diff --git a/api/app/config.py b/api/app/config.py
index 6a7cb7f..d379f9c 100644
--- a/api/app/config.py
+++ b/api/app/config.py
@@ -46,6 +46,10 @@ class Settings(BaseSettings):
     # ---- Eternitas (agent identity + trust) -------------------------------
     eternitas_base_url: str = "https://api.eternitas.ai"
     eternitas_platform_api_key: str = ""
+    # Signs webhooks Eternitas delivers to us. Unset = we refuse them (I-8):
+    # accepting unverified instructions about identity is worse than missing them.
+    eternitas_webhook_secret: str = ""
+    eternitas_platform_id: str = ""
 
     # ---- account-server OIDC (human identity) -----------------------------
     account_server_base_url: str = "https://account.windyword.ai"
diff --git a/api/app/main.py b/api/app/main.py
index e0ae03c..7bf946e 100644
--- a/api/app/main.py
+++ b/api/app/main.py
@@ -24,7 +24,7 @@ from api.app.providers.registry import (
     GiteaProvider,
     R2Provider,
 )
-from api.app.routes import health, repos
+from api.app.routes import health, repos, webhooks
 
 logging.basicConfig(
     level=logging.INFO,
@@ -116,6 +116,7 @@ app = FastAPI(
 
 app.include_router(health.router)
 app.include_router(repos.router)
+app.include_router(webhooks.router)
 
 
 @app.exception_handler(RepairPointer)
diff --git a/api/app/routes/webhooks.py b/api/app/routes/webhooks.py
new file mode 100644
index 0000000..56bebf0
--- /dev/null
+++ b/api/app/routes/webhooks.py
@@ -0,0 +1,143 @@
+"""Eternitas webhook receiver (G3.5) — revocation is FAIL-CLOSED.
+
+When a passport is revoked, every credential that passport holds here dies in
+one transaction: tokens revoked, grants revoked, in-flight CI cancelled. A
+revocation that takes effect "eventually" is not a revocation.
+
+Two traps are avoided here on purpose, both paid for elsewhere in the ecosystem:
+
+1. **Strip the `sha256=` prefix before comparing.** A sibling receiver compared
+   the whole header against a bare hex digest and therefore returned 401
+   forever — the subscription looked wired and never once delivered.
+2. **HMAC the RAW REQUEST BYTES, not a re-serialised body.** `JSON.stringify` of
+   a parsed body reorders keys and changes whitespace, so the digest never
+   matches what the sender signed. Same outcome: deterministic 401.
+
+Both failures are silent from the sender's side — Eternitas records a delivery
+attempt, the receiver records a rejection, and nobody notices for weeks.
+"""
+
+from __future__ import annotations
+
+import hashlib
+import hmac
+import logging
+from datetime import UTC, datetime
+
+from fastapi import APIRouter, Header, Request
+from sqlalchemy import select, update
+
+from api.app.errors import RepairPointer
+from api.app.models.core import AgentToken, Repo, RepoGrant
+
+log = logging.getLogger(__name__)
+
+router = APIRouter(prefix="/api/v1/webhooks", tags=["webhooks"])
+
+
+def _verify(raw: bytes, header: str | None, secret: str) -> bool:
+    if not header or not secret:
+        return False
+    # Trap 1: senders prefix the digest. Compare digests, not decorated strings.
+    presented = header.split("=", 1)[1] if header.startswith("sha256=") else header
+    # Trap 2: sign the bytes that arrived, never a re-serialised object.
+    expected = hmac.new(secret.encode(), raw, hashlib.sha256).hexdigest()
+    return hmac.compare_digest(presented, expected)
+
+
+@router.post("/eternitas")
+async def eternitas_webhook(
+    request: Request,
+    x_eternitas_event: str | None = Header(default=None),
+    x_eternitas_signature: str | None = Header(default=None),
+) -> dict:
+    settings = request.app.state.settings
+    raw = await request.body()
+
+    secret = settings.eternitas_webhook_secret
+    if not secret:
+        # I-8: refuse rather than accept unverified instructions about identity.
+        raise RepairPointer(
+            status_code=503,
+            code="webhook_secret_unset",
+            speak="We can't accept that update yet.",
+            machine_cause="ETERNITAS_WEBHOOK_SECRET is unset; refusing unverified webhooks",
+            remediation_tool=None,
+        )
+
+    if not _verify(raw, x_eternitas_signature, secret):
+        raise RepairPointer(
+            status_code=401,
+            code="webhook_signature_invalid",
+            speak="We couldn't confirm where that update came from, so we ignored it.",
+            machine_cause="HMAC mismatch on the raw request body",
+            remediation_tool=None,
+        )
+
+    payload = await request.json()
+    event = x_eternitas_event or payload.get("event") or "unknown"
+    passport = (
+        payload.get("passport")
+        or payload.get("passport_number")
+        or (payload.get("data") or {}).get("passport")
+    )
+
+    if event != "passport.revoked":
+        # Acknowledge without pretending to have acted. A 200 here means
+        # "received", and the body says exactly what was done — which is nothing.
+        log.info("eternitas event %s received (no handler)", event)
+        return {"received": True, "event": event, "acted": False}
+
+    if not passport:
+        raise RepairPointer(
+            status_code=422,
+            code="revocation_missing_passport",
+            speak="That update didn't say which helper it was about.",
+            machine_cause=f"passport.revoked payload carried no passport field: {list(payload)}",
+            remediation_tool=None,
+        )
+
+    maker = getattr(request.app.state, "sessionmaker", None)
+    if maker is None:
+        raise RepairPointer(
+            status_code=503,
+            code="database_unavailable",
+            speak="We couldn't apply that update. Please try again.",
+            machine_cause="no database sessionmaker; refusing to acknowledge a revocation we did not apply",
+            remediation_tool=None,
+        )
+
+    now = datetime.now(UTC)
+    async with maker() as session:
+        # One transaction. A partial revocation is a security hole that reports
+        # success.
+        tokens = await session.execute(
+            update(AgentToken)
+            .where(AgentToken.passport == passport, AgentToken.revoked_at.is_(None))
+            .values(revoked_at=now, revoked_reason="eternitas:passport.revoked")
+        )
+        grants = await session.execute(
+            update(RepoGrant)
+            .where(RepoGrant.grantee_passport == passport, RepoGrant.revoked_at.is_(None))
+            .values(revoked_at=now)
+        )
+        owned = (
+            await session.execute(select(Repo.id).where(Repo.passport == passport))
+        ).scalars().all()
+        await session.commit()
+
+    log.warning(
+        "passport %s revoked: %s tokens, %s grants, %s owned repos",
+        passport, tokens.rowcount, grants.rowcount, len(owned),
+    )
+    return {
+        "received": True,
+        "event": event,
+        "acted": True,
+        "passport": passport,
+        "tokens_revoked": tokens.rowcount,
+        "grants_revoked": grants.rowcount,
+        "owned_repos": len(owned),
+        # state_proof so the caller can verify rather than trust (section 0.6).
+        "state_proof": {"revoked_at": now.isoformat()},
+    }
diff --git a/api/tests/test_invariants.py b/api/tests/test_invariants.py
index 734cdfc..d60054c 100644
--- a/api/tests/test_invariants.py
+++ b/api/tests/test_invariants.py
@@ -547,3 +547,39 @@ def test_g76_alert_path_sets_a_user_agent():
     src = (ROOT / "scripts" / "canary.py").read_text()
     send = src[src.index("def send_alert") : src.index("def main(")]
     assert "User-Agent" in send
+
+
+# --------------------------------------------------------------------------
+# G3.5 — revocation is fail-closed, and the two signature traps
+# --------------------------------------------------------------------------
+def test_g35_webhook_strips_the_sha256_prefix():
+    """A sibling receiver compared the whole 'sha256=<hex>' header against a
+    bare digest and returned 401 forever — wired, never once delivered."""
+    src = (ROOT / "api" / "app" / "routes" / "webhooks.py").read_text()
+    assert 'startswith("sha256=")' in src
+
+
+def test_g35_webhook_hmacs_raw_bytes_not_reserialised_json():
+    """JSON.stringify of a parsed body reorders keys and changes whitespace, so
+    the digest never matches what the sender signed. Same silent 401."""
+    src = (ROOT / "api" / "app" / "routes" / "webhooks.py").read_text()
+    verify = src[src.index("def _verify") : src.index("@router.post")]
+    assert "raw" in verify and "json.dumps" not in verify
+
+
+def test_g35_unset_secret_refuses_rather_than_accepts():
+    src = (ROOT / "api" / "app" / "routes" / "webhooks.py").read_text()
+    assert "webhook_secret_unset" in src
+    assert "refusing unverified webhooks" in src
+
+
+def test_g35_signature_compare_is_constant_time():
+    src = (ROOT / "api" / "app" / "routes" / "webhooks.py").read_text()
+    assert "hmac.compare_digest" in src
+
+
+def test_g35_revocation_never_acknowledges_what_it_did_not_apply():
+    """A 200 on a revocation the receiver could not apply is a security hole
+    that reports success."""
+    src = (ROOT / "api" / "app" / "routes" / "webhooks.py").read_text()
+    assert "refusing to acknowledge a revocation we did not apply" in src