diff --git a/core/src/main/java/com/google/adk/codeexecutors/BaseCodeExecutor.java b/core/src/main/java/com/google/adk/codeexecutors/BaseCodeExecutor.java
index 92c3fe28..b13d69a9 100644
--- a/core/src/main/java/com/google/adk/codeexecutors/BaseCodeExecutor.java
+++ b/core/src/main/java/com/google/adk/codeexecutors/BaseCodeExecutor.java
@@ -21,7 +21,6 @@
 import com.google.adk.codeexecutors.CodeExecutionUtils.CodeExecutionInput;
 import com.google.adk.codeexecutors.CodeExecutionUtils.CodeExecutionResult;
 import com.google.common.collect.ImmutableList;
-import java.util.List;
 
 /**
  * Abstract base class for all code executors.
@@ -30,6 +29,13 @@
  * the execution results into the final response.
  */
 public abstract class BaseCodeExecutor extends JsonBaseModel {
+
+  private static final ImmutableList<ImmutableList<String>> CODE_BLOCK_DELIMITERS =
+      ImmutableList.of(
+          ImmutableList.of("```tool_code\n", "\n```"), ImmutableList.of("```python\n", "\n```"));
+  private static final ImmutableList<String> EXECUTION_RESULT_DELIMITERS =
+      ImmutableList.of("```tool_output\n", "\n```");
+
   /**
    * If true, extract and process data files from the model request and attach them to the code
    * executor.
@@ -57,6 +63,9 @@ public int errorRetryAttempts() {
   /**
    * The list of the enclosing delimiters to identify the code blocks.
    *
+   * <p>Each inner list contains a pair of start and end delimiters. This supports multiple pairs of
+   * delimiters.
+   *
    * <p>For example, the delimiter ('```python\n', '\n```') can be used to identify code blocks with
    * the following format:
    *
@@ -66,19 +75,20 @@ public int errorRetryAttempts() {
    *
    * <p>```
    */
-  public List<List<String>> codeBlockDelimiters() {
-    return ImmutableList.of(
-        ImmutableList.of("```tool_code\n", "\n```"), ImmutableList.of("```python\n", "\n```"));
+  public ImmutableList<ImmutableList<String>> codeBlockDelimiters() {
+    return CODE_BLOCK_DELIMITERS;
   }
 
   /** The delimiters to format the code execution result. */
-  public List<String> executionResultDelimiters() {
-    return ImmutableList.of("```tool_output\n", "\n```");
+  public ImmutableList<String> executionResultDelimiters() {
+    return EXECUTION_RESULT_DELIMITERS;
   }
 
   /**
    * Executes code and return the code execution result.
    *
+   * <p>This method may perform blocking operations.
+   *
    * @param invocationContext The invocation context of the code execution.
    * @param codeExecutionInput The code execution input.
    * @return The code execution result.
diff --git a/core/src/main/java/com/google/adk/codeexecutors/BuiltInCodeExecutor.java b/core/src/main/java/com/google/adk/codeexecutors/BuiltInCodeExecutor.java
index f92c79a8..972082dd 100644
--- a/core/src/main/java/com/google/adk/codeexecutors/BuiltInCodeExecutor.java
+++ b/core/src/main/java/com/google/adk/codeexecutors/BuiltInCodeExecutor.java
@@ -46,9 +46,8 @@ public void processLlmRequest(LlmRequest.Builder llmRequestBuilder) {
     if (ModelNameUtils.isGemini2Model(llmRequest.model().orElse(null))) {
       GenerateContentConfig.Builder configBuilder =
           llmRequest.config().map(c -> c.toBuilder()).orElseGet(GenerateContentConfig::builder);
-      ImmutableList.Builder<Tool> toolsBuilder =
-          ImmutableList.<Tool>builder()
-              .addAll(configBuilder.build().tools().orElse(ImmutableList.<Tool>of()));
+      ImmutableList.Builder<Tool> toolsBuilder = ImmutableList.<Tool>builder();
+      llmRequest.config().ifPresent(c -> c.tools().ifPresent(toolsBuilder::addAll));
       toolsBuilder.add(Tool.builder().codeExecution(ToolCodeExecution.builder().build()).build());
       configBuilder.tools(toolsBuilder.build());
       llmRequestBuilder.config(configBuilder.build());
diff --git a/core/src/main/java/com/google/adk/codeexecutors/CodeExecutionUtils.java b/core/src/main/java/com/google/adk/codeexecutors/CodeExecutionUtils.java
index ee5be789..b9afdcaf 100644
--- a/core/src/main/java/com/google/adk/codeexecutors/CodeExecutionUtils.java
+++ b/core/src/main/java/com/google/adk/codeexecutors/CodeExecutionUtils.java
@@ -128,7 +128,7 @@ public static Content convertCodeExecutionParts(
    * @return The extracted code if found.
    */
   public static Optional<String> extractCodeAndTruncateContent(
-      Content.Builder contentBuilder, List<List<String>> codeBlockDelimiters) {
+      Content.Builder contentBuilder, List<? extends List<String>> codeBlockDelimiters) {
     Content content = contentBuilder.build();
     if (content.parts().isEmpty() || content.parts().get().isEmpty()) {
       return Optional.empty();
diff --git a/core/src/main/java/com/google/adk/codeexecutors/VertexAiCodeExecutor.java b/core/src/main/java/com/google/adk/codeexecutors/VertexAiCodeExecutor.java
index 03110df6..5268edf3 100644
--- a/core/src/main/java/com/google/adk/codeexecutors/VertexAiCodeExecutor.java
+++ b/core/src/main/java/com/google/adk/codeexecutors/VertexAiCodeExecutor.java
@@ -37,8 +37,8 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
-import java.util.logging.Level;
-import java.util.logging.Logger;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * A code executor that uses Vertex Code Interpreter Extension to execute code.
@@ -51,7 +51,7 @@
  * setup.
  */
 public final class VertexAiCodeExecutor extends BaseCodeExecutor {
-  private static final Logger logger = Logger.getLogger(VertexAiCodeExecutor.class.getName());
+  private static final Logger logger = LoggerFactory.getLogger(VertexAiCodeExecutor.class);
 
   private static final ImmutableList<String> SUPPORTED_IMAGE_TYPES =
       ImmutableList.of("png", "jpg", "jpeg");
@@ -108,7 +108,8 @@ public final class VertexAiCodeExecutor extends BaseCodeExecutor {
           + "{df_info}\"\"\")";
 
   private final String resourceName;
-  private final ExtensionExecutionServiceClient codeInterpreterExtension;
+  private ExtensionExecutionServiceClient codeInterpreterExtension;
+  private final Object extensionClientLock = new Object();
 
   /**
    * Initializes the VertexAiCodeExecutor.
@@ -123,26 +124,11 @@ public VertexAiCodeExecutor(String resourceName) {
     }
 
     if (resolvedResourceName == null || resolvedResourceName.isEmpty()) {
-      logger.warning(
+      logger.warn(
           "No resource name found for Vertex AI Code Interpreter. It will not be available.");
       this.resourceName = null;
-      this.codeInterpreterExtension = null;
     } else {
       this.resourceName = resolvedResourceName;
-      try {
-        String[] parts = this.resourceName.split("/");
-        if (parts.length < 4 || !parts[2].equals("locations")) {
-          throw new IllegalArgumentException("Invalid resource name format: " + this.resourceName);
-        }
-        String location = parts[3];
-        String endpoint = String.format("%s-aiplatform.googleapis.com:443", location);
-        ExtensionExecutionServiceSettings settings =
-            ExtensionExecutionServiceSettings.newBuilder().setEndpoint(endpoint).build();
-        this.codeInterpreterExtension = ExtensionExecutionServiceClient.create(settings);
-      } catch (IOException e) {
-        logger.log(Level.SEVERE, "Failed to create ExtensionExecutionServiceClient", e);
-        throw new IllegalStateException("Failed to create ExtensionExecutionServiceClient", e);
-      }
     }
   }
 
@@ -188,9 +174,9 @@ public CodeExecutionResult executeCode(
 
   private Map<String, Object> executeCodeInterpreter(
       String code, List<File> inputFiles, Optional<String> sessionId) {
+    ExtensionExecutionServiceClient codeInterpreterExtension = getCodeInterpreterExtension();
     if (codeInterpreterExtension == null) {
-      logger.warning(
-          "Vertex AI Code Interpreter execution is not available. Returning empty result.");
+      logger.warn("Vertex AI Code Interpreter execution is not available. Returning empty result.");
       return ImmutableMap.of(
           "execution_result", "", "execution_error", "", "output_files", new ArrayList<>());
     }
@@ -231,7 +217,7 @@ private Map<String, Object> executeCodeInterpreter(
       ObjectMapper mapper = new ObjectMapper();
       return mapper.readValue(jsonOutput, new TypeReference<Map<String, Object>>() {});
     } catch (IOException e) {
-      logger.log(Level.SEVERE, "Failed to parse JSON from code interpreter: " + jsonOutput, e);
+      logger.error("Failed to parse JSON from code interpreter: " + jsonOutput, e);
       return ImmutableMap.of(
           "execution_result",
           "",
@@ -242,6 +228,31 @@ private Map<String, Object> executeCodeInterpreter(
     }
   }
 
+  private ExtensionExecutionServiceClient getCodeInterpreterExtension() {
+    if (this.resourceName == null) {
+      return null;
+    }
+    synchronized (extensionClientLock) {
+      if (this.codeInterpreterExtension == null) {
+        try {
+          String[] parts = this.resourceName.split("/");
+          if (parts.length < 4 || !parts[2].equals("locations")) {
+            throw new IllegalArgumentException(
+                "Invalid resource name format: " + this.resourceName);
+          }
+          String location = parts[3];
+          String endpoint = String.format("%s-aiplatform.googleapis.com:443", location);
+          ExtensionExecutionServiceSettings settings =
+              ExtensionExecutionServiceSettings.newBuilder().setEndpoint(endpoint).build();
+          this.codeInterpreterExtension = ExtensionExecutionServiceClient.create(settings);
+        } catch (IOException e) {
+          throw new IllegalStateException("Failed to create ExtensionExecutionServiceClient", e);
+        }
+      }
+      return this.codeInterpreterExtension;
+    }
+  }
+
   private String getCodeWithImports(String code) {
     return String.format("%s\n\n%s", IMPORTED_LIBRARIES, code);
   }