diff --git a/tide/normalize.go b/tide/normalize.go
index 07ee71d..b847447 100644
--- a/tide/normalize.go
+++ b/tide/normalize.go
@@ -55,6 +55,16 @@ func maskLeaf(path string, val any, step Step, diffs *[]Diff) any {
 	if key == "slug" || disabledPath(step, path, key) {
 		return val
 	}
+	if key == "collection_key" {
+		if val == nil {
+			return nil
+		}
+		if _, ok := val.(string); !ok {
+			*diffs = append(*diffs, Diff{Path: path, Expected: "string collection_key", Actual: formatValue(val)})
+			return val
+		}
+		return maskID
+	}
 	if isDateKey(key) {
 		return maskDate(path, val, diffs)
 	}
@@ -97,7 +107,7 @@ func maskIDValue(path string, val any, diffs *[]Diff) any {
 }
 
 func isDateKey(key string) bool {
-	return strings.HasSuffix(key, "_at")
+	return strings.HasSuffix(key, "_at") || key == "checkpoint"
 }
 
 func isIDKey(key string) bool {
diff --git a/tide/normalize_test.go b/tide/normalize_test.go
index 944de80..bc59445 100644
--- a/tide/normalize_test.go
+++ b/tide/normalize_test.go
@@ -20,6 +20,11 @@ func TestNormalizeDateIDAndDisable(t *testing.T) {
 	if diffs := compareBodies(Response{Headers: jsonCT(), Body: Body(nullID)}, Response{Headers: jsonCT(), Body: Body(nullID)}, step); len(diffs) != 0 {
 		t.Fatalf("null *_id must pass: %+v", diffs)
 	}
+	syncA := []byte(`{"collection_key":"aaa","checkpoint":"2026-01-01T00:00:00+00:00","total_estimate":0}`)
+	syncB := []byte(`{"collection_key":"bbb","checkpoint":"2026-02-02T00:00:00+00:00","total_estimate":0}`)
+	if diffs := compareBodies(Response{Headers: jsonCT(), Body: Body(syncA)}, Response{Headers: jsonCT(), Body: Body(syncB)}, step); len(diffs) != 0 {
+		t.Fatalf("collection_key/checkpoint must mask: %+v", diffs)
+	}
 	zdiffs := compareBodies(Response{Headers: jsonCT(), Body: Body(want)}, Response{Headers: jsonCT(), Body: Body(gotZ)}, step)
 	if len(zdiffs) == 0 {
 		t.Fatal("Z date must fail")