Add none chunking strategy to disable automatic chunking for inference endpoints

This introduces a `none` chunking strategy that disables automatic chunking when using an inference endpoint.
It enables users to provide pre-chunked input directly to a `semantic_text` field without any additional splitting.

The chunking strategy can be configured either on the inference endpoint or directly in the `semantic_text` field definition.

**Example:**

```json
PUT test-index
{
  "mappings": {
    "properties": {
      "my_semantic_field": {
        "type": "semantic_text",
        "chunking_settings": {
          "strategy": "none"    <1>
        }
      }
    }
  }
}
```

<1> Disables automatic chunking on `my_semantic_field`.

```json
PUT test-index/_doc/1
{
    "my_semantic_field": ["my first chunk", "my second chunk", ...]    <1>
    ...
}
```

<1> Pre-chunked input provided as an array of strings.
Each array element represents a single chunk that will be sent directly to the inference service without further processing.

Author: jimczi

docs/reference/elasticsearch/mapping-reference/semantic-text.md

@@ -117,15 +117,16 @@ If specified, these will override the chunking settings set in the {{infer-cap}}
 endpoint associated with `inference_id`.
 If chunking settings are updated, they will not be applied to existing documents
 until they are reindexed.
+To completely disable chunking, use the `none` chunking strategy.
 
     **Valid values for `chunking_settings`**:
 
     `type`
-    :   Indicates the type of chunking strategy to use. Valid values are `word` or
+    :   Indicates the type of chunking strategy to use. Valid values are `none`, `word` or
     `sentence`. Required.
 
     `max_chunk_size`
-    :   The maximum number of works in a chunk. Required.
+    :   The maximum number of works in a chunk. Required for `word` and `sentence` strategies.
 
     `overlap`
     :   The number of overlapping words allowed in chunks. This cannot be defined as
@@ -136,6 +137,12 @@ until they are reindexed.
     :   The number of overlapping sentences allowed in chunks. Valid values are `0`
     or `1`. Required for `sentence` type chunking settings
 
+::::{warning}
+When using the `none` chunking strategy, if the input exceeds the maximum token limit of the underlying model,
+some services (such as OpenAI) may return an error. In contrast, the elastic and elasticsearch services
+will automatically truncate the input to fit within the model's limit.
+::::
+
 ## {{infer-cap}} endpoint validation [infer-endpoint-validation]
 
 The `inference_id` will not be validated when the mapping is created, but when
@@ -166,10 +173,49 @@ For more details on chunking and how to configure chunking settings,
 see [Configuring chunking](https://www.elastic.co/docs/api/doc/elasticsearch/group/endpoint-inference)
 in the Inference API documentation.
 
+You can pre-chunk the input by sending it to Elasticsearch as an array of strings.
+Example:
+
+```console
+PUT test-index
+{
+  "mappings": {
+    "properties": {
+      "my_semantic_field": {
+        "type": "semantic_text",
+        "chunking_settings": {
+          "strategy": "none"    <1>
+        }
+      }
+    }
+  }
+}
+```
+
+1. Disable chunking on `my_semantic_field`.
+
+```console
+PUT test-index/_doc/1
+{
+    "my_semantic_field": ["my first chunk", "my second chunk", ...]    <1>
+    ...
+}
+```
+
+1. The text is pre-chunked and provided as an array of strings.
+   Each element in the array represents a single chunk that will be sent directly to the inference service without further chunking.
+
+**Important considerations**:
+
+* When providing pre-chunked input, ensure that you set the chunking strategy to `none` to avoid additional processing.
+* Each chunk should be sized carefully, staying within the token limit of the inference service and the underlying model.
+* If a chunk exceeds the model's token limit, the behavior depends on the service:
+  * Some services (such as OpenAI) will return an error.
+  * Others (such as `elastic` and `elasticsearch`) will automatically truncate the input.
+
 Refer
 to [this tutorial](docs-content://solutions/search/semantic-search/semantic-search-semantic-text.md)
-to learn more about semantic search using `semantic_text` and the `semantic`
-query.
+to learn more about semantic search using `semantic_text`.
 
 ## Extracting Relevant Fragments from Semantic Text [semantic-text-highlighting]
 

server/src/main/java/org/elasticsearch/TransportVersions.java

@@ -288,6 +288,7 @@ static TransportVersion def(int id) {
     public static final TransportVersion ML_INFERENCE_MISTRAL_CHAT_COMPLETION_ADDED = def(9_090_0_00);
     public static final TransportVersion IDP_CUSTOM_SAML_ATTRIBUTES_ALLOW_LIST = def(9_091_0_00);
     public static final TransportVersion SEARCH_SOURCE_EXCLUDE_VECTORS_PARAM = def(9_092_0_00);
+    public static final TransportVersion NONE_CHUNKING_STRATEGY = def(9_093_0_00);
     /*
      * STOP! READ THIS FIRST! No, really,
      *        ____ _____ ___  ____  _        ____  _____    _    ____    _____ _   _ ___ ____    _____ ___ ____  ____ _____ _

server/src/main/java/org/elasticsearch/inference/ChunkingStrategy.java

@@ -15,7 +15,8 @@
 
 public enum ChunkingStrategy {
     WORD("word"),
-    SENTENCE("sentence");
+    SENTENCE("sentence"),
+    NONE("none");
 
     private final String chunkingStrategy;
 

x-pack/plugin/inference/qa/test-service-plugin/src/main/java/org/elasticsearch/xpack/inference/mock/AbstractTestInferenceService.java

@@ -126,7 +126,9 @@ protected List<ChunkedInput> chunkInputs(ChunkInferenceInput input) {
         }
 
         List<ChunkedInput> chunkedInputs = new ArrayList<>();
-        if (chunkingSettings.getChunkingStrategy() == ChunkingStrategy.WORD) {
+        if (chunkingSettings.getChunkingStrategy() == ChunkingStrategy.NONE) {
+            return List.of(new ChunkedInput(inputText, 0, inputText.length()));
+        } else if (chunkingSettings.getChunkingStrategy() == ChunkingStrategy.WORD) {
             WordBoundaryChunker chunker = new WordBoundaryChunker();
             WordBoundaryChunkingSettings wordBoundaryChunkingSettings = (WordBoundaryChunkingSettings) chunkingSettings;
             List<WordBoundaryChunker.ChunkOffset> offsets = chunker.chunk(

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

@@ -26,6 +26,7 @@
 import org.elasticsearch.xpack.core.inference.results.TextEmbeddingByteResults;
 import org.elasticsearch.xpack.core.inference.results.TextEmbeddingFloatResults;
 import org.elasticsearch.xpack.inference.action.task.StreamingTaskManager;
+import org.elasticsearch.xpack.inference.chunking.NoneChunkingSettings;
 import org.elasticsearch.xpack.inference.chunking.SentenceBoundaryChunkingSettings;
 import org.elasticsearch.xpack.inference.chunking.WordBoundaryChunkingSettings;
 import org.elasticsearch.xpack.inference.common.amazon.AwsSecretSettings;
@@ -552,6 +553,9 @@ private static void addInternalNamedWriteables(List<NamedWriteableRegistry.Entry
     }
 
     private static void addChunkingSettingsNamedWriteables(List<NamedWriteableRegistry.Entry> namedWriteables) {
+        namedWriteables.add(
+            new NamedWriteableRegistry.Entry(ChunkingSettings.class, NoneChunkingSettings.NAME, in -> NoneChunkingSettings.INSTANCE)
+        );
         namedWriteables.add(
             new NamedWriteableRegistry.Entry(ChunkingSettings.class, WordBoundaryChunkingSettings.NAME, WordBoundaryChunkingSettings::new)
         );

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/chunking/ChunkerBuilder.java

@@ -16,6 +16,7 @@ public static Chunker fromChunkingStrategy(ChunkingStrategy chunkingStrategy) {
         }
 
         return switch (chunkingStrategy) {
+            case NONE -> NoopChunker.INSTANCE;
             case WORD -> new WordBoundaryChunker();
             case SENTENCE -> new SentenceBoundaryChunker();
         };

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/chunking/ChunkingSettingsBuilder.java

@@ -45,6 +45,7 @@ public static ChunkingSettings fromMap(Map<String, Object> settings, boolean ret
             settings.get(ChunkingSettingsOptions.STRATEGY.toString()).toString()
         );
         return switch (chunkingStrategy) {
+            case NONE -> NoneChunkingSettings.INSTANCE;
             case WORD -> WordBoundaryChunkingSettings.fromMap(new HashMap<>(settings));
             case SENTENCE -> SentenceBoundaryChunkingSettings.fromMap(new HashMap<>(settings));
         };

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/chunking/NoneChunkingSettings.java

@@ -0,0 +1,104 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+package org.elasticsearch.xpack.inference.chunking;
+
+import org.elasticsearch.TransportVersion;
+import org.elasticsearch.TransportVersions;
+import org.elasticsearch.common.Strings;
+import org.elasticsearch.common.ValidationException;
+import org.elasticsearch.common.io.stream.StreamInput;
+import org.elasticsearch.common.io.stream.StreamOutput;
+import org.elasticsearch.inference.ChunkingSettings;
+import org.elasticsearch.inference.ChunkingStrategy;
+import org.elasticsearch.xcontent.XContentBuilder;
+
+import java.io.IOException;
+import java.util.Arrays;
+import java.util.Locale;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Set;
+
+public class NoneChunkingSettings implements ChunkingSettings {
+    public static final String NAME = "NoneChunkingSettings";
+    public static NoneChunkingSettings INSTANCE = new NoneChunkingSettings();
+
+    private static final ChunkingStrategy STRATEGY = ChunkingStrategy.NONE;
+    private static final Set<String> VALID_KEYS = Set.of(ChunkingSettingsOptions.STRATEGY.toString());
+
+    private NoneChunkingSettings() {}
+
+    public NoneChunkingSettings(StreamInput in) throws IOException {}
+
+    @Override
+    public ChunkingStrategy getChunkingStrategy() {
+        return STRATEGY;
+    }
+
+    @Override
+    public String getWriteableName() {
+        return NAME;
+    }
+
+    @Override
+    public TransportVersion getMinimalSupportedVersion() {
+        return TransportVersions.NONE_CHUNKING_STRATEGY;
+    }
+
+    @Override
+    public void writeTo(StreamOutput out) throws IOException {}
+
+    @Override
+    public Map<String, Object> asMap() {
+        return Map.of(ChunkingSettingsOptions.STRATEGY.toString(), STRATEGY.toString().toLowerCase(Locale.ROOT));
+    }
+
+    public static NoneChunkingSettings fromMap(Map<String, Object> map) {
+        ValidationException validationException = new ValidationException();
+
+        var invalidSettings = map.keySet().stream().filter(key -> VALID_KEYS.contains(key) == false).toArray();
+        if (invalidSettings.length > 0) {
+            validationException.addValidationError(
+                Strings.format("Sentence based chunking settings can not have the following settings: %s", Arrays.toString(invalidSettings))
+            );
+        }
+
+        if (validationException.validationErrors().isEmpty() == false) {
+            throw validationException;
+        }
+
+        return new NoneChunkingSettings();
+    }
+
+    @Override
+    public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+        builder.startObject();
+        {
+            builder.field(ChunkingSettingsOptions.STRATEGY.toString(), STRATEGY);
+        }
+        builder.endObject();
+        return builder;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        return true;
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(getClass());
+    }
+
+    @Override
+    public String toString() {
+        return Strings.toString(this);
+    }
+}

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/chunking/NoopChunker.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+package org.elasticsearch.xpack.inference.chunking;
+
+import org.elasticsearch.common.Strings;
+import org.elasticsearch.inference.ChunkingSettings;
+import org.elasticsearch.xpack.inference.services.openai.embeddings.OpenAiEmbeddingsModel;
+
+import java.util.List;
+
+/**
+ * A {@link Chunker} implementation that returns the input unchanged (no chunking is performed).
+ *
+ * <p><b>WARNING</b>If the input exceeds the maximum token limit, some services (such as {@link OpenAiEmbeddingsModel})
+ * may return an error.
+ * </p>
+ */
+public class NoopChunker implements Chunker {
+    static final NoopChunker INSTANCE = new NoopChunker();
+
+    private NoopChunker() {}
+
+    @Override
+    public List<ChunkOffset> chunk(String input, ChunkingSettings chunkingSettings) {
+        if (chunkingSettings instanceof NoneChunkingSettings) {
+            return List.of(new ChunkOffset(0, input.length()));
+        } else {
+            throw new IllegalArgumentException(
+                Strings.format("NoopChunker can't use ChunkingSettings with strategy [%s]", chunkingSettings.getChunkingStrategy())
+            );
+        }
+    }
+}

x-pack/plugin/inference/src/test/java/org/elasticsearch/xpack/inference/chunking/ChunkerBuilderTests.java

@@ -27,6 +27,13 @@ public void testValidChunkingStrategy() {
     }
 
     private Map<ChunkingStrategy, Class<? extends Chunker>> chunkingStrategyToExpectedChunkerClassMap() {
-        return Map.of(ChunkingStrategy.WORD, WordBoundaryChunker.class, ChunkingStrategy.SENTENCE, SentenceBoundaryChunker.class);
+        return Map.of(
+            ChunkingStrategy.NONE,
+            NoopChunker.class,
+            ChunkingStrategy.WORD,
+            WordBoundaryChunker.class,
+            ChunkingStrategy.SENTENCE,
+            SentenceBoundaryChunker.class
+        );
     }
 }