diff --git a/apps/compute-gateway/tests/test_masking_policy_values.py b/apps/compute-gateway/tests/test_masking_policy_values.py
new file mode 100644
index 00000000..dd1234fc
--- /dev/null
+++ b/apps/compute-gateway/tests/test_masking_policy_values.py
@@ -0,0 +1,115 @@
+"""The DEPLOYED masking policy must actually mask.
+
+These tests read `GATEWAY_MASKING_POLICY` out of deploy/values/compute-gateway.yaml — the
+literal string the cluster will receive — rather than a fixture written to agree with them.
+A policy that is only asserted to be correct in a test fixture proves nothing about the one
+that ships, and this whole layer exists because the policy was previously absent and the PDP
+silently returned outputs unchanged. A YAML typo must fail CI, not quietly revert the gateway
+to passthrough.
+"""
+from __future__ import annotations
+
+import json
+import sys
+from pathlib import Path
+
+import pytest
+import yaml
+
+ROOT = Path(__file__).resolve().parents[3]
+VALUES = ROOT / "deploy" / "values" / "compute-gateway.yaml"
+sys.path.insert(0, str(ROOT / "apps" / "compute-gateway" / "src"))
+
+from compute_gateway import masking  # noqa: E402
+from compute_gateway.contract import ComputeOutput  # noqa: E402
+
+
+@pytest.fixture(scope="module")
+def deployed_policy() -> dict:
+    raw = yaml.safe_load(VALUES.read_text(encoding="utf-8"))["config"]["GATEWAY_MASKING_POLICY"]
+    return json.loads(raw)
+
+
+@pytest.fixture(autouse=True)
+def _install_policy(monkeypatch, deployed_policy):
+    monkeypatch.setenv("GATEWAY_MASKING_POLICY", json.dumps(deployed_policy))
+    monkeypatch.delenv("GATEWAY_MASKING_POLICIES", raising=False)
+
+
+def _records(*recs) -> list[ComputeOutput]:
+    """The PDP walks `data["nodes"]` and nothing else — see test_masking_coverage_boundary."""
+    return [ComputeOutput(type="graph", data={"nodes": [dict(r) for r in recs]})]
+
+
+def test_policy_is_parseable_and_non_empty(deployed_policy):
+    """The failure this guards is a YAML/JSON typo silently disabling enforcement."""
+    assert deployed_policy["mask_fields"], "an empty mask_fields set is passthrough with extra steps"
+    assert deployed_policy["forbidden_mixtures"], "the no_health_adtech veto must be configured"
+    assert deployed_policy.get("policy_version"), "a decision with no policy version is unattributable"
+
+
+def test_direct_identifiers_are_masked_not_returned_raw(deployed_policy):
+    out = masking.apply(
+        _records({"email": "ada@example.com", "ssn": "123-45-6789", "city": "Cambridge"}),
+        kind="graph-query", project="default", actor="analyst", entitlement=None)
+    rec = out[0].data["nodes"][0]
+    assert rec["email"] != "ada@example.com", "email left in cleartext by the deployed policy"
+    assert rec["ssn"] != "123-45-6789", "ssn left in cleartext by the deployed policy"
+    # A masking policy that redacts everything is useless; non-identifiers must survive.
+    assert rec["city"] == "Cambridge"
+
+
+def test_the_masking_decision_is_emitted_as_a_sealed_output():
+    """The decision IS the evidence — that is the whole claim against WKC-style masking,
+    whose enforcement leaves no verifiable artifact."""
+    out = masking.apply(_records({"email": "ada@example.com"}),
+                        kind="graph-query", project="default", actor="analyst", entitlement=None)
+    decisions = [o for o in out if o.type == "masking-decision"]
+    assert len(decisions) == 1, "no masking-decision output — the enforcement left no artifact"
+    d = decisions[0].data
+    assert d["schema_version"] == "identity-prime.masking-decision.v1"
+    assert d["verdict"] == "allow_masked"
+    assert d["applied_transforms"], "verdict claims masking but names no transformed field"
+
+
+def test_forbidden_mixture_withholds_the_records_entirely():
+    """no_health_adtech is a veto, not a masking rule: the records are withheld, not masked."""
+    out = masking.apply(
+        _records({"topic": "health", "email": "ada@example.com"}),
+        kind="graph-query", project="default", actor="adtech-bot", entitlement="adtech")
+    assert len(out) == 1 and out[0].type == "masking-decision", \
+        "records survived a forbidden identity mixture — the veto did not withhold them"
+    assert out[0].data["verdict"] == "deny"
+    assert out[0].data["forbidden_mixture"]
+
+
+def test_unknown_scheme_fails_closed(monkeypatch, deployed_policy):
+    """Fail-closed under a bad policy: an unrecognised scheme must redact, never pass raw
+    through. A typo in a scheme name is otherwise an invisible hole."""
+    broken = json.loads(json.dumps(deployed_policy))
+    broken["mask_fields"]["email"] = "not-a-real-scheme"
+    monkeypatch.setenv("GATEWAY_MASKING_POLICY", json.dumps(broken))
+    out = masking.apply(_records({"email": "ada@example.com"}),
+                        kind="graph-query", project="default", actor="analyst", entitlement=None)
+    assert out[0].data["nodes"][0]["email"] != "ada@example.com"
+
+
+def test_non_read_kinds_are_untouched():
+    """The PDP governs reads. A write path must not be silently rewritten by it."""
+    assert "notebook" not in masking.READ_KINDS
+
+
+def test_masking_coverage_boundary_is_nodes_only():
+    """KNOWN LIMIT, asserted so it cannot be mistaken for coverage.
+
+    masking._iter_records reads `data["nodes"]` and returns [] for anything else, so a result
+    shaped as rows / table / edges is returned UNMASKED even with this policy active. That is
+    the current boundary of the moat, not a claim about it. Widening it means changing
+    _iter_records, which is a larger change than turning the policy on; this test exists so
+    the next person reads the boundary from a failing assertion rather than from an incident.
+    """
+    rows = [ComputeOutput(type="table", data={"rows": [{"email": "ada@example.com"}]})]
+    out = masking.apply(rows, kind="graph-query", project="default",
+                        actor="analyst", entitlement=None)
+    assert out[0].data["rows"][0]["email"] == "ada@example.com", \
+        "row-shaped outputs are now masked — good; update this test and the PR note"
diff --git a/deploy/values/compute-gateway.yaml b/deploy/values/compute-gateway.yaml
index 98550b45..08f34fa7 100644
--- a/deploy/values/compute-gateway.yaml
+++ b/deploy/values/compute-gateway.yaml
@@ -21,6 +21,29 @@ config:
   COMPUTE_ENTITLEMENTS: "demo,default,graph-query,graph-stats,gyg-reporting,materialize,governance,engine-seal"
   # every ok run writes its ComputeRun/Receipt subgraph to hellgraph (best-effort).
   GATEWAY_WRITE_PROVENANCE: "true"
+  # READ-PATH MASKING PDP — ON. Until now this was code that never ran: masking.py has been
+  # mounted in the dispatch path since it landed, but with no policy configured it returned
+  # outputs unchanged, so the moat existed and was switched off. The 2026-08-04 catalog/masking
+  # audit found GATEWAY_MASKING_POLICY set nowhere in deploy/ — not a missing mechanism, an
+  # unused one.
+  #
+  # Applies to graph-query/graph-stats reads only (masking.READ_KINDS). Direct identifiers are
+  # masked before the outputs are sealed, so the Ed25519 receipt attests exactly what the
+  # caller received, and the masking decision is appended as a sealed output — the property
+  # WKC-style dynamic masking does not produce, because there the enforcement leaves no
+  # verifiable artifact.
+  #
+  # forbidden_mixtures encodes the no_health_adtech veto: a requester whose realm carries the
+  # adtech topic reading records that carry the health topic is DENIED outright and the records
+  # are withheld, not merely masked.
+  #
+  # The exact JSON below is parsed out of this file and exercised by
+  # apps/compute-gateway/tests/test_masking_policy_values.py, so a typo here fails CI rather
+  # than silently reverting the gateway to passthrough. A policy nobody tests is a policy that
+  # is one YAML mistake away from being off again.
+  GATEWAY_MASKING_POLICY: >-
+    {"policy_version":"masking-v1-2026-08-04","decided_by":"prophet-platform/compute-gateway","record_topic_fields":["topic","domain"],"requesting_realm_topics":{"adtech":"adtech","marketing":"adtech"},"forbidden_mixtures":[["health","adtech"]],"mask_fields":{"email":"hmac_pseudonym","email_address":"hmac_pseudonym","phone":"hmac_pseudonym","msisdn":"hmac_pseudonym","ssn":"one_way_hash","national_id":"one_way_hash","patient_mrn":"chameleon_token","date_of_birth":"generalize","dob":"generalize","full_name":"redact","street_address":"redact"}}
+
   # Durable proof store: receipts + content-addressed artifacts + the doc→SQL sink land on
   # the PVC mounted here, so a governed run survives a restart (compute-gateway persistence).
   GATEWAY_STORE_DIR: "/data"