Updates to the documentation.

Co-authored-by: Copilot <copilot@github.com>

documentation/GUIDE__AE_API_V3_for_Frontend.md

@@ -102,6 +102,22 @@ Modify data in the system.
     *   **Header:** `x-ae-ignore-extra-fields: true`
     *   **Behavior:** When set to `true`, the backend will automatically strip any fields from the payload that are not defined in the object's model before attempting to save to the database.
 
+#### `*_json` field serialization — do NOT pre-stringify in route/component code
+
+The frontend API wrappers (`src/lib/ae_api/api_post__crud_obj.ts` for V3, `src/lib/api/api.ts` for legacy CRUD) automatically serialize any field whose name ends in `_json` (e.g. `cfg_json`, `data_json`) before sending. They pretty-print with 2-space indent via an internal `serialize_json_field_pretty()` helper.
+
+**Pass `*_json` fields as plain JS objects from routes and components.** The serialization layer handles the rest.
+
+```ts
+// ✅ Correct — pass as plain object; V3 wrapper serializes it
+await update_ae_obj__site({ site_id, data_kv: { cfg_json: { jitsi_token_endpoint: url } } });
+
+// ❌ Wrong — double-encodes the JSON string (the wrapper would stringify an already-stringified value)
+await update_ae_obj__site({ site_id, data_kv: { cfg_json: JSON.stringify({ jitsi_token_endpoint: url }) } });
+```
+
+The V3 wrapper (`api_post__crud_obj.ts`) only serializes when `typeof value === 'object'`, so it will not double-encode a plain string. The legacy wrapper (`api.ts`) stringifies unconditionally, so pre-stringifying there **will** produce double-encoded JSON. In both cases, the right answer is to pass the raw object and let the layer handle it.
+
 ### D. ID Fields in Responses (Vision ID Convention)
 
 > [!IMPORTANT]