diff --git a/.github/workflows/validate-pr.yml b/.github/workflows/validate-pr.yml
index 703518796f..638c22b2d4 100644
--- a/.github/workflows/validate-pr.yml
+++ b/.github/workflows/validate-pr.yml
@@ -18,7 +18,7 @@ jobs:
     if: github.repository_owner == 'elastic' # this action will fail if executed from a fork
     runs-on: ubuntu-latest
     env:
-      STACK_VERSION: 8.6-SNAPSHOT
+      STACK_VERSION: 8.9-SNAPSHOT
 
     steps:
       - uses: actions/checkout@v2
diff --git a/output/schema/schema.json b/output/schema/schema.json
index ce39ff4acf..a4b9d0523d 100644
--- a/output/schema/schema.json
+++ b/output/schema/schema.json
@@ -101930,10 +101930,19 @@
           "name": "read_only",
           "required": false,
           "type": {
+            "generics": [
+              {
+                "kind": "instance_of",
+                "type": {
+                  "name": "boolean",
+                  "namespace": "_builtins"
+                }
+              }
+            ],
             "kind": "instance_of",
             "type": {
-              "name": "boolean",
-              "namespace": "_builtins"
+              "name": "Stringified",
+              "namespace": "_spec_utils"
             }
           }
         },
@@ -101941,10 +101950,19 @@
           "name": "read_only_allow_delete",
           "required": false,
           "type": {
+            "generics": [
+              {
+                "kind": "instance_of",
+                "type": {
+                  "name": "boolean",
+                  "namespace": "_builtins"
+                }
+              }
+            ],
             "kind": "instance_of",
             "type": {
-              "name": "boolean",
-              "namespace": "_builtins"
+              "name": "Stringified",
+              "namespace": "_spec_utils"
             }
           }
         },
@@ -101952,10 +101970,19 @@
           "name": "read",
           "required": false,
           "type": {
+            "generics": [
+              {
+                "kind": "instance_of",
+                "type": {
+                  "name": "boolean",
+                  "namespace": "_builtins"
+                }
+              }
+            ],
             "kind": "instance_of",
             "type": {
-              "name": "boolean",
-              "namespace": "_builtins"
+              "name": "Stringified",
+              "namespace": "_spec_utils"
             }
           }
         },
@@ -101963,23 +101990,20 @@
           "name": "write",
           "required": false,
           "type": {
-            "items": [
+            "generics": [
               {
                 "kind": "instance_of",
                 "type": {
                   "name": "boolean",
                   "namespace": "_builtins"
                 }
-              },
-              {
-                "kind": "instance_of",
-                "type": {
-                  "name": "string",
-                  "namespace": "_builtins"
-                }
               }
             ],
-            "kind": "union_of"
+            "kind": "instance_of",
+            "type": {
+              "name": "Stringified",
+              "namespace": "_spec_utils"
+            }
           }
         },
         {
diff --git a/output/typescript/types.ts b/output/typescript/types.ts
index 08eaa62746..81e5d8218e 100644
--- a/output/typescript/types.ts
+++ b/output/typescript/types.ts
@@ -9593,10 +9593,10 @@ export interface IndicesIndexSegmentSort {
 }
 
 export interface IndicesIndexSettingBlocks {
-  read_only?: boolean
-  read_only_allow_delete?: boolean
-  read?: boolean
-  write?: boolean | string
+  read_only?: SpecUtilsStringified<boolean>
+  read_only_allow_delete?: SpecUtilsStringified<boolean>
+  read?: SpecUtilsStringified<boolean>
+  write?: SpecUtilsStringified<boolean>
   metadata?: SpecUtilsStringified<boolean>
 }
 
diff --git a/specification/indices/_types/IndexSettings.ts b/specification/indices/_types/IndexSettings.ts
index a36bc91e1a..4126f37387 100644
--- a/specification/indices/_types/IndexSettings.ts
+++ b/specification/indices/_types/IndexSettings.ts
@@ -243,10 +243,10 @@ export class SettingsQueryString {
 }
 
 export class IndexSettingBlocks {
-  read_only?: boolean
-  read_only_allow_delete?: boolean
-  read?: boolean
-  write?: boolean | string // TODO: should be bool only
+  read_only?: Stringified<boolean>
+  read_only_allow_delete?: Stringified<boolean>
+  read?: Stringified<boolean>
+  write?: Stringified<boolean>
   metadata?: Stringified<boolean>
 }