model=…`.
+
+    For WEB_APP_VIA_SERVER with a captured browser UA, the browser string
+    is encoded under `device-model` with the rest of the fields defaulting
+    to our pod's values — HMRC's validator accepts this hybrid shape.
+    """
+    model = session.user_agent or rt.device_model or "pod"
+    pairs = [
+        f"os-family={_pct(rt.os_family)}",
+        f"os-version={_pct(rt.os_version)}",
+        f"device-manufacturer={_pct(rt.device_manufacturer)}",
+        f"device-model={_pct(model)}",
+    ]
+    return "&".join(pairs)
+
+
+def _format_ip_list(ips: list[str]) -> str:
+    """IPv6 addresses percent-encoded; IPv4 passes through. Joined with ','.
+
+    HMRC's validator accepts an empty header only if the request truly
+    has no IPs; on a live pod we always have at least one — if the list
+    comes back empty we fall back to the loopback so the header is
+    syntactically valid.
+    """
+    if not ips:
+        return "127.0.0.1"
+    out = []
+    for ip in ips:
+        out.append(_pct(ip) if ":" in ip else ip)
+    return ",".join(out)
+
+
+def _format_mac_list(macs: list[str]) -> str:
+    """Each MAC percent-encoded (colons → %3A), comma-joined.
+
+    Empty list → single dummy MAC so we never ship a blank header;
+    HMRC's validator treats blank as a violation.
+    """
+    if not macs:
+        return _pct("02:00:00:00:00:00")
+    return ",".join(_pct(m) for m in macs)
+
+
+def _fallback_device_id() -> str:
+    """Deterministic UUID derived from hostname when no Vault-backed
+    Device-ID is seeded. Stable across restarts on the same node."""
+    return str(uuid.uuid5(uuid.NAMESPACE_DNS, f"hmrc-sync-{platform.node()}"))
+
+
+def _pct(s: str) -> str:
+    return urllib.parse.quote(s, safe="")
+
+
+def as_validator_payload(headers: dict[str, str]) -> dict[str, Any]:
+    """Reshape headers for the HMRC fraud-header validator API body."""
+    return {"headers": [{"name": k, "value": v} for k, v in headers.items()]}

hmrc_sync/oauth.py

@@ -0,0 +1,125 @@
+"""HMRC OAuth token persistence — Vault-backed refresh-token store.
+
+The refresh_token is long-lived (HMRC grants 18 months). We keep it in
+Vault at `secret/viktor/hmrc_refresh_token` and let ESO sync it to a K8s
+Secret the pod mounts as an env var. On every refresh, we write the new
+token back to Vault so the next pod restart picks it up.
+
+Writing back requires Vault write access — the pod uses a short-lived
+K8s-auth Vault token with a narrow policy that only allows writing
+`secret/viktor/hmrc_refresh_token`.
+"""
+from __future__ import annotations
+
+import logging
+import os
+from dataclasses import dataclass
+
+import httpx
+
+log = logging.getLogger(__name__)
+
+VAULT_KEY = "secret/viktor/hmrc_refresh_token"
+PROD_BASE = "https://api.service.hmrc.gov.uk"
+TOKEN_PATH = "/oauth/token"
+
+
+@dataclass(frozen=True)
+class OAuthCreds:
+    client_id: str
+    client_secret: str
+    redirect_uri: str
+
+
+@dataclass
+class TokenBundle:
+    access_token: str
+    refresh_token: str
+    expires_in: int
+    scope: str
+
+    @classmethod
+    def from_json(cls, data: dict[str, object]) -> TokenBundle:
+        return cls(
+            access_token=str(data["access_token"]),
+            refresh_token=str(data["refresh_token"]),
+            expires_in=int(data["expires_in"]),  # type: ignore[arg-type]
+            scope=str(data.get("scope", "")),
+        )
+
+
+def load_creds_from_env() -> OAuthCreds:
+    return OAuthCreds(
+        client_id=os.environ["HMRC_PROD_CLIENT_ID"],
+        client_secret=os.environ["HMRC_PROD_CLIENT_SECRET"],
+        redirect_uri=os.environ["HMRC_PROD_REDIRECT_URI"],
+    )
+
+
+async def exchange_code(creds: OAuthCreds, code: str) -> TokenBundle:
+    """Swap a fresh authorization_code for an access+refresh token pair."""
+    async with httpx.AsyncClient(timeout=30.0) as client:
+        resp = await client.post(
+            f"{PROD_BASE}{TOKEN_PATH}",
+            data={
+                "grant_type": "authorization_code",
+                "client_id": creds.client_id,
+                "client_secret": creds.client_secret,
+                "redirect_uri": creds.redirect_uri,
+                "code": code,
+            },
+            headers={"Accept": "application/vnd.hmrc.1.0+json"},
+        )
+        resp.raise_for_status()
+        return TokenBundle.from_json(resp.json())
+
+
+async def refresh(creds: OAuthCreds, refresh_token: str) -> TokenBundle:
+    """Exchange an old refresh_token for a fresh access+refresh pair.
+
+    HMRC rotates the refresh_token on every refresh — the old one becomes
+    invalid immediately after this call returns. Persist the new one to
+    Vault atomically; a failure between the refresh and the Vault write
+    leaves us stranded.
+    """
+    async with httpx.AsyncClient(timeout=30.0) as client:
+        resp = await client.post(
+            f"{PROD_BASE}{TOKEN_PATH}",
+            data={
+                "grant_type": "refresh_token",
+                "client_id": creds.client_id,
+                "client_secret": creds.client_secret,
+                "refresh_token": refresh_token,
+            },
+            headers={"Accept": "application/vnd.hmrc.1.0+json"},
+        )
+        resp.raise_for_status()
+        return TokenBundle.from_json(resp.json())
+
+
+def persist_to_vault(token: TokenBundle) -> None:
+    """Write the new refresh_token back to Vault.
+
+    Uses the hvac client with K8s-auth — the pod's service-account token
+    at /var/run/secrets/kubernetes.io/serviceaccount/token logs into
+    Vault's kubernetes auth method and receives a short-lived Vault token
+    with write access to `secret/viktor/hmrc_refresh_token` only.
+    """
+    import hvac
+
+    addr = os.environ.get("VAULT_ADDR", "https://vault.viktorbarzin.me")
+    role = os.environ.get("VAULT_K8S_ROLE", "hmrc-sync")
+    jwt_path = "/var/run/secrets/kubernetes.io/serviceaccount/token"
+    with open(jwt_path, encoding="utf-8") as fh:
+        jwt = fh.read()
+    client = hvac.Client(url=addr)
+    client.auth.kubernetes.login(role=role, jwt=jwt)
+    client.secrets.kv.v2.create_or_update_secret(
+        path="viktor/hmrc_refresh_token",
+        secret={
+            "refresh_token": token.refresh_token,
+            "expires_in": token.expires_in,
+            "scope": token.scope,
+        },
+    )
+    log.info("Rotated HMRC refresh_token persisted to Vault")