You can preview and debug your server-side container in Tag Manager. ## Understand the preview layout To open the debugger, in Tag Manager select your server container and click **Preview**. The left panel lists incoming HTTP requests, e.g. `collect?v=2&....`. If the client that claimed the request created an event data object, e.g. `scroll`, `page_view`, `add_to_cart`, this event displays as a sub item under the request. The tabs on the right include details about the item selected in the left panel. Each tab is discussed in a separate section. > [!NOTE] > **Note:** You can click any box in a tab to open a slider panel with full details. ![Screenshot showing Tag Manager in preview mode](https://developers.google.com/static/tag-platform/tag-manager/server-side/img/debug/tag-manager-preview-layout.png) > [!NOTE] > **Note:** Google Analytics bundles events within one request and you may see more than one event under a top-level incoming request (`collect?v=...`). These events represent all events sent in that request. However, when you run your client-side container in preview mode at the same time, the hits are not bundled. ## Debug requests The **Request** tab contains boxes for the: - Client that claimed the request - Outgoing HTTP requests - Incoming HTTP request ![Screenshot showing Tag Manager Request tab](https://developers.google.com/static/tag-platform/tag-manager/server-side/img/debug/tag-manager-preview-request-tab.png) ### Check the right client claimed the request Server containers can have many clients, but only one client can claim a request. The correct client recognizes the format of the request from the browser and claims it if there is a match. Click the **Client** box to check the client's details. This information should correspond with the client configuration in **Clients \> *Client name***. ![Screenshot showing Tag Manager client details](https://developers.google.com/static/tag-platform/tag-manager/server-side/img/debug/tag-manager-preview-client-details.png) > [!NOTE] > **Note:** The client that claimed the request is not necessarily the client that generated the event data. ### Check incoming requests Click the **Incoming HTTP Request** box to view the full details of the incoming request. The request (`collect?v=...`) should be a 1:1 representation of what you see in the Chrome network tab. Confirm that what you see in the browser is a match and that the server container is receiving the right parameters. ![Screenshot showing Tag Manager incoming request details](https://developers.google.com/static/tag-platform/tag-manager/server-side/img/debug/tag-manager-preview-incoming-request-details.png) > [!NOTE] > **Note:** AppEngine appends additional headers that do not show up in the browser. Other cloud services may also append their own data. ### Check outgoing requests Click any **Outgoing HTTP Requests** box to view details about the: - Tag that generated the request - Outgoing request - HTTP response returned by the vendor endpoint The outgoing request details show the data sent by the server container to the vendor endpoint. ![Screenshot showing Tag Manager outgoing request details](https://developers.google.com/static/tag-platform/tag-manager/server-side/img/debug/tag-manager-preview-outgoing-request-details.png) The Response shows the status code, headers and body (if any) returned by the vendor endpoint. ![Screenshot showing Tag Manager outgoing response details](https://developers.google.com/static/tag-platform/tag-manager/server-side/img/debug/tag-manager-preview-outgoing-request-response.png) ## Debug tags The **Tags** tab lists all configured tags: - **Tags Fired**: Tags fired by the event. Each box indicates whether the tag failed or fired successfully, and the number of times the tag fired. - **Tags Not Fired**: Available tags that did not fire. ![Screenshot showing Tag Manager Tags tab](https://developers.google.com/static/tag-platform/tag-manager/server-side/img/debug/tag-manager-preview-tags-tab.png) Click any box to view the tag properties, outgoing HTTP requests and firing triggers. ![Screenshot showing Tag Manager tag details](https://developers.google.com/static/tag-platform/tag-manager/server-side/img/debug/tag-manager-preview-tags-detail.png) Read [How to build a server tag](https://developers.google.com/tag-platform/tag-manager/server-side/how-to-build-a-server-tag) for more information about configuring and implementing server tags. ## Debug variables The **Variables** tag lists the event's variables and includes the variable type, return type, and value. For example, the screenshot below for a `user-engagement` event shows the `var_screen_resolution` variable set to "1536x864". ![Screenshot showing Tag Manager Variables tab](https://developers.google.com/static/tag-platform/tag-manager/server-side/img/debug/tag-manager-preview-variables-tab.png) The data used to set the variable values is shown in the [**Event Data** tab](https://developers.google.com/tag-platform/tag-manager/server-side/debug#debug_event_data). Read [Variables](https://support.google.com/tagmanager/topic/7683268) for more information about configuring [built-in variables](https://support.google.com/tagmanager/topic/7182737) and [user-defined variables](https://support.google.com/tagmanager/topic/9125128). ## Debug event data The **Event Data** tab shows full details about the event. This data is used to set the variable values in the [**Variables** tab](https://developers.google.com/tag-platform/tag-manager/server-side/debug#debug_variables). ![Screenshot showing Tag Manager Event Data tab](https://developers.google.com/static/tag-platform/tag-manager/server-side/img/debug/tag-manager-preview-event-data-tab.png) ## Console The **Console** tab shows all errors originating from fired tags. The log includes each error message and its source, e.g. an invalid endpoint or target API call. ![Screenshot showing Tag Manager console](https://developers.google.com/static/tag-platform/tag-manager/server-side/img/debug/tag-manager-preview-console.png)