diff --git a/api/tests/test_webhooks_behavior.py b/api/tests/test_webhooks_behavior.py
new file mode 100644
index 0000000..3f056bd
--- /dev/null
+++ b/api/tests/test_webhooks_behavior.py
@@ -0,0 +1,106 @@
+"""G3.5 — the Eternitas webhook receiver, driven over HTTP (audit 2026-08-13).
+
+The G3.5 invariants in test_invariants.py grep webhooks.py for strings; a
+refactor that kept the strings and broke the behaviour would pass them all.
+These send real requests through the real route (no DB: every case here stops
+before the revocation handler) and assert what the receiver DOES.
+"""
+
+from __future__ import annotations
+
+import hashlib
+import hmac
+import json
+
+import httpx
+import pytest
+from fastapi import FastAPI
+from fastapi.responses import JSONResponse
+
+from api.app.config import Settings
+from api.app.errors import RepairPointer
+from api.app.routes import webhooks
+
+SECRET = "s" * 64
+URL = "/api/v1/webhooks/eternitas"
+
+
+def _app(secret: str = SECRET) -> FastAPI:
+    app = FastAPI()
+    app.include_router(webhooks.router)
+    app.state.settings = Settings(eternitas_webhook_secret=secret)
+
+    @app.exception_handler(RepairPointer)
+    async def _h(_, exc: RepairPointer) -> JSONResponse:
+        return JSONResponse(status_code=exc.status_code, content=exc.detail)
+
+    return app
+
+
+async def _post(body: bytes, headers: dict, secret: str = SECRET) -> httpx.Response:
+    transport = httpx.ASGITransport(app=_app(secret))
+    async with httpx.AsyncClient(transport=transport, base_url="http://t") as c:
+        return await c.post(
+            URL, content=body, headers={"content-type": "application/json", **headers}
+        )
+
+
+def _sig(raw: bytes, secret: str = SECRET) -> str:
+    return hmac.new(secret.encode(), raw, hashlib.sha256).hexdigest()
+
+
+# Deliberately odd spacing/key order: a receiver that re-serialises before
+# hashing produces a different digest and must fail.
+RAW = b'{"event":"windygit.selftest",  "data": {"b": 2, "a": 1}}'
+EVENT = {"x-eternitas-event": "windygit.selftest"}
+
+
+@pytest.mark.asyncio
+async def test_prefixed_and_bare_digests_are_both_accepted():
+    for header in (f"sha256={_sig(RAW)}", _sig(RAW)):
+        r = await _post(RAW, {**EVENT, "x-eternitas-signature": header})
+        assert r.status_code == 200, r.text
+        assert r.json()["acted"] is False  # unknown event: received, nothing done
+
+
+@pytest.mark.asyncio
+async def test_digest_of_reserialised_json_is_refused():
+    reserialised = json.dumps(json.loads(RAW)).encode()
+    assert reserialised != RAW
+    r = await _post(RAW, {**EVENT, "x-eternitas-signature": f"sha256={_sig(reserialised)}"})
+    assert r.status_code == 401 and r.json()["code"] == "webhook_signature_invalid"
+
+
+@pytest.mark.asyncio
+async def test_forged_or_wrong_key_signature_is_refused():
+    for header in ("sha256=" + "0" * 64, f"sha256={_sig(RAW, 'other-secret')}", "garbage"):
+        r = await _post(RAW, {**EVENT, "x-eternitas-signature": header})
+        assert r.status_code == 401, header
+
+
+@pytest.mark.asyncio
+async def test_signed_event_without_signature_is_refused():
+    r = await _post(RAW, EVENT)
+    assert r.status_code == 401
+
+
+@pytest.mark.asyncio
+async def test_unset_secret_refuses_rather_than_accepts():
+    r = await _post(RAW, {**EVENT, "x-eternitas-signature": f"sha256={_sig(RAW)}"}, secret="")
+    assert r.status_code == 503 and r.json()["code"] == "webhook_secret_unset"
+
+
+@pytest.mark.asyncio
+async def test_revocation_with_bad_signature_never_reaches_the_handler():
+    body = b'{"event":"passport.revoked","passport":"ET26-TEST-GOOD"}'
+    r = await _post(
+        body,
+        {"x-eternitas-event": "passport.revoked", "x-eternitas-signature": "sha256=" + "f" * 64},
+    )
+    assert r.status_code == 401  # refused before any DB work
+
+
+@pytest.mark.asyncio
+async def test_reachability_ping_acknowledges_but_never_acts():
+    r = await _post(b'{"anything": "at all"}', {"x-eternitas-event": "platform.test_ping"})
+    assert r.status_code == 200 and r.json()["acted"] is False