feat(flags): the registry and a fail-closed Unleash client

Unleash holds flag state; it does not hold the list of flags. REGISTRY
is that list, because the SDK evaluates an unknown flag to False and
without a registry that is an undeclared False — indistinguishable from
a typo in a flag name.

Fail-closed throughout, and never raises: an unset UNLEASH_URL, an
unreachable server, a client that throws, an undeclared name — all
False. A flag layer that can 500 a request path or stop the API booting
is worse than one that is switched off.

Every flag defaults to False, with no per-flag override, because a flag
that defaults on is a kill switch and this is deliberately not one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015mWQnpye9F299NVRCCSRvj

backend/config.py

@@ -42,6 +42,16 @@ class Settings(BaseSettings):
     typesense_url: str = "http://localhost:8108"
     typesense_api_key: str = ""
 
+    # Feature flags (Unleash). An empty unleash_url disables flags entirely and
+    # every flag evaluates False — the correct behaviour for local development
+    # and CI, and the reason no test needs a running Unleash.
+    unleash_url: str = ""
+    unleash_api_token: str = ""
+    unleash_app_name: str = "schoolcompare-backend"
+    # On a named volume, so a restart during an Unleash outage keeps
+    # last-known state instead of reverting a released feature to dark.
+    unleash_cache_directory: str = "/app/.unleash"
+
     # Analytics
     ga_measurement_id: Optional[str] = "G-J0PCVT14NY"  # Google Analytics 4 Measurement ID
 

backend/flags.py

@@ -0,0 +1,111 @@
+"""Feature flags: what can be switched, and what is switched right now.
+
+Ship-dark, not a kill switch. Flags let work merge and deploy without becoming
+visible; they are expected to flip about monthly, by a person, deliberately.
+Nothing here does percentage rollouts or user targeting — the site has no user
+identity to target.
+
+Unleash holds the state. It does not hold the list. REGISTRY below is that
+list, and it exists for three reasons: the SDK evaluates an unknown flag to
+False, so without a registry that is an *undeclared* False, indistinguishable
+from a typo; /api/flags needs a key set to return when Unleash is unreachable;
+and a flag in the UI but not in the registry is orphaned and should be visibly
+so rather than quietly authoritative.
+
+Every flag defaults to False. There is no per-flag default, because a flag that
+defaults on is a kill switch, and this is not one.
+"""
+
+from __future__ import annotations
+
+import logging
+from dataclasses import dataclass
+from datetime import date
+
+from .config import settings
+
+logger = logging.getLogger(__name__)
+
+# A flag is temporary scaffolding. See test_a_flag_older_than_the_limit.
+MAX_FLAG_AGE_DAYS = 90
+
+
+@dataclass(frozen=True)
+class Flag:
+    # One string: the registry key, the Unleash flag name, and the JSON key in
+    # /api/flags. snake_case, matching the API's existing convention. No case
+    # transformation anywhere, so there is no mapping layer to get wrong.
+    name: str
+    description: str          # one line: what turning this on reveals
+    added: date               # for the staleness tripwire
+
+
+REGISTRY: dict[str, Flag] = {
+    f.name: f for f in (
+        Flag(
+            name="admission_distance",
+            description=(
+                "The last-distance-offered figure on the Admissions tile and "
+                "the 'How far away are you?' section on school pages."
+            ),
+            added=date(2026, 8, 23),
+        ),
+    )
+}
+
+
+_client = None
+
+
+def init() -> None:
+    """Start the Unleash client, or log why flags are all off.
+
+    Called once from the app lifespan. Never raises: a flag system that can
+    stop the API from booting is worse than one that is switched off.
+    """
+    global _client
+    if not settings.unleash_url or not settings.unleash_api_token:
+        logger.warning(
+            "Unleash is not configured (UNLEASH_URL / UNLEASH_API_TOKEN); "
+            "every feature flag evaluates to False.")
+        return
+
+    try:
+        from UnleashClient import UnleashClient
+
+        _client = UnleashClient(
+            url=settings.unleash_url,
+            app_name=settings.unleash_app_name,
+            custom_headers={"Authorization": settings.unleash_api_token},
+            cache_directory=settings.unleash_cache_directory,
+            refresh_interval=15,
+        )
+        _client.initialize_client()
+        logger.info("Unleash client initialised against %s", settings.unleash_url)
+    except Exception:
+        # Fail closed and keep serving. The SDK also evaluates everything False
+        # until its first successful sync, so this is the same direction.
+        _client = None
+        logger.exception("Unleash client failed to start; flags are all False.")
+
+
+def is_enabled(name: str) -> bool:
+    """Whether `name` is on. False for anything unknown, unreachable or broken."""
+    if name not in REGISTRY:
+        logger.error(
+            "undeclared feature flag %r was evaluated; returning False. "
+            "Add it to backend/flags.py REGISTRY or fix the name.", name)
+        return False
+    if _client is None:
+        return False
+    try:
+        return bool(_client.is_enabled(
+            name, fallback_function=lambda feature_name, context: False))
+    except Exception:
+        logger.exception("flag %r failed to evaluate; returning False", name)
+        return False
+
+
+def all_flags() -> dict[str, bool]:
+    """Every declared flag and its current value. Serves /api/flags."""
+    return {name: is_enabled(name) for name in REGISTRY}

backend/tests/test_flags.py

@@ -0,0 +1,74 @@
+"""Tests for the feature flag layer (spec 2026-08-23).
+
+None of these need a running Unleash. That is the point: an unset UNLEASH_URL
+means every flag is False, which is what local development and CI get.
+"""
+
+from datetime import date, timedelta
+
+from backend import flags
+
+
+def test_every_declared_flag_is_keyed_by_its_own_name():
+    # One string is the registry key, the Unleash flag name and the JSON key.
+    # A mismatch here would mean the UI toggles a flag the code never reads.
+    for key, flag in flags.REGISTRY.items():
+        assert key == flag.name
+
+
+def test_flag_names_are_snake_case():
+    # Matches the API's existing convention (admission_distance,
+    # rwm_expected_pct) so no case transformation exists to get wrong.
+    for name in flags.REGISTRY:
+        assert name == name.lower()
+        assert "-" not in name and " " not in name
+
+
+def test_an_unconfigured_client_evaluates_every_flag_false(monkeypatch):
+    monkeypatch.setattr(flags, "_client", None)
+    for name in flags.REGISTRY:
+        assert flags.is_enabled(name) is False
+
+
+def test_an_undeclared_flag_is_false_rather_than_an_error(monkeypatch):
+    # A typo'd flag name must not raise in a request path. It is logged as an
+    # error, because an undeclared flag is always a bug.
+    monkeypatch.setattr(flags, "_client", None)
+    assert flags.is_enabled("no_such_flag") is False
+
+
+def test_an_exploding_client_is_false_rather_than_a_500(monkeypatch):
+    class Boom:
+        def is_enabled(self, *a, **kw):
+            raise RuntimeError("unleash is on fire")
+
+    monkeypatch.setattr(flags, "_client", Boom())
+    name = next(iter(flags.REGISTRY))
+    assert flags.is_enabled(name) is False
+
+
+def test_all_flags_reports_every_declared_flag(monkeypatch):
+    monkeypatch.setattr(flags, "_client", None)
+    assert set(flags.all_flags()) == set(flags.REGISTRY)
+    assert all(v is False for v in flags.all_flags().values())
+
+
+def test_a_flag_older_than_the_limit_fails_this_test():
+    """A tripwire, not an assertion about correctness.
+
+    Flags are temporary scaffolding and the failure mode of every flag system
+    is accumulation. This fails on the day a flag turns 90, on whatever PR
+    happens to be open — which is the point: someone has to decide.
+
+    To fix: delete the flag and the branches that read it, or, if it genuinely
+    still needs to exist, move its `added` date and say why in the commit.
+    """
+    stale = [
+        f.name for f in flags.REGISTRY.values()
+        if date.today() - f.added > timedelta(days=flags.MAX_FLAG_AGE_DAYS)
+    ]
+    assert not stale, (
+        f"Flags older than {flags.MAX_FLAG_AGE_DAYS} days: {stale}. "
+        "Remove the flag and the code branches it guards, or move its `added` "
+        "date deliberately."
+    )

requirements.txt

@@ -12,4 +12,4 @@ slowapi==0.1.9
 secure==0.3.0
 typesense==0.21.0
 numpy==1.26.4
-
+UnleashClient==6.0.1