feat: Add chat-completions API support to ApigeeLlm

core/src/main/java/com/google/adk/models/ApigeeLlm.java

@@ -19,7 +19,10 @@
 import static com.google.common.base.Strings.isNullOrEmpty;
 
 import com.google.adk.Version;
+import com.google.adk.models.chat.ChatCompletionsClient;
+import com.google.adk.models.chat.ChatCompletionsHttpClient;
 import com.google.common.annotations.VisibleForTesting;
+import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableMap;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import com.google.genai.Client;
@@ -28,6 +31,9 @@
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Objects;
+import org.jspecify.annotations.Nullable;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * A {@link BaseLlm} implementation for calling an Apigee proxy.
@@ -36,6 +42,7 @@
  * allows for specifying the provider (Gemini or Vertex AI), API version, and model ID.
  */
 public class ApigeeLlm extends BaseLlm {
+  private static final Logger logger = LoggerFactory.getLogger(ApigeeLlm.class);
   private static final String GOOGLE_GENAI_USE_VERTEXAI_ENV_VARIABLE_NAME =
       "GOOGLE_GENAI_USE_VERTEXAI";
   private static final String APIGEE_PROXY_URL_ENV_VARIABLE_NAME = "APIGEE_PROXY_URL";
@@ -51,9 +58,18 @@ public class ApigeeLlm extends BaseLlm {
             "user-agent", versionHeaderValue);
   }
 
+  /** Defines the type of API to be used by the Apigee proxy. */
+  public enum ApiType {
+    UNKNOWN,
+    CHAT_COMPLETIONS,
+    GENAI
+  }
+
   private final Gemini geminiDelegate;
+  private final ChatCompletionsClient chatCompletionsClient;
   private final Client apiClient;
   private final HttpOptions httpOptions;
+  private final ApiType apiType;
 
   /**
    * Constructs a new ApigeeLlm instance.
@@ -62,7 +78,8 @@ public class ApigeeLlm extends BaseLlm {
    * @param proxyUrl The URL of the Apigee proxy.
    * @param customHeaders A map of custom headers to be sent with the request.
    */
-  private ApigeeLlm(String modelName, String proxyUrl, Map<String, String> customHeaders) {
+  private ApigeeLlm(
+      String modelName, String proxyUrl, Map<String, String> customHeaders, ApiType apiType) {
     super(modelName);
 
     if (!validateModelString(modelName)) {
@@ -71,6 +88,16 @@ private ApigeeLlm(String modelName, String proxyUrl, Map<String, String> customH
               + modelName);
     }
 
+    if (apiType == ApiType.UNKNOWN) {
+      if (modelName.startsWith("apigee/openai/")) {
+        this.apiType = ApiType.CHAT_COMPLETIONS;
+      } else {
+        this.apiType = ApiType.GENAI;
+      }
+    } else {
+      this.apiType = apiType;
+    }
+
     String effectiveProxyUrl = proxyUrl;
     if (isNullOrEmpty(effectiveProxyUrl)) {
       effectiveProxyUrl = System.getenv(APIGEE_PROXY_URL_ENV_VARIABLE_NAME);
@@ -96,13 +123,26 @@ private ApigeeLlm(String modelName, String proxyUrl, Map<String, String> customH
               .buildOrThrow());
     }
     this.httpOptions = httpOptionsBuilder.build();
-    Client.Builder apiClientBuilder = Client.builder().httpOptions(this.httpOptions);
-    if (isVertexAiModel(modelName)) {
-      apiClientBuilder.vertexAI(true);
+
+    if (this.apiType == ApiType.CHAT_COMPLETIONS) {
+      this.apiClient = null;
+      this.geminiDelegate = null;
+      this.chatCompletionsClient = new ChatCompletionsHttpClient(this.httpOptions);
+    } else {
+      Client.Builder apiClientBuilder = Client.builder().httpOptions(this.httpOptions);
+      if (isVertexAiModel(modelName)) {
+        apiClientBuilder.vertexAI(true);
+      }
+      this.apiClient = apiClientBuilder.build();
+      this.geminiDelegate = new Gemini(modelName, apiClient);
+      this.chatCompletionsClient = null;
     }
 
-    this.apiClient = apiClientBuilder.build();
-    this.geminiDelegate = new Gemini(modelName, apiClient);
+    logger.trace(
+        "ApigeeLlm constructed: modelName={} apiType={} effectiveProxyUrl={}",
+        modelName,
+        this.apiType,
+        effectiveProxyUrl);
   }
 
   /**
@@ -113,10 +153,31 @@ private ApigeeLlm(String modelName, String proxyUrl, Map<String, String> customH
    */
   @VisibleForTesting
   ApigeeLlm(String modelName, Gemini geminiDelegate) {
+    this(modelName, geminiDelegate, null);
+  }
+
+  /**
+   * Constructs a new ApigeeLlm instance for testing purposes.
+   *
+   * @param modelName The name of the Apigee model to use.
+   * @param geminiDelegate The Gemini delegate to use for making API calls.
+   * @param chatCompletionsClient The ChatCompletionsClient to use for making API calls.
+   */
+  @VisibleForTesting
+  ApigeeLlm(
+      String modelName,
+      Gemini geminiDelegate,
+      @Nullable ChatCompletionsClient chatCompletionsClient) {
     super(modelName);
     this.apiClient = null;
     this.httpOptions = null;
     this.geminiDelegate = geminiDelegate;
+    this.chatCompletionsClient = chatCompletionsClient;
+    if (chatCompletionsClient != null) {
+      this.apiType = ApiType.CHAT_COMPLETIONS;
+    } else {
+      this.apiType = ApiType.GENAI;
+    }
   }
 
   /**
@@ -178,6 +239,7 @@ public static class Builder {
     private String modelName;
     private String proxyUrl;
     private Map<String, String> customHeaders = new HashMap<>();
+    private ApiType apiType = ApiType.UNKNOWN;
 
     protected Builder() {}
 
@@ -243,6 +305,19 @@ public Builder customHeaders(Map<String, String> customHeaders) {
       return this;
     }
 
+    /**
+     * Sets the explicit {@link ApiType} to use (e.g., CHAT_COMPLETIONS or GENAI).
+     *
+     * @param apiType the type of API.
+     * @return this builder.
+     * @throws NullPointerException if {@code apiType} is null.
+     */
+    @CanIgnoreReturnValue
+    public Builder apiType(ApiType apiType) {
+      this.apiType = Preconditions.checkNotNull(apiType);
+      return this;
+    }
+
     /**
      * Builds the {@link ApigeeLlm} instance.
      *
@@ -255,7 +330,7 @@ public ApigeeLlm build() {
         throw new IllegalArgumentException("Invalid model string: " + modelName);
       }
 
-      return new ApigeeLlm(modelName, proxyUrl, customHeaders);
+      return new ApigeeLlm(modelName, proxyUrl, customHeaders, apiType);
     }
   }
 
@@ -264,11 +339,23 @@ public Flowable<LlmResponse> generateContent(LlmRequest llmRequest, boolean stre
     String modelToUse = llmRequest.model().orElse(model());
     String modelId = getModelId(modelToUse);
     LlmRequest newLlmRequest = llmRequest.toBuilder().model(modelId).build();
+
+    logger.debug("ApigeeLlm.generateContent routing through {} for model {}", apiType, modelId);
+
+    if (apiType == ApiType.CHAT_COMPLETIONS) {
+      return chatCompletionsClient.complete(newLlmRequest, stream);
+    }
+
     return geminiDelegate.generateContent(newLlmRequest, stream);
   }
 
   @Override
   public BaseLlmConnection connect(LlmRequest llmRequest) {
+    if (apiType == ApiType.CHAT_COMPLETIONS) {
+      throw new UnsupportedOperationException(
+          "Streaming connections are not supported for chat completions.");
+    }
+
     String modelToUse = llmRequest.model().orElse(model());
     String modelId = getModelId(modelToUse);
     LlmRequest newLlmRequest = llmRequest.toBuilder().model(modelId).build();
@@ -297,7 +384,9 @@ private static boolean validateModelString(String model) {
       return components[1].startsWith("v");
     }
     if (components.length == 2) {
-      if (components[0].equals("vertex_ai") || components[0].equals("gemini")) {
+      if (components[0].equals("vertex_ai")
+          || components[0].equals("gemini")
+          || components[0].equals("openai")) {
         return true;
       }
       return components[0].startsWith("v");

core/src/main/java/com/google/adk/models/chat/ChatCompletionsClient.java

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.adk.models.chat;
+
+import com.google.adk.models.LlmRequest;
+import com.google.adk.models.LlmResponse;
+import io.reactivex.rxjava3.core.Flowable;
+
+/**
+ * A client for interacting with OpenAI-compatible chat completions endpoints.
+ *
+ * <p>Supports both non-streaming responses (single {@link LlmResponse} emission) and streaming
+ * Server-Sent Events (SSE) responses (multiple incremental {@link LlmResponse} emissions). See the
+ * <a href="https://developers.openai.com/api/reference/resources/chat">OpenAI Chat Completions API
+ * reference</a> for the wire protocol.
+ */
+public interface ChatCompletionsClient {
+
+  /**
+   * Generates a conversational response from the chat completions endpoint based on the provided
+   * messages. This encapsulates building the payload, sending the request to the completions
+   * endpoint, and initiating the handling of complete calls.
+   *
+   * @param llmRequest The request containing the model, configuration, and sequence of messages.
+   * @param stream Whether to request a streaming response.
+   * @return A {@link Flowable} emitting the discrete (or combined) {@link LlmResponse} objects.
+   */
+  Flowable<LlmResponse> complete(LlmRequest llmRequest, boolean stream);
+}

core/src/main/java/com/google/adk/models/chat/ChatCompletionsHttpClient.java

@@ -46,14 +46,12 @@
 import org.slf4j.LoggerFactory;
 
 /**
- * An HTTP client for interacting with OpenAI-compatible chat completions endpoints.
- *
- * <p>Supports both non-streaming responses (single {@link LlmResponse} emission) and streaming
- * Server-Sent Events (SSE) responses (multiple incremental {@link LlmResponse} emissions). See the
- * <a href="https://developers.openai.com/api/reference/resources/chat">OpenAI Chat Completions API
- * reference</a> for the wire protocol.
+ * An OkHttp-based implementation of {@link ChatCompletionsClient} that targets OpenAI-compatible
+ * chat completions endpoints. Both non-streaming responses (single {@link LlmResponse} emission)
+ * and streaming Server-Sent Events (SSE) responses (multiple incremental {@link LlmResponse}
+ * emissions) are supported.
  */
-public final class ChatCompletionsHttpClient {
+public final class ChatCompletionsHttpClient implements ChatCompletionsClient {
   private static final Logger logger = LoggerFactory.getLogger(ChatCompletionsHttpClient.class);
   private static final ObjectMapper objectMapper = JsonBaseModel.getMapper();
 
@@ -178,15 +176,7 @@ private static Duration resolveCallTimeout(HttpOptions httpOptions) {
     return timeoutMs == 0L ? Duration.ZERO : Duration.ofMillis(timeoutMs);
   }
 
-  /**
-   * Generates a conversational response from the chat completions endpoint based on the provided
-   * messages. This encapsulates building the HTTP payload, sending the request to the completions
-   * endpoint, and initiating the handling of complete calls.
-   *
-   * @param llmRequest The request containing the model, configuration, and sequence of messages.
-   * @param stream Whether to request a streaming response.
-   * @return A {@link Flowable} emitting the discrete (or combined) {@link LlmResponse} objects.
-   */
+  @Override
   public Flowable<LlmResponse> complete(LlmRequest llmRequest, boolean stream) {
     return Flowable.defer(
         () -> {