Support Gemini thinking budget in inference API

Adding support for configuring the thinkingBudget for Gemini 2.5 models
when creating chat completion inference endpoints. The thinking_budget
field is nested inside the thinking_config object in service_settings.

- Added ThinkingConfig class to contain the thinking_budget field. This
  results in a less flat structure for the PUT
  _inference/chat_completion/ call but will make adding support for
  include_thoughts easier in future
- Added extractOptionalInteger() method to ServiceUtils
- Unit tests for ThinkingConfig class
- Updated existing tests to account for the new object and field

These changes enable elastic/kibana#227590 to be completed

Author: DonalEvans

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

@@ -369,6 +369,7 @@ static TransportVersion def(int id) {
     public static final TransportVersion SCRIPT_RESCORER = def(9_143_0_00);
     public static final TransportVersion ESQL_LOOKUP_OPERATOR_EMITTED_ROWS = def(9_144_0_00);
     public static final TransportVersion ALLOCATION_DECISION_NOT_PREFERRED = def(9_145_0_00);
+    public static final TransportVersion GEMINI_THINKING_BUDGET_ADDED = def(9_146_0_00);
 
     /*
      * STOP! READ THIS FIRST! No, really,

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

@@ -780,6 +780,25 @@ public static Integer extractOptionalPositiveInteger(
         String settingName,
         String scope,
         ValidationException validationException
+    ) {
+        return extractOptionalInteger(map, settingName, scope, validationException, true);
+    }
+
+    public static Integer extractOptionalInteger(
+        Map<String, Object> map,
+        String settingName,
+        String scope,
+        ValidationException validationException
+    ) {
+        return extractOptionalInteger(map, settingName, scope, validationException, false);
+    }
+
+    private static Integer extractOptionalInteger(
+        Map<String, Object> map,
+        String settingName,
+        String scope,
+        ValidationException validationException,
+        boolean mustBePositive
     ) {
         int initialValidationErrorCount = validationException.validationErrors().size();
         Integer optionalField = ServiceUtils.removeAsType(map, settingName, Integer.class, validationException);
@@ -788,7 +807,7 @@ public static Integer extractOptionalPositiveInteger(
             return null;
         }
 
-        if (optionalField != null && optionalField <= 0) {
+        if (optionalField != null && mustBePositive && optionalField <= 0) {
             validationException.addValidationError(ServiceUtils.mustBeAPositiveIntegerErrorMessage(settingName, scope, optionalField));
             return null;
         }

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/services/googlevertexai/completion/GoogleVertexAiChatCompletionModel.java

@@ -80,7 +80,8 @@ public static GoogleVertexAiChatCompletionModel of(GoogleVertexAiChatCompletionM
             originalModelServiceSettings.projectId(),
             originalModelServiceSettings.location(),
             Objects.requireNonNullElse(request.model(), originalModelServiceSettings.modelId()),
-            originalModelServiceSettings.rateLimitSettings()
+            originalModelServiceSettings.rateLimitSettings(),
+            originalModelServiceSettings.thinkingConfig()
         );
 
         return new GoogleVertexAiChatCompletionModel(

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/services/googlevertexai/completion/GoogleVertexAiChatCompletionServiceSettings.java

@@ -9,6 +9,7 @@
 
 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;
@@ -44,12 +45,14 @@ public class GoogleVertexAiChatCompletionServiceSettings extends FilteredXConten
     private final String projectId;
 
     private final RateLimitSettings rateLimitSettings;
+    private final ThinkingConfig thinkingConfig;
 
     // https://cloud.google.com/vertex-ai/docs/quotas#eval-quotas
     private static final RateLimitSettings DEFAULT_RATE_LIMIT_SETTINGS = new RateLimitSettings(1000);
+    private static final ThinkingConfig EMPTY_THINKING_CONFIG = new ThinkingConfig();
 
     public GoogleVertexAiChatCompletionServiceSettings(StreamInput in) throws IOException {
-        this(in.readString(), in.readString(), in.readString(), new RateLimitSettings(in));
+        this(in.readString(), in.readString(), in.readString(), new RateLimitSettings(in), new ThinkingConfig(in));
     }
 
     @Override
@@ -58,6 +61,7 @@ protected XContentBuilder toXContentFragmentOfExposedFields(XContentBuilder buil
         builder.field(LOCATION, location);
         builder.field(MODEL_ID, modelId);
         rateLimitSettings.toXContent(builder, params);
+        thinkingConfig.toXContent(builder, params);
         return builder;
     }
 
@@ -78,23 +82,34 @@ public static GoogleVertexAiChatCompletionServiceSettings fromMap(Map<String, Ob
             context
         );
 
+        // Extract optional thinkingConfig settings
+        ThinkingConfig thinkingConfig = ThinkingConfig.of(
+            map,
+            EMPTY_THINKING_CONFIG,
+            validationException,
+            GoogleVertexAiService.NAME,
+            context
+        );
+
         if (validationException.validationErrors().isEmpty() == false) {
             throw validationException;
         }
 
-        return new GoogleVertexAiChatCompletionServiceSettings(projectId, location, modelId, rateLimitSettings);
+        return new GoogleVertexAiChatCompletionServiceSettings(projectId, location, modelId, rateLimitSettings, thinkingConfig);
     }
 
     public GoogleVertexAiChatCompletionServiceSettings(
         String projectId,
         String location,
         String modelId,
-        @Nullable RateLimitSettings rateLimitSettings
+        @Nullable RateLimitSettings rateLimitSettings,
+        @Nullable ThinkingConfig thinkingConfig
     ) {
         this.projectId = projectId;
         this.location = location;
         this.modelId = modelId;
         this.rateLimitSettings = Objects.requireNonNullElse(rateLimitSettings, DEFAULT_RATE_LIMIT_SETTINGS);
+        this.thinkingConfig = Objects.requireNonNullElse(thinkingConfig, EMPTY_THINKING_CONFIG);
     }
 
     public String location() {
@@ -116,6 +131,15 @@ public String getWriteableName() {
         return NAME;
     }
 
+    @Override
+    public RateLimitSettings rateLimitSettings() {
+        return rateLimitSettings;
+    }
+
+    public ThinkingConfig thinkingConfig() {
+        return thinkingConfig;
+    }
+
     @Override
     public TransportVersion getMinimalSupportedVersion() {
         assert false : "should never be called when supportsVersion is used";
@@ -134,6 +158,7 @@ public void writeTo(StreamOutput out) throws IOException {
         out.writeString(location);
         out.writeString(modelId);
         rateLimitSettings.writeTo(out);
+        thinkingConfig.writeTo(out);
     }
 
     @Override
@@ -152,16 +177,17 @@ public boolean equals(Object o) {
         return Objects.equals(location, that.location)
             && Objects.equals(modelId, that.modelId)
             && Objects.equals(projectId, that.projectId)
-            && Objects.equals(rateLimitSettings, that.rateLimitSettings);
+            && Objects.equals(rateLimitSettings, that.rateLimitSettings)
+            && Objects.equals(thinkingConfig, that.thinkingConfig);
     }
 
     @Override
     public int hashCode() {
-        return Objects.hash(location, modelId, projectId, rateLimitSettings);
+        return Objects.hash(location, modelId, projectId, rateLimitSettings, thinkingConfig);
     }
 
     @Override
-    public RateLimitSettings rateLimitSettings() {
-        return rateLimitSettings;
+    public String toString() {
+        return Strings.toString(this);
     }
 }

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/services/googlevertexai/completion/ThinkingConfig.java

@@ -0,0 +1,121 @@
+/*
+ * 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.services.googlevertexai.completion;
+
+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.common.io.stream.Writeable;
+import org.elasticsearch.inference.ModelConfigurations;
+import org.elasticsearch.xcontent.ToXContentFragment;
+import org.elasticsearch.xcontent.XContentBuilder;
+import org.elasticsearch.xpack.inference.services.ConfigurationParseContext;
+import org.elasticsearch.xpack.inference.services.ServiceUtils;
+
+import java.io.IOException;
+import java.util.Map;
+import java.util.Objects;
+
+import static org.elasticsearch.xpack.inference.services.ServiceUtils.removeFromMapOrDefaultEmpty;
+import static org.elasticsearch.xpack.inference.services.ServiceUtils.throwIfNotEmptyMap;
+
+/**
+ * This class encapsulates the ThinkingConfig object contained within GenerationConfig. Only the thinkingBudget field is currently
+ * supported, but the includeThoughts field may be added in the future
+ */
+public class ThinkingConfig implements Writeable, ToXContentFragment {
+    public static final String THINKING_CONFIG_FIELD = "thinking_config";
+    public static final String THINKING_BUDGET_FIELD = "thinking_budget";
+
+    private final Integer thinkingBudget;
+
+    /**
+     * Constructor for an empty {@code ThinkingConfig}
+     */
+    public ThinkingConfig() {
+        this.thinkingBudget = null;
+    }
+
+    public ThinkingConfig(Integer thinkingBudget) {
+        this.thinkingBudget = thinkingBudget;
+    }
+
+    public ThinkingConfig(StreamInput in) throws IOException {
+        if (in.getTransportVersion().onOrAfter(TransportVersions.GEMINI_THINKING_BUDGET_ADDED)) {
+            thinkingBudget = in.readOptionalVInt();
+        } else {
+            thinkingBudget = null;
+        }
+    }
+
+    public static ThinkingConfig of(
+        Map<String, Object> map,
+        ThinkingConfig defaultValue,
+        ValidationException validationException,
+        String serviceName,
+        ConfigurationParseContext context
+    ) {
+        Map<String, Object> thinkingConfigSettings = removeFromMapOrDefaultEmpty(map, THINKING_CONFIG_FIELD);
+        Integer thinkingBudget = ServiceUtils.extractOptionalInteger(
+            thinkingConfigSettings,
+            THINKING_BUDGET_FIELD,
+            ModelConfigurations.SERVICE_SETTINGS,
+            validationException
+        );
+
+        if (ConfigurationParseContext.isRequestContext(context)) {
+            throwIfNotEmptyMap(thinkingConfigSettings, serviceName);
+        }
+
+        return thinkingBudget == null ? defaultValue : new ThinkingConfig(thinkingBudget);
+    }
+
+    public boolean isEmpty() {
+        return thinkingBudget == null;
+    }
+
+    public Integer getThinkingBudget() {
+        return thinkingBudget;
+    }
+
+    @Override
+    public void writeTo(StreamOutput out) throws IOException {
+        if (out.getTransportVersion().onOrAfter(TransportVersions.GEMINI_THINKING_BUDGET_ADDED)) {
+            out.writeOptionalVInt(thinkingBudget);
+        }
+    }
+
+    @Override
+    public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+        if (thinkingBudget != null) {
+            builder.startObject(THINKING_CONFIG_FIELD);
+            builder.field(THINKING_BUDGET_FIELD, thinkingBudget);
+            builder.endObject();
+        }
+        return builder;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (o == null || getClass() != o.getClass()) return false;
+        ThinkingConfig that = (ThinkingConfig) o;
+        return Objects.equals(thinkingBudget, that.thinkingBudget);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hashCode(thinkingBudget);
+    }
+
+    @Override
+    public String toString() {
+        return Strings.toString(this);
+    }
+}

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/services/googlevertexai/request/GoogleVertexAiUnifiedChatCompletionRequest.java

@@ -37,7 +37,10 @@ public GoogleVertexAiUnifiedChatCompletionRequest(UnifiedChatInput unifiedChatIn
     public HttpRequest createHttpRequest() {
         HttpPost httpPost = new HttpPost(uri);
 
-        var requestEntity = new GoogleVertexAiUnifiedChatCompletionRequestEntity(unifiedChatInput);
+        var requestEntity = new GoogleVertexAiUnifiedChatCompletionRequestEntity(
+            unifiedChatInput,
+            model.getServiceSettings().thinkingConfig()
+        );
 
         ByteArrayEntity byteEntity = new ByteArrayEntity(Strings.toString(requestEntity).getBytes(StandardCharsets.UTF_8));
         httpPost.setEntity(byteEntity);

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/services/googlevertexai/request/GoogleVertexAiUnifiedChatCompletionRequestEntity.java

@@ -19,6 +19,7 @@
 import org.elasticsearch.xcontent.XContentParserConfiguration;
 import org.elasticsearch.xcontent.XContentType;
 import org.elasticsearch.xpack.inference.external.http.sender.UnifiedChatInput;
+import org.elasticsearch.xpack.inference.services.googlevertexai.completion.ThinkingConfig;
 
 import java.io.IOException;
 import java.util.Locale;
@@ -37,6 +38,8 @@ public class GoogleVertexAiUnifiedChatCompletionRequestEntity implements ToXCont
     private static final String TEMPERATURE = "temperature";
     private static final String MAX_OUTPUT_TOKENS = "maxOutputTokens";
     private static final String TOP_P = "topP";
+    private static final String THINKING_CONFIG = "thinkingConfig";
+    private static final String THINKING_BUDGET = "thinkingBudget";
 
     private static final String TOOLS = "tools";
     private static final String FUNCTION_DECLARATIONS = "functionDeclarations";
@@ -56,6 +59,7 @@ public class GoogleVertexAiUnifiedChatCompletionRequestEntity implements ToXCont
     private static final String FUNCTION_CALL_ARGS = "args";
 
     private final UnifiedChatInput unifiedChatInput;
+    private final ThinkingConfig thinkingConfig;
 
     private static final String USER_ROLE = "user";
     private static final String MODEL_ROLE = "model";
@@ -66,8 +70,9 @@ public class GoogleVertexAiUnifiedChatCompletionRequestEntity implements ToXCont
 
     private static final String SYSTEM_INSTRUCTION = "systemInstruction";
 
-    public GoogleVertexAiUnifiedChatCompletionRequestEntity(UnifiedChatInput unifiedChatInput) {
+    public GoogleVertexAiUnifiedChatCompletionRequestEntity(UnifiedChatInput unifiedChatInput, ThinkingConfig thinkingConfig) {
         this.unifiedChatInput = Objects.requireNonNull(unifiedChatInput);
+        this.thinkingConfig = Objects.requireNonNull(thinkingConfig);
     }
 
     private String messageRoleToGoogleVertexAiSupportedRole(String messageRole) {
@@ -316,7 +321,8 @@ private void buildGenerationConfig(XContentBuilder builder) throws IOException {
         boolean hasAnyConfig = request.stop() != null
             || request.temperature() != null
             || request.maxCompletionTokens() != null
-            || request.topP() != null;
+            || request.topP() != null
+            || thinkingConfig.isEmpty() == false;
 
         if (hasAnyConfig == false) {
             return;
@@ -336,6 +342,11 @@ private void buildGenerationConfig(XContentBuilder builder) throws IOException {
         if (request.topP() != null) {
             builder.field(TOP_P, request.topP());
         }
+        if (thinkingConfig.isEmpty() == false) {
+            builder.startObject(THINKING_CONFIG);
+            builder.field(THINKING_BUDGET, thinkingConfig.getThinkingBudget());
+            builder.endObject();
+        }
 
         builder.endObject();
     }