Document Mustache template support in Search Relevance Workbench

_search-plugins/search-relevance/query-sets.md

@@ -85,6 +85,7 @@ All query sets comprise one or more entries. Each entry is a JSON object contain
 | :--- | :--- | :--- |
 | `queryText` | String | The user query string. Required. |
 | `referenceAnswer` | String | The expected or correct answer to the user query. This field is used for generating judgments, especially with large language models (LLMs). Optional. |
+| Custom fields | String | Any additional field (beyond `queryText` and `referenceAnswer`) is stored as a custom field on the entry. Custom fields are available as [Mustache template variables]({{site.url}}{{site.baseurl}}/search-plugins/search-relevance/search-configurations/#using-mustache-templates) in a search configuration, referenced by field name. Optional. |
 
 ### Basic query set example
 
@@ -117,6 +118,20 @@ This format includes the `referenceAnswer` field alongside the `queryText`. It i
 
 The `referenceAnswer` field is particularly useful when using [LLMs to generate judgments]({{site.url}}{{site.baseurl}}/search-plugins/search-relevance/judgments/). The LLM can use the reference answer as a ground truth to compare against the retrieved search results, allowing it to accurately score the relevance of the response.
 
+### Query set with custom fields example
+
+In addition to `queryText`, each entry can include arbitrary custom fields. These values are available as Mustache template variables in a [search configuration]({{site.url}}{{site.baseurl}}/search-plugins/search-relevance/search-configurations/#using-mustache-templates), letting a single search configuration vary its filters or parameters for each query.
+
+#### Example query set with custom fields
+
+```json
+{"queryText": "phone", "category": "electronics"}
+{"queryText": "steel", "category": "materials"}
+{"queryText": "keyboard", "category": "electronics"}
+```
+
+For example, a search configuration whose query references `{{category}}` filters each query's results by the category value provided in the corresponding entry.
+
 ## Managing query sets
 
 You can retrieve or delete query sets using the following APIs.

_search-plugins/search-relevance/search-configurations.md

@@ -29,7 +29,7 @@
 :---  | :--- | :---
 `name` | String | The name of the search configuration.
 `description` | String | Description of the search configuration.
-`query` | Object | Defines the query in OpenSearch query DSL. Use `%SearchText%` as a placeholder for the user query. Needs to be escaped.
+`query` | Object | Defines the query in OpenSearch query DSL. Use the `%SearchText%` placeholder or [Mustache](https://mustache.github.io/) template variables (such as `{{queryText}}`) to substitute query set values at runtime. Needs to be escaped. For more information, see [Using Mustache templates](#using-mustache-templates).
 `index` | String | The target index queried by this search configuration.
 `searchPipeline` | String | Specifies an existing search pipeline. Optional.
 
@@ -45,6 +45,34 @@
 }
 ```
 
+### Using Mustache templates
+
+Instead of the `%SearchText%` placeholder, a query can use [Mustache](https://mustache.github.io/) template variables. If the `query` contains `{{`, it is rendered as a Mustache template; otherwise, the `%SearchText%` placeholder is substituted. Existing `%SearchText%` configurations continue to work unchanged.
+
+The following variables are available when a template is rendered:
+
+* `{{queryText}}`: The `queryText` of the current query set entry.
+* Any custom field defined on the query set entry, referenced by its field name (for example, `{{category}}`). For more information, see [Query sets]({{site.url}}{{site.baseurl}}/search-plugins/search-relevance/query-sets/).
+
+Substituted values are automatically JSON-escaped, so query text containing quotation marks or other special characters does not break the query.
+
+Mustache partials (`{{>...}}`) are not supported and cause the query to be rejected.
+{: .note}
+
+#### Example request: Creating a search configuration that uses a Mustache template
+
+The following request creates a search configuration that matches the user query against the `title` field and filters results by a `category` custom field provided by each query set entry:
+
+```json
+PUT _plugins/_search_relevance/search_configurations
+{
+  "name": "mustache_filter",
+  "query": "{\"query\":{\"bool\":{\"must\":[{\"match\":{\"title\":\"{{queryText}}\"}}],\"filter\":[{\"term\":{\"category\":\"{{category}}\"}}]}}}",
+  "description": "Title match filtered by a category custom field",
+  "index": "ecommerce"
+}
+```
+
 ## Managing search configurations
 
 You can retrieve or delete configurations using the following APIs.