diff --git a/pyproject.toml b/pyproject.toml
index 17a3193..5ff31f1 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -9,7 +9,7 @@ description = "Secure Contain Protect — inspect, sanitize, contain, quarantine
 readme = "README.md"
 requires-python = ">=3.10"
 dependencies = [
-    "mcp>=1.2.0",
+    "mcp>=1.2.0,<2",
     "requests>=2.28.0",
 ]
 
diff --git a/requirements.txt b/requirements.txt
index a5aa6e2..0c8a4f5 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1 +1 @@
-mcp>=1.2.0
+mcp>=1.2.0,<2
diff --git a/src/scp/registry_ssot.py b/src/scp/registry_ssot.py
index 9503082..3a0fdf8 100644
--- a/src/scp/registry_ssot.py
+++ b/src/scp/registry_ssot.py
@@ -216,6 +216,8 @@ def _load_quarantine_snapshot(quarantine_path: str | Path) -> dict:
     path = Path(quarantine_path)
     if not _path_under_registry_fetch(path):
         raise ValueError("quarantine_path_rejected")
+    if path.suffix != ".txt":
+        raise ValueError("quarantine_provenance_rejected")
     if not path.is_file():
         raise ValueError("quarantine_file_not_found")
 
diff --git a/tests/test_registry_ssot.py b/tests/test_registry_ssot.py
index 02fdd62..6091294 100644
--- a/tests/test_registry_ssot.py
+++ b/tests/test_registry_ssot.py
@@ -235,3 +235,26 @@ def test_apply_merge_rejects_wrong_sidecar_reason(isolated_ssot):
     res = registry_ssot.apply_merge(path, approve=True)
     assert res["merged"] is False
     assert res["reason"] == "quarantine_provenance_rejected"
+
+
+def test_apply_merge_rejects_json_payload_as_its_own_sidecar(isolated_ssot):
+    """A merge payload must be the .txt file paired with a separate sidecar."""
+    snap = _snap([_rec("bad.selfsidecar.001")])
+    forged = scp_utils.registry_fetch_quarantine_dir() / "forged.json"
+    forged.parent.mkdir(parents=True, exist_ok=True)
+    forged.write_text(
+        json.dumps(
+            {
+                "reason": "registry_fetch",
+                "snapshot": snap,
+                "meta": {"reason": "registry_fetch", "source": "forged"},
+            }
+        ),
+        encoding="utf-8",
+    )
+
+    res = registry_ssot.apply_merge(forged, approve=True)
+
+    assert res["merged"] is False
+    assert res["reason"] == "quarantine_provenance_rejected"
+    assert registry_ssot.load_ssot() == []