diff --git a/core/src/main/java/com/google/adk/models/ApigeeLlm.java b/core/src/main/java/com/google/adk/models/ApigeeLlm.java
index 088e0af76..c2ad27b01 100644
--- a/core/src/main/java/com/google/adk/models/ApigeeLlm.java
+++ b/core/src/main/java/com/google/adk/models/ApigeeLlm.java
@@ -19,7 +19,9 @@
 import static com.google.common.base.Strings.isNullOrEmpty;
 
 import com.google.adk.Version;
+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 +30,8 @@
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Objects;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * A {@link BaseLlm} implementation for calling an Apigee proxy.
@@ -36,6 +40,7 @@
  * 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 +56,18 @@ 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 ChatCompletionsHttpClient chatCompletionsHttpClient;
   private final Client apiClient;
   private final HttpOptions httpOptions;
+  private final ApiType apiType;
 
   /**
    * Constructs a new ApigeeLlm instance.
@@ -62,7 +76,8 @@ 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 +86,16 @@ 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 +121,26 @@ 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.chatCompletionsHttpClient = 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.chatCompletionsHttpClient = null;
     }
 
-    this.apiClient = apiClientBuilder.build();
-    this.geminiDelegate = new Gemini(modelName, apiClient);
+    logger.trace(
+        "ApigeeLlm constructed: modelName={} apiType={} effectiveProxyUrl={}",
+        modelName,
+        this.apiType,
+        effectiveProxyUrl);
   }
 
   /**
@@ -113,10 +151,31 @@ 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 chatCompletionsHttpClient The ChatCompletionsHttpClient to use for making API calls.
+   */
+  @VisibleForTesting
+  ApigeeLlm(
+      String modelName,
+      Gemini geminiDelegate,
+      ChatCompletionsHttpClient chatCompletionsHttpClient) {
     super(modelName);
     this.apiClient = null;
     this.httpOptions = null;
     this.geminiDelegate = geminiDelegate;
+    this.chatCompletionsHttpClient = chatCompletionsHttpClient;
+    if (chatCompletionsHttpClient != null) {
+      this.apiType = ApiType.CHAT_COMPLETIONS;
+    } else {
+      this.apiType = ApiType.GENAI;
+    }
   }
 
   /**
@@ -178,6 +237,7 @@ 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 +303,19 @@ 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 +328,7 @@ 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 +337,23 @@ 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 chatCompletionsHttpClient.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 +382,9 @@ 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");
diff --git a/core/src/test/java/com/google/adk/models/ApigeeLlmTest.java b/core/src/test/java/com/google/adk/models/ApigeeLlmTest.java
index 6ba2832c0..fce26e812 100644
--- a/core/src/test/java/com/google/adk/models/ApigeeLlmTest.java
+++ b/core/src/test/java/com/google/adk/models/ApigeeLlmTest.java
@@ -18,6 +18,7 @@
 import static com.google.common.truth.Truth.assertThat;
 import static org.junit.Assert.assertThrows;
 import static org.junit.Assume.assumeNotNull;
+import static org.junit.Assume.assumeTrue;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.anyBoolean;
 import static org.mockito.ArgumentMatchers.eq;
@@ -25,6 +26,8 @@
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
 
+import com.google.adk.models.ApigeeLlm.ApiType;
+import com.google.adk.models.chat.ChatCompletionsHttpClient;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.google.genai.types.Content;
@@ -47,6 +50,7 @@ public class ApigeeLlmTest {
 
   @Rule public final MockitoRule mocks = MockitoJUnit.rule();
   @Mock private Gemini mockGeminiDelegate;
+  @Mock private ChatCompletionsHttpClient mockCcClient;
 
   private static final String PROXY_URL = "https://test.apigee.net";
 
@@ -62,7 +66,8 @@ public void build_withValidModelStrings_succeeds() {
       "apigee/v1/whatever-model",
       "apigee/vertex_ai/whatever-model",
       "apigee/gemini/v1/whatever-model",
-      "apigee/vertex_ai/v1beta/whatever-model"
+      "apigee/vertex_ai/v1beta/whatever-model",
+      "apigee/openai/gpt-4"
     };
 
     for (String modelName : validModelStrings) {
@@ -84,9 +89,10 @@ public void build_withInvalidModelStrings_throwsException() {
     };
 
     for (String modelName : invalidModelStrings) {
-      assertThrows(
-          IllegalArgumentException.class,
-          () -> ApigeeLlm.builder().modelName(modelName).proxyUrl(PROXY_URL).build());
+      ApigeeLlm.Builder builder = ApigeeLlm.builder().modelName(modelName).proxyUrl(PROXY_URL);
+      IllegalArgumentException e =
+          assertThrows(IllegalArgumentException.class, () -> builder.build());
+      assertThat(e).hasMessageThat().contains("Invalid model string: " + modelName);
     }
   }
 
@@ -108,6 +114,47 @@ public void generateContent_stripsApigeePrefixAndSendsToDelegate() {
     assertThat(requestCaptor.getValue().model()).hasValue("whatever-model");
   }
 
+  @Test
+  public void generateContent_withChatCompletionsApiType_sendsToCcClient() {
+    when(mockCcClient.complete(any(), anyBoolean())).thenReturn(Flowable.empty());
+
+    ApigeeLlm llm = new ApigeeLlm("apigee/openai/gpt-4", mockGeminiDelegate, mockCcClient);
+
+    LlmRequest request =
+        LlmRequest.builder()
+            .model("apigee/openai/gpt-4")
+            .contents(ImmutableList.of(Content.builder().parts(Part.fromText("hello")).build()))
+            .build();
+    llm.generateContent(request, false).test().assertNoErrors();
+
+    ArgumentCaptor<LlmRequest> requestCaptor = ArgumentCaptor.forClass(LlmRequest.class);
+    verify(mockCcClient).complete(requestCaptor.capture(), eq(false));
+    verify(mockGeminiDelegate, never()).generateContent(any(), anyBoolean());
+    assertThat(requestCaptor.getValue().model()).hasValue("gpt-4");
+  }
+
+  @Test
+  public void connect_withChatCompletionsApiType_throwsUnsupportedOperationException() {
+    ApigeeLlm llm = new ApigeeLlm("apigee/openai/gpt-4", mockGeminiDelegate, mockCcClient);
+    LlmRequest request = LlmRequest.builder().model("apigee/openai/gpt-4").build();
+    UnsupportedOperationException e =
+        assertThrows(UnsupportedOperationException.class, () -> llm.connect(request));
+    assertThat(e)
+        .hasMessageThat()
+        .contains("Streaming connections are not supported for chat completions.");
+  }
+
+  @Test
+  public void build_withExplicitChatCompletionsApiType_success() {
+    ApigeeLlm llm =
+        ApigeeLlm.builder()
+            .modelName("apigee/whatever-model")
+            .proxyUrl(PROXY_URL)
+            .apiType(ApiType.CHAT_COMPLETIONS)
+            .build();
+    assertThat(llm).isNotNull();
+  }
+
   // Add a test to verify the vertexAI flag is set correctly.
   @Test
   public void generateContent_setsVertexAiFlagCorrectly_withVertexAi() {
@@ -187,23 +234,163 @@ public void build_withTrailingSlashInModel_parsesVersionAndModelId() {
         LlmRequest.builder()
             .contents(ImmutableList.of(Content.builder().parts(Part.fromText("hi")).build()))
             .build();
-    assertThrows(IllegalArgumentException.class, () -> llm.generateContent(request, false));
+    IllegalArgumentException e =
+        assertThrows(IllegalArgumentException.class, () -> llm.generateContent(request, false));
+    assertThat(e)
+        .hasMessageThat()
+        .contains(
+            "Invalid model string, expected apigee/[<provider>/][<version>/]<model_id>: "
+                + "apigee/gemini/v1/");
     verify(mockGeminiDelegate, never()).generateContent(any(), anyBoolean());
   }
 
   @Test
-  public void build_withoutProxyUrl_readsFromEnvironment() {
+  public void build_withoutProxyUrlAndEnvVarSet_readsFromEnvironment() {
+    assumeNotNull(System.getenv("APIGEE_PROXY_URL"));
     String envProxyUrl = System.getenv("APIGEE_PROXY_URL");
-    if (envProxyUrl != null) {
-      ApigeeLlm llm = ApigeeLlm.builder().modelName("apigee/whatever-model").build();
-      assertThat(llm.getHttpOptions().baseUrl()).hasValue(envProxyUrl);
-    } else {
-      assertThrows(
-          IllegalArgumentException.class,
-          () -> ApigeeLlm.builder().modelName("apigee/whatever-model").build());
-      ApigeeLlm llm =
-          ApigeeLlm.builder().proxyUrl(PROXY_URL).modelName("apigee/whatever-model").build();
-      assertThat(llm.getHttpOptions().baseUrl()).hasValue(PROXY_URL);
+    ApigeeLlm llm = ApigeeLlm.builder().modelName("apigee/whatever-model").build();
+    assertThat(llm.getHttpOptions().baseUrl()).hasValue(envProxyUrl);
+  }
+
+  @Test
+  public void build_withoutProxyUrlAndEnvVarNotSet_throwsException() {
+    assumeTrue(System.getenv("APIGEE_PROXY_URL") == null);
+    ApigeeLlm.Builder builder = ApigeeLlm.builder().modelName("apigee/whatever-model");
+    IllegalArgumentException e =
+        assertThrows(IllegalArgumentException.class, () -> builder.build());
+    assertThat(e)
+        .hasMessageThat()
+        .contains(
+            "Apigee proxy URL is not set and not found in the environment variable"
+                + " APIGEE_PROXY_URL.");
+  }
+
+  @Test
+  public void build_withProxyUrl_usesProvidedUrl() {
+    ApigeeLlm llm =
+        ApigeeLlm.builder().proxyUrl(PROXY_URL).modelName("apigee/whatever-model").build();
+    assertThat(llm.getHttpOptions().baseUrl()).hasValue(PROXY_URL);
+  }
+
+  @Test
+  public void generateContent_withChatCompletionsApiType_sendsToCcClient_streaming() {
+    when(mockCcClient.complete(any(), anyBoolean())).thenReturn(Flowable.empty());
+
+    ApigeeLlm llm = new ApigeeLlm("apigee/openai/gpt-4o", mockGeminiDelegate, mockCcClient);
+    LlmRequest request =
+        LlmRequest.builder()
+            .model("apigee/openai/gpt-4o")
+            .contents(ImmutableList.of(Content.builder().parts(Part.fromText("hello")).build()))
+            .build();
+    llm.generateContent(request, true).test().assertNoErrors();
+
+    ArgumentCaptor<LlmRequest> requestCaptor = ArgumentCaptor.forClass(LlmRequest.class);
+    verify(mockCcClient).complete(requestCaptor.capture(), eq(true));
+    verify(mockGeminiDelegate, never()).generateContent(any(), anyBoolean());
+    assertThat(requestCaptor.getValue().model()).hasValue("gpt-4o");
+  }
+
+  @Test
+  public void generateContent_requestLevelModelOverride_extractedCorrectly() {
+    when(mockCcClient.complete(any(), anyBoolean())).thenReturn(Flowable.empty());
+
+    ApigeeLlm llm = new ApigeeLlm("apigee/openai/gpt-4o", mockGeminiDelegate, mockCcClient);
+    LlmRequest request =
+        LlmRequest.builder()
+            .model("apigee/openai/gpt-3.5-turbo")
+            .contents(ImmutableList.of(Content.builder().parts(Part.fromText("hello")).build()))
+            .build();
+
+    llm.generateContent(request, false).test().assertNoErrors();
+
+    ArgumentCaptor<LlmRequest> requestCaptor = ArgumentCaptor.forClass(LlmRequest.class);
+    verify(mockCcClient).complete(requestCaptor.capture(), eq(false));
+    assertThat(requestCaptor.getValue().model()).hasValue("gpt-3.5-turbo");
+  }
+
+  @Test
+  public void validateModelString_rejectsOpenAiWithVersion() {
+    // 3-component model string (e.g. apigee/openai/v1/gpt-4o) fails because "openai" != "vertex_ai"
+    // and != "gemini"
+    ApigeeLlm.Builder builder =
+        ApigeeLlm.builder().modelName("apigee/openai/v1/gpt-4o").proxyUrl(PROXY_URL);
+    IllegalArgumentException e = assertThrows(IllegalArgumentException.class, builder::build);
+    assertThat(e).hasMessageThat().contains("Invalid model string: apigee/openai/v1/gpt-4o");
+  }
+
+  @Test
+  public void build_withCustomHeadersOverlappingTrackingHeaders_throwsException() {
+    ImmutableMap<String, String> overlappingHeaders = ImmutableMap.of("user-agent", "custom-agent");
+    ApigeeLlm.Builder builder =
+        ApigeeLlm.builder()
+            .modelName("apigee/openai/gpt-4o")
+            .proxyUrl(PROXY_URL)
+            .customHeaders(overlappingHeaders);
+    IllegalArgumentException e = assertThrows(IllegalArgumentException.class, builder::build);
+    assertThat(e).hasMessageThat().contains("Multiple entries with same key: user-agent=");
+  }
+
+  @Test
+  public void build_withNullApiType_throwsNullPointerException() {
+    ApigeeLlm.Builder builder =
+        ApigeeLlm.builder().modelName("apigee/whatever-model").proxyUrl(PROXY_URL);
+    assertThrows(NullPointerException.class, () -> builder.apiType(null));
+  }
+
+  @Test
+  public void generateContent_crossApiTypeRequestOverride_routesBasedOnOriginalApiType() {
+    when(mockGeminiDelegate.generateContent(any(), anyBoolean())).thenReturn(Flowable.empty());
+
+    // Original ApiType is GENAI implicitly
+    ApigeeLlm llm = new ApigeeLlm("apigee/gemini/gemini-pro", mockGeminiDelegate);
+
+    // Override specifies openai models
+    LlmRequest request =
+        LlmRequest.builder()
+            .model("apigee/openai/gpt-4o")
+            .contents(ImmutableList.of(Content.builder().parts(Part.fromText("hello")).build()))
+            .build();
+
+    llm.generateContent(request, false).test().assertNoErrors();
+
+    ArgumentCaptor<LlmRequest> requestCaptor = ArgumentCaptor.forClass(LlmRequest.class);
+    verify(mockGeminiDelegate).generateContent(requestCaptor.capture(), eq(false));
+    // It still goes to gemini delegate, but model is stripped
+    assertThat(requestCaptor.getValue().model()).hasValue("gpt-4o");
+  }
+
+  @Test
+  public void generateContent_invalidRequestLevelOverride_throwsException() {
+    ApigeeLlm llm = new ApigeeLlm("apigee/openai/gpt-4o", mockGeminiDelegate, mockCcClient);
+
+    LlmRequest request =
+        LlmRequest.builder()
+            .model("invalid-no-apigee-prefix")
+            .contents(ImmutableList.of(Content.builder().parts(Part.fromText("hello")).build()))
+            .build();
+
+    IllegalArgumentException e =
+        assertThrows(IllegalArgumentException.class, () -> llm.generateContent(request, false));
+    assertThat(e)
+        .hasMessageThat()
+        .contains(
+            "Invalid model string, expected apigee/[<provider>/][<version>/]<model_id>: "
+                + "invalid-no-apigee-prefix");
+  }
+
+  @Test
+  public void build_nullModelName_throwsNullPointerException() {
+    ApigeeLlm.Builder builder = ApigeeLlm.builder().modelName(null).proxyUrl(PROXY_URL);
+    assertThrows(NullPointerException.class, builder::build);
+  }
+
+  @Test
+  public void build_malformedModelsWithTrailingSlashes_throwsException() {
+    String[] malformedModels = {"apigee/openai/", "apigee/openai/gpt-4o/"};
+    for (String modelName : malformedModels) {
+      ApigeeLlm.Builder builder = ApigeeLlm.builder().modelName(modelName).proxyUrl(PROXY_URL);
+      IllegalArgumentException e = assertThrows(IllegalArgumentException.class, builder::build);
+      assertThat(e).hasMessageThat().contains("Invalid model string: " + modelName);
     }
   }
 }
diff --git a/core/src/test/java/com/google/adk/models/chat/ChatCompletionsRequestTest.java b/core/src/test/java/com/google/adk/models/chat/ChatCompletionsRequestTest.java
index eb00b3770..ec4246f90 100644
--- a/core/src/test/java/com/google/adk/models/chat/ChatCompletionsRequestTest.java
+++ b/core/src/test/java/com/google/adk/models/chat/ChatCompletionsRequestTest.java
@@ -710,10 +710,8 @@ private static void assertThoughtSignatureExtraContent(
     assertThat(extraContent).containsKey("google");
     @SuppressWarnings("unchecked") // This code won't run in production and it is a JSON object.
     Map<String, Object> google = (Map<String, Object>) extraContent.get("google");
-    assertThat(google).containsKey("thought_signature");
-    Object sigObj = google.get("thought_signature");
-    assertThat(sigObj).isInstanceOf(String.class);
-    assertThat(Base64.getDecoder().decode((String) sigObj)).isEqualTo(expected);
+    String expectedB64 = Base64.getEncoder().encodeToString(expected);
+    assertThat(google).containsEntry("thought_signature", expectedB64);
   }
 
   @Test