Default semantic_text fields to ELSER on EIS when available

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/mapper/SemanticTextFieldMapper.java

@@ -108,6 +108,7 @@
 import static org.elasticsearch.inference.TaskType.TEXT_EMBEDDING;
 import static org.elasticsearch.lucene.search.uhighlight.CustomUnifiedHighlighter.MULTIVAL_SEP_CHAR;
 import static org.elasticsearch.search.SearchService.DEFAULT_SIZE;
+import static org.elasticsearch.xpack.inference.services.elastic.ElasticInferenceService.DEFAULT_ELSER_ENDPOINT_ID_V2;
 import static org.elasticsearch.xpack.inference.mapper.SemanticTextField.CHUNKED_EMBEDDINGS_FIELD;
 import static org.elasticsearch.xpack.inference.mapper.SemanticTextField.CHUNKED_OFFSET_FIELD;
 import static org.elasticsearch.xpack.inference.mapper.SemanticTextField.CHUNKING_SETTINGS_FIELD;
@@ -153,9 +154,22 @@ public class SemanticTextFieldMapper extends FieldMapper implements InferenceFie
 
     public static final String CONTENT_TYPE = "semantic_text";
     public static final String DEFAULT_ELSER_2_INFERENCE_ID = DEFAULT_ELSER_ID;
+    private static final String EIS_ELSER_INFERENCE_ID = DEFAULT_ELSER_ENDPOINT_ID_V2;
 
     public static final float DEFAULT_RESCORE_OVERSAMPLE = 3.0f;
 
+    /**
+     * Determines the preferred ELSER inference ID based on EIS availability.
+     * Returns .elser-2-elastic (EIS) when available, otherwise falls back to .elser-2-elasticsearch (ML nodes).
+     * This enables automatic selection of EIS for better performance while maintaining compatibility with on-prem deployments.
+     */
+    private static String getPreferredElserInferenceId(ModelRegistry modelRegistry) {
+        if (modelRegistry != null && modelRegistry.containsDefaultConfigId(EIS_ELSER_INFERENCE_ID)) {
+            return EIS_ELSER_INFERENCE_ID;
+        }
+        return DEFAULT_ELSER_ID;
+    }
+
     static final String INDEX_OPTIONS_FIELD = "index_options";
 
     public static final TypeParser parser(Supplier<ModelRegistry> modelRegistry) {
@@ -242,7 +256,7 @@ public Builder(
                 INFERENCE_ID_FIELD,
                 false,
                 mapper -> ((SemanticTextFieldType) mapper.fieldType()).inferenceId,
-                DEFAULT_ELSER_2_INFERENCE_ID
+                getPreferredElserInferenceId(modelRegistry)
             ).addValidator(v -> {
                 if (Strings.isEmpty(v)) {
                     throw new IllegalArgumentException(

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/services/elastic/ElasticInferenceService.java

@@ -115,7 +115,7 @@ public class ElasticInferenceService extends SenderService {
 
     // elser-2
     static final String DEFAULT_ELSER_2_MODEL_ID = "elser_model_2";
-    static final String DEFAULT_ELSER_ENDPOINT_ID_V2 = defaultEndpointId("elser-2");
+    public static final String DEFAULT_ELSER_ENDPOINT_ID_V2 = defaultEndpointId("elser-2");
 
     // multilingual-text-embed
     static final String DEFAULT_MULTILINGUAL_EMBED_MODEL_ID = "multilingual-embed-v1";

x-pack/plugin/inference/src/test/java/org/elasticsearch/xpack/inference/mapper/SemanticTextFieldMapperTests.java

@@ -298,6 +298,44 @@ public void testDefaults() throws Exception {
         assertTrue(fields.isEmpty());
     }
 
+    public void testDynamicElserDefaultSelection() throws Exception {
+        final String fieldName = "field";
+        final XContentBuilder fieldMapping = fieldMapping(this::minimalMapping);
+
+        // Test 1: When EIS is available, should default to .elser-2-elastic
+        when(globalModelRegistry.containsDefaultConfigId(".elser-2-elastic")).thenReturn(true);
+        MapperService mapperServiceWithEis = createMapperService(fieldMapping, useLegacyFormat);
+        assertInferenceEndpoints(mapperServiceWithEis, fieldName, ".elser-2-elastic", ".elser-2-elastic");
+
+        // Test 2: When EIS is not available, should fallback to .elser-2-elasticsearch
+        when(globalModelRegistry.containsDefaultConfigId(".elser-2-elastic")).thenReturn(false);
+        MapperService mapperServiceWithoutEis = createMapperService(fieldMapping, useLegacyFormat);
+        assertInferenceEndpoints(mapperServiceWithoutEis, fieldName, DEFAULT_ELSER_2_INFERENCE_ID, DEFAULT_ELSER_2_INFERENCE_ID);
+    }
+
+    public void testDynamicElserDefaultSelectionEdgeCases() throws Exception {
+        final String fieldName = "field";
+        final XContentBuilder fieldMapping = fieldMapping(this::minimalMapping);
+
+        // Test: ModelRegistry throws exception - should fallback gracefully
+        when(globalModelRegistry.containsDefaultConfigId(".elser-2-elastic")).thenThrow(new RuntimeException("Registry error"));
+        MapperService mapperServiceWithError = createMapperService(fieldMapping, useLegacyFormat);
+        assertInferenceEndpoints(mapperServiceWithError, fieldName, DEFAULT_ELSER_2_INFERENCE_ID, DEFAULT_ELSER_2_INFERENCE_ID);
+    }
+
+    public void testExplicitInferenceIdOverridesDynamicSelection() throws Exception {
+        final String fieldName = "field";
+        final String explicitInferenceId = "my-custom-model";
+        final XContentBuilder fieldMapping = fieldMapping(
+            b -> b.field("type", "semantic_text").field(INFERENCE_ID_FIELD, explicitInferenceId)
+        );
+
+        // Even when EIS is available, explicit inference_id should take precedence
+        when(globalModelRegistry.containsDefaultConfigId(".elser-2-elastic")).thenReturn(true);
+        MapperService mapperService = createMapperService(fieldMapping, useLegacyFormat);
+        assertInferenceEndpoints(mapperService, fieldName, explicitInferenceId, explicitInferenceId);
+    }
+
     @Override
     public void testFieldHasValue() {
         MappedFieldType fieldType = getMappedFieldType();