diff --git a/gitea_saas/config.py b/gitea_saas/config.py
new file mode 100644
index 0000000..f3dcd13
--- /dev/null
+++ b/gitea_saas/config.py
@@ -0,0 +1,58 @@
+"""Environment-backed configuration for the gitea-saas-core HTTP service.
+
+Reads only the SAAS_* variables.  It deliberately never reads real Stripe
+keys (STRIPE_* / *_SECRET) or any other external credentials: deployment-time
+secrets belong to the platform layers, not to this config module.
+"""
+
+from __future__ import annotations
+
+import os
+from dataclasses import dataclass
+from pathlib import Path
+from typing import Optional
+
+# Fallback used only when SAAS_PORT is unset / empty.
+DEFAULT_PORT = 8080
+
+# When SAAS_DB_PATH is unset the CLI entrypoint defaults to an ephemeral
+# (in-memory) SQLite database.  Real deployments must set SAAS_DB_PATH.
+DEFAULT_DB_PATH: Optional[Path] = None
+
+# Header name carrying the API key on every authenticated request.
+API_KEY_HEADER = "X-API-Key"
+
+
+def _env_nonempty(name: str) -> Optional[str]:
+    value = os.environ.get(name, "").strip()
+    return value or None
+
+
+@dataclass(frozen=True)
+class ServerConfig:
+    """Resolved configuration for the Flask API process."""
+
+    api_key: Optional[str]
+    port: int
+    db_path: Optional[Path]
+
+
+def load_config() -> ServerConfig:
+    """Read the SAAS_* environment into a frozen config object.
+
+    Sane defaults, never average anything: the only mutable knob that has a
+    bare fallback is the port; the API key and DB path are explicit so that a
+    misconfigured deployment fails loudly instead of serving unauthenticated.
+    """
+    raw_port = os.environ.get("SAAS_PORT", "").strip()
+    db_path_raw = os.environ.get("SAAS_DB_PATH", "").strip()
+    return ServerConfig(
+        api_key=_env_nonempty("SAAS_API_KEY"),
+        port=int(raw_port) if raw_port else DEFAULT_PORT,
+        db_path=Path(db_path_raw).expanduser() if db_path_raw else DEFAULT_DB_PATH,
+    )
+
+
+def api_key() -> Optional[str]:
+    """Convenience accessor matching :func:`load_config` behaviour."""
+    return _env_nonempty("SAAS_API_KEY")
diff --git a/gitea_saas/server.py b/gitea_saas/server.py
new file mode 100644
index 0000000..efecfb9
--- /dev/null
+++ b/gitea_saas/server.py
@@ -0,0 +1,365 @@
+"""Flask HTTP API over the billing core.
+
+Consumed core interface (issue #1, ``gitea_saas/billing_core.py``):
+
+    Every operation returns ``(ok, details)`` and never raises for domain
+    failures; ``ok`` is bool and ``details`` is a dict.  Domain errors carry
+    ``details = {"error": {"code": "...", "message": "..."}}`` with codes
+    such as ``UNKNOWN_CLIENT``, ``INSUFFICIENT_BALANCE``, ``INVALID_AMOUNT``,
+    ``HOLD_NOT_FOUND``, ``INTERNAL`` (see ``billing_core`` for the full set).
+
+    create_client(name, tier="free", status="active", meta=None)
+                                          -> {"client_id", "name", "tier",
+                                             "status", "meta", "created_at"}
+    get_client(client_id, include_balance=True)
+                                          -> {"client": {...}, "balance": N}
+    credit(client_id, amount_minor, idempotency_key,
+           currency="USD", ref_type=None, ref_id=None)
+                                          -> {"ledger_id", "client_id",
+                                             "balance", ...}
+    debit(...)                            -> same shape as credit
+    reserve_hold(client_id, amount_minor, idempotency_key,
+                 ref_type=None, ref_id=None, expires_in_minutes=30)
+                                          -> {"hold_id", "client_id",
+                                             "amount_minor", "balance", ...}
+    release_hold(hold_id, idempotency_key)
+                                          -> {"hold_id", "released_at",
+                                             "balance", ...}
+    list_ledger(client_id, from_=None, to=None)
+                                          -> {"entries": [ledger row, ...],
+                                             "count": N}
+
+No ``list_clients`` exists on the core (issue #1 did not land one), so this
+API has no list-all-clients endpoint.
+
+Duplicate ``idempotency_key`` mutations are no-ops that return the original
+result with ``ok=True`` and ``replayed=True`` (never an error).  Balances and
+amounts are integers in minor units.
+
+The factory is deliberately pure HTTP: the core is injected, which keeps the
+tests hermetic (Flask test client + temp DB) and free of any network/Stripe.
+"""
+
+from __future__ import annotations
+
+import hmac
+import sys
+from pathlib import Path
+from typing import Any, Dict, Optional, Tuple
+
+from flask import Flask, jsonify, request
+
+from .config import API_KEY_HEADER, load_config
+
+
+class ApiError(Exception):
+    """An error that maps to a concrete HTTP response."""
+
+    def __init__(self, status: int, code: str, message: str):
+        super().__init__(message)
+        self.status = status
+        self.code = code
+        self.message = message
+
+
+def _core(app: Flask) -> Any:
+    return app.config["CORE"]
+
+
+def _error(status: int, code: str, message: str):
+    payload = {"error": message, "code": code}
+    resp = jsonify(payload)
+    resp.status_code = status
+    return resp
+
+
+# --------------------------------------------------------------------------
+# Core result handling
+# --------------------------------------------------------------------------
+
+def _call(app: Flask, fn: str, **kwargs) -> Dict[str, Any]:
+    """Invoke a core operation, enforcing the (ok, details) contract."""
+    try:
+        op = getattr(_core(app), fn)
+    except AttributeError:
+        raise ApiError(501, "not_implemented",
+                       f"core does not implement {fn}()")
+    try:
+        result = op(**kwargs)
+    except TypeError as exc:
+        # Guard against core variants with different signatures surfacing as
+        # 500s instead of a clear 400/501.
+        raise ApiError(500, "core_contract", f"core.{fn}() failed: {exc}")
+    except Exception as exc:  # noqa: BLE001 - fail closed, never leak internals
+        raise ApiError(500, "core_error", f"core.{fn}() raised: {exc!r}")
+    if not isinstance(result, tuple) or len(result) != 2:
+        raise ApiError(500, "core_contract",
+                       f"core.{fn}() must return (ok, details)")
+    ok, details = result
+    if not ok:
+        raise _map_core_error(fn, details)
+    return details if isinstance(details, dict) else {"result": details}
+
+
+def _map_core_error(fn: str, details: Any) -> ApiError:
+    """Map a real-core error payload to an HTTP error.
+
+    The real ``BillingCore`` reports failures as
+    ``(False, {"error": {"code": ..., "message": ...}})`` with codes such as
+    ``UNKNOWN_CLIENT`` / ``INSUFFICIENT_BALANCE`` / ``INVALID_AMOUNT`` /
+    ``HOLD_NOT_FOUND``.  Translate those to HTTP statuses while keeping the
+    response body a plain ``{"error", "code"}`` pair.
+    """
+    err = details.get("error") if isinstance(details, dict) else None
+    if isinstance(err, dict):
+        code = err.get("code")
+        message = err.get("message")
+    else:
+        # Defensive: tolerate the older flat {code, error} shape.
+        code = details.get("code") if isinstance(details, dict) else None
+        message = details.get("error") if isinstance(details, dict) else str(details)
+    message = message or f"{fn} failed"
+
+    status_map = {
+        "UNKNOWN_CLIENT": 404,
+        "HOLD_NOT_FOUND": 404,
+        "INSUFFICIENT_BALANCE": 409,
+        "DUPLICATE_CLIENT": 409,
+        "HOLD_ALREADY_RELEASED": 409,
+        "IDEMPOTENCY_CONFLICT": 409,
+        "INVALID_AMOUNT": 400,
+        "INVALID_KEY": 400,
+        "INVALID_CURRENCY": 400,
+        "INVALID_NAME": 400,
+        "INVALID_HOLD_TTL": 400,
+        "INTERNAL": 500,
+    }
+    if code in status_map:
+        return ApiError(status_map[code], code.lower(), message)
+    # Unknown structured errors are still surfaced (fail-closed), not 500'd.
+    if code:
+        return ApiError(400, code, message)
+    return ApiError(500, "core_error", message)
+
+
+def _bad_request(message: str) -> ApiError:
+    return ApiError(400, "invalid_request", message)
+
+
+def _require_int_field(body: Dict[str, Any], name: str, *, positive: bool = True) -> int:
+    value = body.get(name)
+    if isinstance(value, bool) or not isinstance(value, int):
+        raise _bad_request(f"'{name}' must be an integer (minor units)")
+    if positive and value <= 0:
+        raise _bad_request(f"'{name}' must be a positive integer")
+    if not positive and value < 0:
+        raise _bad_request(f"'{name}' must be a non-negative integer")
+    return value
+
+
+def _require_key(body: Dict[str, Any], name: str) -> str:
+    value = body.get(name)
+    if not isinstance(value, str) or not value.strip():
+        raise _bad_request(f"'{name}' must be a non-empty string")
+    return value.strip()
+
+
+def _optional_str(body: Dict[str, Any], name: str) -> Optional[str]:
+    value = body.get(name)
+    if value is None or value == "":
+        return None
+    if not isinstance(value, str):
+        raise _bad_request(f"'{name}' must be a string")
+    return value
+
+
+def _json_body() -> Dict[str, Any]:
+    body = request.get_json(silent=True)
+    if body is None or not isinstance(body, dict):
+        raise _bad_request("request body must be a JSON object")
+    return body
+
+
+def _client_out(client: Dict[str, Any]) -> Dict[str, Any]:
+    """Normalise a real-core client payload for the wire.
+
+    The real ``BillingCore`` returns two shapes:
+      * flat     from ``create_client``: ``{client_id, name, tier, status,
+                 meta, created_at}``
+      * nested   from ``get_client``:    ``{"client": {...}, "balance": N}``
+
+    This handles both and always exposes the API's ``id`` field (the core
+    calls it ``client_id``) plus the balance when present.  Unknown keys are
+    passed through rather than dropped, so a contract mismatch becomes a
+    visible test failure instead of silent data loss.
+    """
+    if not isinstance(client, dict):
+        return client
+    # Unwrap the nested get_client shape.
+    inner = client.get("client")
+    if isinstance(inner, dict):
+        out = dict(inner)
+        if "balance" in client:
+            out["balance"] = client["balance"]
+        client = out
+    out = dict(client)
+    if "client_id" in out and "id" not in out:
+        out["id"] = out.pop("client_id")
+    return out
+
+
+# --------------------------------------------------------------------------
+# App factory
+# --------------------------------------------------------------------------
+
+def create_app(core: Any, api_key: str) -> Flask:
+    """Build the Flask app around a billing ``core`` instance.
+
+    ``api_key`` is matched against the ``X-API-Key`` header on every route
+    except ``/health``.  Missing or wrong keys get ``401``.
+    """
+    app = Flask(__name__)
+    app.config["CORE"] = core
+    app.config["API_KEY"] = api_key or ""  # empty -> deny all (fail-closed)
+
+    @app.before_request
+    def _require_api_key():
+        if request.path.rstrip("/") == "/health":
+            return None
+        sent = request.headers.get(API_KEY_HEADER, "")
+        expected = app.config["API_KEY"]
+        if not expected or not isinstance(sent, str) or not hmac.compare_digest(
+                sent, expected):
+            return _error(401, "unauthorized", "missing or invalid API key")
+        return None
+
+    @app.errorhandler(ApiError)
+    def _handle_api_error(exc: ApiError):
+        return _error(exc.status, exc.code, exc.message)
+
+    @app.errorhandler(404)
+    def _handle_404(exc):
+        return _error(404, "not_found", "resource not found")
+
+    # -- public ----------------------------------------------------------
+    @app.get("/health")
+    def health():
+        return jsonify({"ok": True})
+
+    # -- clients ---------------------------------------------------------
+    @app.post("/api/clients")
+    def create_client():
+        body = _json_body()
+        name = _require_key(body, "name")
+        tier = _optional_str(body, "tier")
+        meta = body.get("meta")
+        kwargs: Dict[str, Any] = {"name": name}
+        if tier is not None:
+            kwargs["tier"] = tier
+        if meta is not None:
+            kwargs["meta"] = meta
+        created = _call(app, "create_client", **kwargs)
+        client_id = created.get("client_id") or created.get("id")
+        # Re-fetch through the real core so the response carries the API's
+        # `id` field plus the current balance (the real create_client does
+        # not return a balance).
+        client = _call(app, "get_client", client_id=client_id, include_balance=True)
+        resp = jsonify(_client_out(client))
+        resp.status_code = 201
+        return resp
+
+    @app.get("/api/clients/<int:client_id>")
+    def get_client(client_id: int):
+        client = _call(app, "get_client", client_id=client_id, include_balance=True)
+        return jsonify(_client_out(client))
+
+    # -- money movement --------------------------------------------------
+    @app.post("/api/clients/<int:client_id>/credits")
+    def add_credit(client_id: int):
+        body = _json_body()
+        return jsonify(_call(app, "credit",
+                             client_id=client_id,
+                             amount_minor=_require_int_field(body, "amount_minor"),
+                             idempotency_key=_require_key(body, "idempotency_key"),
+                             ref_type=_optional_str(body, "ref_type"),
+                             ref_id=_optional_str(body, "ref_id")))
+
+    @app.post("/api/clients/<int:client_id>/debits")
+    def add_debit(client_id: int):
+        body = _json_body()
+        return jsonify(_call(app, "debit",
+                             client_id=client_id,
+                             amount_minor=_require_int_field(body, "amount_minor"),
+                             idempotency_key=_require_key(body, "idempotency_key"),
+                             ref_type=_optional_str(body, "ref_type"),
+                             ref_id=_optional_str(body, "ref_id")))
+
+    # -- holds -----------------------------------------------------------
+    @app.post("/api/clients/<int:client_id>/holds")
+    def reserve_hold(client_id: int):
+        body = _json_body()
+        kwargs: Dict[str, Any] = {
+            "client_id": client_id,
+            "amount_minor": _require_int_field(body, "amount_minor"),
+            "idempotency_key": _require_key(body, "idempotency_key"),
+            "ref_type": _optional_str(body, "ref_type"),
+            "ref_id": _optional_str(body, "ref_id"),
+        }
+        ttl = body.get("ttl_s")
+        if ttl is not None:
+            if isinstance(ttl, bool) or not isinstance(ttl, (int, float)) or ttl <= 0:
+                raise _bad_request("'ttl_s' must be a positive number (seconds)")
+            # Real core takes expires_in_minutes (a float, minutes).
+            kwargs["expires_in_minutes"] = ttl / 60.0
+        held = _call(app, "reserve_hold", **kwargs)
+        resp = jsonify(held)
+        resp.status_code = 201
+        return resp
+
+    @app.post("/api/holds/<int:hold_id>/release")
+    def release_hold(hold_id: int):
+        body = _json_body()
+        return jsonify(_call(app, "release_hold",
+                             hold_id=hold_id,
+                             idempotency_key=_require_key(body, "idempotency_key")))
+
+    # -- ledger ----------------------------------------------------------
+    @app.get("/api/clients/<int:client_id>/ledger")
+    def ledger(client_id: int):
+        from_ = request.args.get("from")
+        to_ = request.args.get("to")
+        result = _call(app, "list_ledger", client_id=client_id,
+                       from_=from_ or None, to=to_ or None)
+        entries = result.get("entries", []) if isinstance(result, dict) else []
+        return jsonify({"rows": entries, "count": len(list(entries))})
+
+    return app
+
+
+def _build_core(cfg):
+    """Construct the real billing core from config (issue #1 module)."""
+    from gitea_saas import billing_core  # lazy import: pure-server may omit it
+    db_path = cfg.db_path or Path(":memory:")
+    for ctor_name in ("BillingCore", "Core", "LedgerCore"):
+        ctor = getattr(billing_core, ctor_name, None)
+        if ctor is not None:
+            return ctor(db_path)
+    raise RuntimeError(
+        "gitea_saas.billing_core has no known constructor "
+        "(expected BillingCore/Core/LedgerCore)"
+    )
+
+
+def main() -> None:
+    """Entrypoint for ``python -m gitea_saas.server``."""
+    cfg = load_config()
+    if not cfg.api_key:
+        sys.exit("SAAS_API_KEY is unset; refusing to start (fail-closed).")
+    core = _build_core(cfg)
+    app = create_app(core, cfg.api_key)
+    # Deployed behind a reverse proxy in practice; still default to loopback.
+    app.run(host="127.0.0.1", port=cfg.port)
+
+
+if __name__ == "__main__":
+    main()
diff --git a/requirements.txt b/requirements.txt
index 3f0f5f0..2053737 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,3 +1,4 @@
-# Runtime: pure stdlib (sqlite3, pathlib, threading, json). No third-party deps.
+# Runtime: Flask for the billing HTTP API (#2). Core (#1) is pure stdlib.
 # Test/dev dependency:
+flask>=3.0
 pytest>=8.0
diff --git a/tests/test_server.py b/tests/test_server.py
new file mode 100644
index 0000000..b67466f
--- /dev/null
+++ b/tests/test_server.py
@@ -0,0 +1,325 @@
+"""Tests for gitea_saas.server against the real BillingCore (issue #1).
+
+Hermetic by design: every test builds a temp SQLite DB and drives the app
+through Flask's test client — no network, no Stripe, no Gitea.
+
+Issue #1 (the billing core) has landed on ``main``, so this suite runs
+against the real ``gitea_saas.billing_core.BillingCore``.  The only doubles
+left are two minimal stubs that force error paths the real core cannot
+produce on demand: a core missing an op (501) and a core that raises (500).
+Every endpoint has at least one test against the real core.
+"""
+
+from __future__ import annotations
+
+import datetime
+from pathlib import Path
+
+import pytest
+
+from gitea_saas import billing_core, server
+from gitea_saas.config import API_KEY_HEADER
+
+API_KEY = "test-api-key-secret"
+AUTH = {API_KEY_HEADER: API_KEY}
+
+
+# --------------------------------------------------------------------------
+# Real-core app fixture
+# --------------------------------------------------------------------------
+
+@pytest.fixture()
+def app(tmp_path: Path):
+    core = billing_core.BillingCore(tmp_path / "test.db")
+    application = server.create_app(core, API_KEY)
+    application.config.update(TESTING=True)
+    try:
+        yield application
+    finally:
+        core._conn.close()
+
+
+@pytest.fixture()
+def client(app):
+    return app.test_client()
+
+
+def _post(client, url, body, key=API_KEY):
+    return client.post(url, json=body, headers={API_KEY_HEADER: key})
+
+
+# --------------------------------------------------------------------------
+# Minimal doubles for error paths the real core cannot produce on demand
+# --------------------------------------------------------------------------
+
+class MissingOpCore:
+    """Looks like the core but omits ``credit`` -> drives the 501 path."""
+
+    def create_client(self, name, tier="free", status="active", meta=None):
+        return True, {"client_id": 1, "name": name, "tier": tier,
+                      "status": status, "meta": meta or {},
+                      "created_at": "2026-01-01T00:00:00+00:00"}
+
+    def get_client(self, client_id, include_balance=True):
+        return False, {"error": {"code": "UNKNOWN_CLIENT",
+                                 "message": f"no client with id {client_id}"}}
+
+
+class RaisingCore:
+    """Raises where the real core returns a tuple -> drives the 500 path."""
+
+    def credit(self, **kwargs):
+        raise RuntimeError("boom")
+
+
+# --------------------------------------------------------------------------
+# Auth / health
+# --------------------------------------------------------------------------
+
+def test_health_is_public(client):
+    resp = client.get("/health")
+    assert resp.status_code == 200
+    assert resp.get_json() == {"ok": True}
+
+
+def test_missing_key_is_401(client):
+    resp = client.get("/api/clients/1")
+    assert resp.status_code == 401
+    body = resp.get_json()
+    assert body["code"] == "unauthorized"
+
+
+def test_wrong_key_is_401(client):
+    resp = client.get("/api/clients/1", headers={API_KEY_HEADER: "nope"})
+    assert resp.status_code == 401
+    assert resp.get_json()["code"] == "unauthorized"
+
+
+def test_correct_key_is_200(client):
+    created = _post(client, "/api/clients", {"name": "AuthOK"})
+    assert created.status_code == 201
+    cid = created.get_json()["id"]
+    resp = client.get(f"/api/clients/{cid}", headers=AUTH)
+    assert resp.status_code == 200
+
+
+# --------------------------------------------------------------------------
+# Client lifecycle
+# --------------------------------------------------------------------------
+
+def test_full_lifecycle_via_api(client):
+    # create -> response carries the API's id plus a balance
+    created = _post(client, "/api/clients",
+                    {"name": "Acme", "tier": "enterprise", "meta": {"v": 1}})
+    assert created.status_code == 201
+    out = created.get_json()
+    cid = out["id"]
+    assert out["balance"] == 0
+    assert out["name"] == "Acme"
+    assert out["tier"] == "enterprise"
+    assert out["meta"] == {"v": 1}
+    assert out["status"] == "active"
+
+    # credit
+    cred = _post(client, f"/api/clients/{cid}/credits",
+                 {"amount_minor": 5000, "idempotency_key": "c1"})
+    assert cred.status_code == 200
+    assert cred.get_json()["balance"] == 5000
+
+    # get -> full client record plus balance
+    got = client.get(f"/api/clients/{cid}", headers=AUTH)
+    assert got.status_code == 200
+    g = got.get_json()
+    assert g["balance"] == 5000
+    assert g["name"] == "Acme"
+    assert g["tier"] == "enterprise"
+
+    # hold
+    held = _post(client, f"/api/clients/{cid}/holds",
+                 {"amount_minor": 2000, "idempotency_key": "h1",
+                  "ref_type": "job", "ref_id": "j1"})
+    assert held.status_code == 201
+    hold_id = held.get_json()["hold_id"]
+    got = client.get(f"/api/clients/{cid}", headers=AUTH)
+    assert got.get_json()["balance"] == 3000  # 5000 - 2000 held
+
+    # debit (up to held-away balance)
+    deb = _post(client, f"/api/clients/{cid}/debits",
+                {"amount_minor": 3000, "idempotency_key": "d1"})
+    assert deb.status_code == 200
+    assert deb.get_json()["balance"] == 0  # 5000 - 3000 - 2000(held)
+
+    # release hold -> balance restored
+    rel = _post(client, f"/api/holds/{hold_id}/release",
+                {"idempotency_key": "r1"})
+    assert rel.status_code == 200
+    assert "released_at" in rel.get_json()
+    got = client.get(f"/api/clients/{cid}", headers=AUTH)
+    assert got.get_json()["balance"] == 2000
+
+    # ledger shows all four kinds (credit, hold, debit, release)
+    led = client.get(f"/api/clients/{cid}/ledger", headers=AUTH)
+    assert led.status_code == 200
+    kinds = {r["kind"] for r in led.get_json()["rows"]}
+    assert kinds == {"credit", "hold", "debit", "release"}
+
+
+def test_debit_below_zero_is_409_and_no_change(client):
+    created = _post(client, "/api/clients", {"name": "Broke"})
+    cid = created.get_json()["id"]
+    resp = _post(client, f"/api/clients/{cid}/debits",
+                 {"amount_minor": 100, "idempotency_key": "d-lose"})
+    assert resp.status_code == 409
+    body = resp.get_json()
+    assert body["code"] == "insufficient_balance"
+    assert body["error"]
+    got = client.get(f"/api/clients/{cid}", headers=AUTH)
+    assert got.get_json()["balance"] == 0
+    # idempotency key must not be burned by a rejected debit
+    resp2 = _post(client, f"/api/clients/{cid}/credits",
+                  {"amount_minor": 50, "idempotency_key": "d-lose"})
+    assert resp2.status_code == 200
+    assert resp2.get_json()["balance"] == 50
+
+
+def test_duplicate_idempotency_key_no_double_apply(client):
+    created = _post(client, "/api/clients", {"name": "Dup"})
+    cid = created.get_json()["id"]
+    first = _post(client, f"/api/clients/{cid}/credits",
+                  {"amount_minor": 777, "idempotency_key": "same"})
+    second = _post(client, f"/api/clients/{cid}/credits",
+                   {"amount_minor": 777, "idempotency_key": "same"})
+    assert first.status_code == second.status_code == 200
+    assert first.get_json()["balance"] == second.get_json()["balance"] == 777
+    assert second.get_json().get("replayed") is True
+    got = client.get(f"/api/clients/{cid}", headers=AUTH)
+    assert got.get_json()["balance"] == 777
+    # exactly one credit row in the ledger
+    led = client.get(f"/api/clients/{cid}/ledger", headers=AUTH)
+    credits = [r for r in led.get_json()["rows"] if r["kind"] == "credit"]
+    assert len(credits) == 1
+
+
+def test_hold_insufficient_is_409(client):
+    created = _post(client, "/api/clients", {"name": "Tiny"})
+    cid = created.get_json()["id"]
+    resp = _post(client, f"/api/clients/{cid}/holds",
+                 {"amount_minor": 10, "idempotency_key": "h-x"})
+    assert resp.status_code == 409
+    assert resp.get_json()["code"] == "insufficient_balance"
+
+
+def test_hold_expiry_auto_releases(client, app):
+    created = _post(client, "/api/clients", {"name": "Expiry"})
+    cid = created.get_json()["id"]
+    _post(client, f"/api/clients/{cid}/credits",
+          {"amount_minor": 1000, "idempotency_key": "e1"})
+    held = _post(client, f"/api/clients/{cid}/holds",
+                 {"amount_minor": 400, "idempotency_key": "h2", "ttl_s": 60})
+    hold_id = held.get_json()["hold_id"]
+    assert client.get(f"/api/clients/{cid}",
+                      headers=AUTH).get_json()["balance"] == 600
+    # force expiry by rewriting timestamps back in time
+    core = app.config["CORE"]
+    past = (datetime.datetime.now(datetime.timezone.utc)
+            - datetime.timedelta(seconds=5)).isoformat()
+    with core._lock:
+        core._conn.execute("UPDATE holds SET expires_at = ? WHERE id = ?",
+                           (past, hold_id))
+        core._conn.commit()
+    got = client.get(f"/api/clients/{cid}", headers=AUTH)
+    assert got.get_json()["balance"] == 1000  # auto-released on read
+
+
+def test_release_missing_hold_is_404(client):
+    resp = _post(client, "/api/holds/999999/release",
+                 {"idempotency_key": "r-x"})
+    assert resp.status_code == 404
+    assert resp.get_json()["code"] == "hold_not_found"
+
+
+def test_unknown_client_is_404(client):
+    resp = client.get("/api/clients/424242", headers=AUTH)
+    assert resp.status_code == 404
+    assert resp.get_json()["code"] == "unknown_client"
+
+
+def test_ledger_filter_from_to(client):
+    created = _post(client, "/api/clients", {"name": "Filter"})
+    cid = created.get_json()["id"]
+    _post(client, f"/api/clients/{cid}/credits",
+          {"amount_minor": 10, "idempotency_key": "f1"})
+    _post(client, f"/api/clients/{cid}/credits",
+          {"amount_minor": 20, "idempotency_key": "f2"})
+    # from=epoch start -> both rows; to=something future -> both rows
+    led = client.get(f"/api/clients/{cid}/ledger?from=2000-01-01T00:00:00",
+                     headers=AUTH)
+    assert led.status_code == 200
+    assert led.get_json()["count"] == 2
+
+
+# --------------------------------------------------------------------------
+# Validation / error shape
+# --------------------------------------------------------------------------
+
+def test_errors_are_json_objects(client):
+    resp = _post(client, "/api/clients", {"amount_minor": "x"})  # bad body
+    assert resp.status_code == 400
+    body = resp.get_json()
+    assert set(body) == {"error", "code"}
+    assert isinstance(body["error"], str)
+
+
+def test_invalid_amount_is_400(client):
+    created = _post(client, "/api/clients", {"name": "Bad"})
+    cid = created.get_json()["id"]
+    resp = _post(client, f"/api/clients/{cid}/credits",
+                 {"amount_minor": -5, "idempotency_key": "neg"})
+    assert resp.status_code == 400
+    assert resp.get_json()["code"] == "invalid_request"
+
+
+def test_missing_idempotency_key_is_400(client):
+    created = _post(client, "/api/clients", {"name": "NoKey"})
+    cid = created.get_json()["id"]
+    resp = _post(client, f"/api/clients/{cid}/credits", {"amount_minor": 1})
+    assert resp.status_code == 400
+
+
+def test_create_client_requires_name(client):
+    resp = _post(client, "/api/clients", {"tier": "standard"})
+    assert resp.status_code == 400
+
+
+def test_duplicate_client_name_is_409(client):
+    first = _post(client, "/api/clients", {"name": "DupName"})
+    assert first.status_code == 201
+    second = _post(client, "/api/clients", {"name": "DupName"})
+    assert second.status_code == 409
+    assert second.get_json()["code"] == "duplicate_client"
+
+
+# --------------------------------------------------------------------------
+# Error paths the real core cannot produce on demand (fakes only here)
+# --------------------------------------------------------------------------
+
+def test_missing_core_op_is_501():
+    application = server.create_app(MissingOpCore(), API_KEY)
+    application.config.update(TESTING=True)
+    c = application.test_client()
+    resp = c.post("/api/clients/1/credits",
+                  json={"amount_minor": 100, "idempotency_key": "x"},
+                  headers=AUTH)
+    assert resp.status_code == 501
+    assert resp.get_json()["code"] == "not_implemented"
+
+
+def test_core_raise_is_500():
+    application = server.create_app(RaisingCore(), API_KEY)
+    application.config.update(TESTING=True)
+    c = application.test_client()
+    resp = c.post("/api/clients/1/credits",
+                  json={"amount_minor": 100, "idempotency_key": "x"},
+                  headers=AUTH)
+    assert resp.status_code == 500
+    assert resp.get_json()["code"] == "core_error"