diff --git a/argocd/platform-addons/ops-mason-builder/clusterrole.yaml b/argocd/platform-addons/ops-mason-builder/clusterrole.yaml
new file mode 100644
index 0000000..9b9f8fa
--- /dev/null
+++ b/argocd/platform-addons/ops-mason-builder/clusterrole.yaml
@@ -0,0 +1,29 @@
+# Scoped ops-mason builder (founder decision 2026-10-22, MASON-IN-0006 (b)).
+# Kinds kubernetes_plane applies, plus get/list/watch on services for dependency
+# reads, plus create on self-subject access reviews so kubectl auth can-i works.
+# Excluded: pods, secrets, configmaps, serviceaccounts/token, impersonate,
+# roles, rolebindings, clusterroles, clusterrolebindings, nodes/proxy, pods/exec,
+# escalate, bind, wildcards.
+apiVersion: rbac.authorization.k8s.io/v1
+kind: ClusterRole
+metadata:
+  name: ops-mason-builder
+  labels:
+    app.kubernetes.io/part-of: railiance-gitops
+    railiance-platform/component: ops-mason-builder
+rules:
+  - apiGroups: [""]
+    resources: ["namespaces"]
+    verbs: ["create", "get", "list", "watch", "update", "patch", "delete"]
+  - apiGroups: [""]
+    resources: ["serviceaccounts"]
+    verbs: ["create", "get", "list", "watch", "update", "patch", "delete"]
+  - apiGroups: ["networking.k8s.io"]
+    resources: ["networkpolicies"]
+    verbs: ["create", "get", "list", "watch", "update", "patch", "delete"]
+  - apiGroups: [""]
+    resources: ["services"]
+    verbs: ["get", "list", "watch"]
+  - apiGroups: ["authorization.k8s.io"]
+    resources: ["selfsubjectaccessreviews", "selfsubjectrulesreviews"]
+    verbs: ["create"]
diff --git a/argocd/platform-addons/ops-mason-builder/clusterrolebinding.yaml b/argocd/platform-addons/ops-mason-builder/clusterrolebinding.yaml
new file mode 100644
index 0000000..41ec5d6
--- /dev/null
+++ b/argocd/platform-addons/ops-mason-builder/clusterrolebinding.yaml
@@ -0,0 +1,15 @@
+apiVersion: rbac.authorization.k8s.io/v1
+kind: ClusterRoleBinding
+metadata:
+  name: ops-mason-builder
+  labels:
+    app.kubernetes.io/part-of: railiance-gitops
+    railiance-platform/component: ops-mason-builder
+roleRef:
+  apiGroup: rbac.authorization.k8s.io
+  kind: ClusterRole
+  name: ops-mason-builder
+subjects:
+  - kind: User
+    name: ops-mason-builder
+    apiGroup: rbac.authorization.k8s.io
diff --git a/argocd/platform-addons/ops-mason-builder/kustomization.yaml b/argocd/platform-addons/ops-mason-builder/kustomization.yaml
new file mode 100644
index 0000000..a37d052
--- /dev/null
+++ b/argocd/platform-addons/ops-mason-builder/kustomization.yaml
@@ -0,0 +1,11 @@
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+
+# MASON-IN-0006 choice (b), decision 30d29060-8d14-4a8e-9cde-34aa826eb8d1.
+# Scoped builder for ops-mason. User ops-mason-builder, no ServiceAccount token,
+# no Secret. Applied by argocd/railiance01/applications/ops-mason-builder.application.yaml.
+# Manual sync. targetRevision is pinned to the commit that introduced these manifests.
+
+resources:
+  - clusterrole.yaml
+  - clusterrolebinding.yaml
diff --git a/tests/test_ops_mason_builder_rbac.py b/tests/test_ops_mason_builder_rbac.py
new file mode 100644
index 0000000..1ab0f33
--- /dev/null
+++ b/tests/test_ops_mason_builder_rbac.py
@@ -0,0 +1,89 @@
+"""Offline guard for the ops-mason builder identity (MASON-IN-0006 choice (b)).
+
+Parses the GitOps manifests and asserts the founder's 2026-10-22 boundary:
+the five granted rules only, no forbidden resources, no escalate/bind/impersonate,
+no wildcards, and a User subject with no ServiceAccount.
+"""
+from pathlib import Path
+
+import yaml
+
+ROOT = Path(__file__).resolve().parents[1]
+ADDON = ROOT / "argocd/platform-addons/ops-mason-builder"
+
+WRITE_VERBS = ["create", "get", "list", "watch", "update", "patch", "delete"]
+READ_VERBS = ["get", "list", "watch"]
+FORBIDDEN_RESOURCES = {
+    "secrets", "pods", "serviceaccounts/token", "configmaps", "roles",
+    "rolebindings", "clusterroles", "clusterrolebindings", "nodes",
+    "nodes/proxy", "pods/exec", "pods/log",
+}
+FORBIDDEN_VERBS = {"escalate", "bind", "impersonate"}
+EXPECTED_RULES = [
+    {"apiGroups": [""], "resources": ["namespaces"], "verbs": WRITE_VERBS},
+    {"apiGroups": [""], "resources": ["serviceaccounts"], "verbs": WRITE_VERBS},
+    {"apiGroups": ["networking.k8s.io"], "resources": ["networkpolicies"],
+     "verbs": WRITE_VERBS},
+    {"apiGroups": [""], "resources": ["services"], "verbs": READ_VERBS},
+    {"apiGroups": ["authorization.k8s.io"],
+     "resources": ["selfsubjectaccessreviews", "selfsubjectrulesreviews"],
+     "verbs": ["create"]},
+]
+
+
+def _docs():
+    kust = yaml.safe_load((ADDON / "kustomization.yaml").read_text())
+    docs = []
+    for res in kust["resources"]:
+        docs.extend(d for d in yaml.safe_load_all((ADDON / res).read_text()) if d)
+    return docs
+
+
+def _one(kind):
+    found = [d for d in _docs() if d["kind"] == kind]
+    assert len(found) == 1, kind
+    return found[0]
+
+
+def test_only_expected_kinds_no_secret():
+    kinds = sorted(d["kind"] for d in _docs())
+    assert kinds == ["ClusterRole", "ClusterRoleBinding"]
+    for path in ADDON.iterdir():
+        if path.is_file():
+            assert "kind: Secret" not in path.read_text(), path
+
+
+def test_cluster_role_exact_rules_no_wildcards():
+    role = _one("ClusterRole")
+    assert role["metadata"]["name"] == "ops-mason-builder"
+    assert "aggregationRule" not in role
+    assert role["rules"] == EXPECTED_RULES
+    granted = set()
+    for rule in role["rules"]:
+        values = [*rule["verbs"], *rule.get("apiGroups", []), *rule.get("resources", [])]
+        assert not any("*" in v for v in values), rule
+        assert FORBIDDEN_VERBS.isdisjoint(rule["verbs"]), rule
+        for resource in rule["resources"]:
+            assert resource not in FORBIDDEN_RESOURCES, resource
+            granted.add(resource)
+    assert granted == {
+        "namespaces", "serviceaccounts", "networkpolicies", "services",
+        "selfsubjectaccessreviews", "selfsubjectrulesreviews",
+    }
+    by_resource = {r["resources"][0]: r["verbs"] for r in role["rules"] if len(r["resources"]) == 1}
+    assert by_resource["namespaces"] == WRITE_VERBS
+    assert by_resource["serviceaccounts"] == WRITE_VERBS
+    assert by_resource["networkpolicies"] == WRITE_VERBS
+    assert by_resource["services"] == READ_VERBS
+    review = next(r for r in role["rules"] if "selfsubjectaccessreviews" in r["resources"])
+    assert review["verbs"] == ["create"]
+    assert review["resources"] == ["selfsubjectaccessreviews", "selfsubjectrulesreviews"]
+
+
+def test_binding_targets_only_the_user():
+    binding = _one("ClusterRoleBinding")
+    assert binding["roleRef"]["name"] == "ops-mason-builder"
+    assert binding["roleRef"]["kind"] == "ClusterRole"
+    assert binding["subjects"] == [{"kind": "User", "name": "ops-mason-builder",
+                                    "apiGroup": "rbac.authorization.k8s.io"}]
+    assert all(s["kind"] != "ServiceAccount" for s in binding["subjects"])