diff --git a/tests/test_release_contract.py b/tests/test_release_contract.py
new file mode 100644
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--- /dev/null
+++ b/tests/test_release_contract.py
@@ -0,0 +1,118 @@
+"""Release contract (DESIGN 3, 9.4 and 13): what a release promises its users.
+
+The generated per-operation tests cover requests and responses, and
+``test_contract.py`` covers the operation surface. These tests cover the two
+promises the release itself makes and that nothing else checks:
+
+- every webhook event name the released OpenAPI document accepts is a delivery
+  the SDK verifies, returns and keeps whole (DESIGN 9.3, 9.4);
+- the version a release publishes is the same in every file release-please
+  updates, because ``release.yml`` refuses a tag that disagrees with it.
+"""
+
+from __future__ import annotations
+
+import hashlib
+import hmac
+import importlib
+import json
+import pathlib
+import sys
+from typing import Any
+
+import pytest
+
+import didit
+from didit import webhooks
+from didit.webhooks import UnknownEvent, canonical_json
+from didit.webhooks._events import _BY_TYPE  # pyright: ignore[reportPrivateUsage]
+
+# Python 3.11 has tomllib; 3.10 reads the same files with tomli (a mypy dependency there).
+tomllib: Any = importlib.import_module("tomllib" if sys.version_info >= (3, 11) else "tomli")
+
+ROOT = pathlib.Path(__file__).resolve().parents[1]
+SPEC = json.loads((ROOT / "openapi" / "verification.json").read_text("utf-8"))
+RELEASED_EVENTS: list[str] = SPEC["components"]["schemas"]["WebhookEvent"]["enum"]
+SECRET = "whsec_test_release_contract_0001"
+NOW = 1_760_702_400
+
+# The events DESIGN 9.4 types, plus the registry-search event it names there.
+# Everything else the released document accepts arrives as UnknownEvent, which
+# keeps the whole body in ``payload`` (DESIGN 9.4).
+TYPED_EVENTS = {
+    "status.updated",
+    "data.updated",
+    "user.status.updated",
+    "user.data.updated",
+    "business.status.updated",
+    "business.data.updated",
+    "activity.created",
+    "transaction.created",
+    "transaction.status.updated",
+    "travel_rule.status.updated",
+    "workflow.compliance_changed",
+    "kyb.registry_search.resolved",
+}
+
+
+def _sign(message: bytes) -> str:
+    return hmac.new(SECRET.encode(), message, hashlib.sha256).hexdigest()
+
+
+def _delivery(webhook_type: str) -> tuple[bytes, dict[str, str], dict[str, Any]]:
+    """A delivery signed the way service-didit-verification signs one."""
+    payload: dict[str, Any] = {
+        "application_id": "0b5e8d2c-1f4a-4c3b-9d6e-2a7f8c1b3e5d",
+        "created_at": NOW,
+        "environment": "sandbox",
+        "event_id": "7c9e6679-7425-40de-944b-e07fc1f90ae7",
+        "timestamp": NOW,
+        "webhook_type": webhook_type,
+        "vendor_user_id": "user-1",
+        "vendor_data": "vendor-1",
+        "status": "Approved",
+    }
+    body = json.dumps(payload, sort_keys=True, separators=(",", ":")).encode()
+    headers = {
+        "X-Signature": _sign(body),
+        "X-Signature-V2": _sign(canonical_json(payload).encode()),
+        "X-Timestamp": str(NOW),
+    }
+    return body, headers, payload
+
+
+@pytest.mark.parametrize("webhook_type", RELEASED_EVENTS)
+def test_every_released_webhook_event_verifies(
+    webhook_type: str, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    """A customer may subscribe to any released event; each one verifies and is whole."""
+    monkeypatch.setattr("didit.webhooks._verify.time.time", lambda: float(NOW))
+    body, headers, payload = _delivery(webhook_type)
+
+    event = webhooks.verify(body, headers, SECRET)
+
+    assert event.webhook_type == webhook_type
+    assert event.body_authenticated is True
+    if isinstance(event, UnknownEvent):
+        assert event.payload == payload
+    else:
+        assert webhook_type in TYPED_EVENTS
+        assert getattr(event, "vendor_data", None) == "vendor-1"
+
+
+def test_typed_events_are_the_design_set() -> None:
+    """The hand-written typed union is exactly DESIGN 9.4, so a new one is a design change."""
+    assert set(_BY_TYPE) == TYPED_EVENTS
+    assert TYPED_EVENTS - {"kyb.registry_search.resolved"} <= set(RELEASED_EVENTS)
+
+
+def test_version_contract_agrees_across_release_files() -> None:
+    """release.yml refuses a tag that disagrees with these files; keep them in step."""
+    pyproject = tomllib.loads((ROOT / "pyproject.toml").read_text("utf-8"))
+    manifest = json.loads((ROOT / ".release-please-manifest.json").read_text("utf-8"))
+    lock = tomllib.loads((ROOT / "uv.lock").read_text("utf-8"))
+    locked = [p for p in lock["package"] if p.get("name") == "didit-sdk"]
+
+    assert len(locked) == 1
+    assert didit.__version__ == pyproject["project"]["version"] == manifest["."]
+    assert locked[0]["version"] == didit.__version__