Update the-protocol.mdx

Author: pascalbaljet

v2/core-concepts/the-protocol.mdx

@@ -143,6 +143,10 @@ The following headers are automatically sent by Inertia when making requests. Yo
   Indicates whether the requested data should be appended or prepended when using [Infinite scroll](/v2/data-props/infinite-scroll).
 </ParamField>
 
+<ParamField header="X-Inertia-Page-Once-Props" type="string">
+  Comma-separated list of non-expired [once prop](/v2/data-props/once-props) keys already loaded on the client. The server will skip resolving these props unless explicitly requested via a partial reload or force refreshed server-side.
+</ParamField>
+
 ## Response Headers
 
 The following headers should be sent by your server-side adapter in Inertia responses. If you're using an official server-side adapter, these are handled automatically.
@@ -211,6 +215,10 @@ Inertia shares data between the server and client via a page object. This object
   Configuration for client-side [lazy loading of props](/v2/data-props/deferred-props).
 </ParamField>
 
+<ParamField body="onceProps" type="object">
+  Configuration for [once props](/v2/data-props/once-props) that should only be resolved once and reused on subsequent pages. Each entry maps a key to an object containing the `prop` name and optional `expiresAt` timestamp (in milliseconds).
+</ParamField>
+
 On standard full page visits, the page object is JSON encoded into the `data-page` attribute in the root `<div>`. On Inertia visits (as indicated by the presence of the `X-Inertia` header), the page object is returned as the JSON payload.
 
 ### Basic Page Object
@@ -360,6 +368,78 @@ When using [Infinite scroll](/v2/data-props/infinite-scroll), the page object in
 }
 ```
 
+### Page Object with Once Props
+
+When using [once props](/v2/data-props/once-props), the page object includes an `onceProps` configuration. Each entry maps a key to the prop name and an optional expiration timestamp.
+
+```json
+{
+    "component": "Billing/Plans",
+    "props": {
+        "errors": {},
+        "plans": [
+            {
+                "id": 1,
+                "name": "Basic"
+            },
+            {
+                "id": 2,
+                "name": "Pro"
+            }
+        ]
+    },
+    "url": "/billing/plans",
+    "version": "6b16b94d7c51cbe5b1fa42aac98241d5",
+    "clearHistory": false,
+    "encryptHistory": false,
+    "onceProps": {
+        "plans": {
+            "prop": "plans",
+            "expiresAt": null
+        }
+    }
+}
+```
+
+When navigating to a subsequent page that includes the same once prop, the client sends the loaded keys in the `X-Inertia-Page-Once-Props` header. The server skips resolving these props and excludes them from the response. The client reuses the previously loaded values.
+
+```http
+REQUEST
+GET: https://example.com/billing/upgrade
+Accept: text/html, application/xhtml+xml
+X-Requested-With: XMLHttpRequest
+X-Inertia: true
+X-Inertia-Version: 6b16b94d7c51cbe5b1fa42aac98241d5
+X-Inertia-Page-Once-Props: plans
+
+RESPONSE
+HTTP/1.1 200 OK
+Content-Type: application/json
+
+{
+    "component": "Billing/Upgrade",
+    "props": {
+        "errors": {},
+        "currentPlan": {
+            "id": 1,
+            "name": "Basic"
+        }
+    },
+    "url": "/billing/upgrade",
+    "version": "6b16b94d7c51cbe5b1fa42aac98241d5",
+    "clearHistory": false,
+    "encryptHistory": false,
+    "onceProps": {
+        "plans": {
+            "prop": "plans",
+            "expiresAt": null
+        }
+    }
+}
+```
+
+Note that `plans` is included in `onceProps` but not in `props` since it was already loaded on the client. The `onceProps` key identifies the once prop across pages, while `prop` specifies the actual prop name. These may differ when using [custom keys](/v2/data-props/once-props#custom-keys).
+
 ## Asset Versioning
 
 One common challenge with single-page apps is refreshing site assets when they've been changed. Inertia makes this easy by optionally tracking the current version of the site's assets. In the event that an asset changes, Inertia will automatically make a full-page visit instead of an XHR visit.