diff --git a/core/src/main/java/com/google/adk/agents/LlmAgent.java b/core/src/main/java/com/google/adk/agents/LlmAgent.java
index 78ebe757..f7628df7 100644
--- a/core/src/main/java/com/google/adk/agents/LlmAgent.java
+++ b/core/src/main/java/com/google/adk/agents/LlmAgent.java
@@ -17,6 +17,7 @@
 package com.google.adk.agents;
 
 import static com.google.common.collect.ImmutableList.toImmutableList;
+import static java.util.Objects.requireNonNullElse;
 import static java.util.stream.Collectors.joining;
 
 import com.fasterxml.jackson.core.JsonProcessingException;
@@ -103,12 +104,12 @@ public enum IncludeContents {
   private final Optional<Integer> maxSteps;
   private final boolean disallowTransferToParent;
   private final boolean disallowTransferToPeers;
-  private final Optional<List<? extends BeforeModelCallback>> beforeModelCallback;
-  private final Optional<List<? extends AfterModelCallback>> afterModelCallback;
-  private final Optional<List<? extends OnModelErrorCallback>> onModelErrorCallback;
-  private final Optional<List<? extends BeforeToolCallback>> beforeToolCallback;
-  private final Optional<List<? extends AfterToolCallback>> afterToolCallback;
-  private final Optional<List<? extends OnToolErrorCallback>> onToolErrorCallback;
+  private final ImmutableList<? extends BeforeModelCallback> beforeModelCallback;
+  private final ImmutableList<? extends AfterModelCallback> afterModelCallback;
+  private final ImmutableList<? extends OnModelErrorCallback> onModelErrorCallback;
+  private final ImmutableList<? extends BeforeToolCallback> beforeToolCallback;
+  private final ImmutableList<? extends AfterToolCallback> afterToolCallback;
+  private final ImmutableList<? extends OnToolErrorCallback> onToolErrorCallback;
   private final Optional<Schema> inputSchema;
   private final Optional<Schema> outputSchema;
   private final Optional<Executor> executor;
@@ -126,29 +127,28 @@ protected LlmAgent(Builder builder) {
         builder.beforeAgentCallback,
         builder.afterAgentCallback);
     this.model = Optional.ofNullable(builder.model);
-    this.instruction =
-        builder.instruction == null ? new Instruction.Static("") : builder.instruction;
+    this.instruction = requireNonNullElse(builder.instruction, new Instruction.Static(""));
     this.globalInstruction =
-        builder.globalInstruction == null ? new Instruction.Static("") : builder.globalInstruction;
+        requireNonNullElse(builder.globalInstruction, new Instruction.Static(""));
     this.generateContentConfig = Optional.ofNullable(builder.generateContentConfig);
     this.exampleProvider = Optional.ofNullable(builder.exampleProvider);
-    this.includeContents =
-        builder.includeContents != null ? builder.includeContents : IncludeContents.DEFAULT;
+    this.includeContents = requireNonNullElse(builder.includeContents, IncludeContents.DEFAULT);
     this.planning = builder.planning != null && builder.planning;
     this.maxSteps = Optional.ofNullable(builder.maxSteps);
     this.disallowTransferToParent = builder.disallowTransferToParent;
     this.disallowTransferToPeers = builder.disallowTransferToPeers;
-    this.beforeModelCallback = Optional.ofNullable(builder.beforeModelCallback);
-    this.afterModelCallback = Optional.ofNullable(builder.afterModelCallback);
-    this.onModelErrorCallback = Optional.ofNullable(builder.onModelErrorCallback);
-    this.beforeToolCallback = Optional.ofNullable(builder.beforeToolCallback);
-    this.afterToolCallback = Optional.ofNullable(builder.afterToolCallback);
-    this.onToolErrorCallback = Optional.ofNullable(builder.onToolErrorCallback);
+    this.beforeModelCallback = requireNonNullElse(builder.beforeModelCallback, ImmutableList.of());
+    this.afterModelCallback = requireNonNullElse(builder.afterModelCallback, ImmutableList.of());
+    this.onModelErrorCallback =
+        requireNonNullElse(builder.onModelErrorCallback, ImmutableList.of());
+    this.beforeToolCallback = requireNonNullElse(builder.beforeToolCallback, ImmutableList.of());
+    this.afterToolCallback = requireNonNullElse(builder.afterToolCallback, ImmutableList.of());
+    this.onToolErrorCallback = requireNonNullElse(builder.onToolErrorCallback, ImmutableList.of());
     this.inputSchema = Optional.ofNullable(builder.inputSchema);
     this.outputSchema = Optional.ofNullable(builder.outputSchema);
     this.executor = Optional.ofNullable(builder.executor);
     this.outputKey = Optional.ofNullable(builder.outputKey);
-    this.toolsUnion = builder.toolsUnion != null ? builder.toolsUnion : ImmutableList.of();
+    this.toolsUnion = requireNonNullElse(builder.toolsUnion, ImmutableList.of());
     this.toolsets = extractToolsets(this.toolsUnion);
     this.codeExecutor = Optional.ofNullable(builder.codeExecutor);
 
@@ -850,27 +850,27 @@ public boolean disallowTransferToPeers() {
     return disallowTransferToPeers;
   }
 
-  public Optional<List<? extends BeforeModelCallback>> beforeModelCallback() {
+  public List<? extends BeforeModelCallback> beforeModelCallback() {
     return beforeModelCallback;
   }
 
-  public Optional<List<? extends AfterModelCallback>> afterModelCallback() {
+  public List<? extends AfterModelCallback> afterModelCallback() {
     return afterModelCallback;
   }
 
-  public Optional<List<? extends BeforeToolCallback>> beforeToolCallback() {
+  public List<? extends BeforeToolCallback> beforeToolCallback() {
     return beforeToolCallback;
   }
 
-  public Optional<List<? extends AfterToolCallback>> afterToolCallback() {
+  public List<? extends AfterToolCallback> afterToolCallback() {
     return afterToolCallback;
   }
 
-  public Optional<List<? extends OnModelErrorCallback>> onModelErrorCallback() {
+  public List<? extends OnModelErrorCallback> onModelErrorCallback() {
     return onModelErrorCallback;
   }
 
-  public Optional<List<? extends OnToolErrorCallback>> onToolErrorCallback() {
+  public List<? extends OnToolErrorCallback> onToolErrorCallback() {
     return onToolErrorCallback;
   }
 
@@ -880,7 +880,7 @@ public Optional<List<? extends OnToolErrorCallback>> onToolErrorCallback() {
    * <p>This method is only for use by Agent Development Kit.
    */
   public List<? extends BeforeModelCallback> canonicalBeforeModelCallbacks() {
-    return beforeModelCallback.orElse(ImmutableList.of());
+    return beforeModelCallback;
   }
 
   /**
@@ -889,7 +889,7 @@ public List<? extends BeforeModelCallback> canonicalBeforeModelCallbacks() {
    * <p>This method is only for use by Agent Development Kit.
    */
   public List<? extends AfterModelCallback> canonicalAfterModelCallbacks() {
-    return afterModelCallback.orElse(ImmutableList.of());
+    return afterModelCallback;
   }
 
   /**
@@ -898,7 +898,7 @@ public List<? extends AfterModelCallback> canonicalAfterModelCallbacks() {
    * <p>This method is only for use by Agent Development Kit.
    */
   public List<? extends OnModelErrorCallback> canonicalOnModelErrorCallbacks() {
-    return onModelErrorCallback.orElse(ImmutableList.of());
+    return onModelErrorCallback;
   }
 
   /**
@@ -907,7 +907,7 @@ public List<? extends OnModelErrorCallback> canonicalOnModelErrorCallbacks() {
    * <p>This method is only for use by Agent Development Kit.
    */
   public List<? extends BeforeToolCallback> canonicalBeforeToolCallbacks() {
-    return beforeToolCallback.orElse(ImmutableList.of());
+    return beforeToolCallback;
   }
 
   /**
@@ -916,7 +916,7 @@ public List<? extends BeforeToolCallback> canonicalBeforeToolCallbacks() {
    * <p>This method is only for use by Agent Development Kit.
    */
   public List<? extends AfterToolCallback> canonicalAfterToolCallbacks() {
-    return afterToolCallback.orElse(ImmutableList.of());
+    return afterToolCallback;
   }
 
   /**
@@ -925,7 +925,7 @@ public List<? extends AfterToolCallback> canonicalAfterToolCallbacks() {
    * <p>This method is only for use by Agent Development Kit.
    */
   public List<? extends OnToolErrorCallback> canonicalOnToolErrorCallbacks() {
-    return onToolErrorCallback.orElse(ImmutableList.of());
+    return onToolErrorCallback;
   }
 
   public Optional<Schema> inputSchema() {
diff --git a/core/src/test/java/com/google/adk/agents/ConfigAgentUtilsTest.java b/core/src/test/java/com/google/adk/agents/ConfigAgentUtilsTest.java
index 4825efca..11e07a09 100644
--- a/core/src/test/java/com/google/adk/agents/ConfigAgentUtilsTest.java
+++ b/core/src/test/java/com/google/adk/agents/ConfigAgentUtilsTest.java
@@ -1161,20 +1161,25 @@ public void fromConfig_withConfiguredCallbacks_resolvesCallbacks()
 
     String pfx = "test.callbacks.";
     registry.register(
-        pfx + "before_agent_1", (Callbacks.BeforeAgentCallback) (ctx) -> Maybe.empty());
+        pfx + "before_agent_1", (Callbacks.BeforeAgentCallback) (unusedCtx) -> Maybe.empty());
     registry.register(
-        pfx + "before_agent_2", (Callbacks.BeforeAgentCallback) (ctx) -> Maybe.empty());
-    registry.register(pfx + "after_agent_1", (Callbacks.AfterAgentCallback) (ctx) -> Maybe.empty());
+        pfx + "before_agent_2", (Callbacks.BeforeAgentCallback) (unusedCtx) -> Maybe.empty());
     registry.register(
-        pfx + "before_model_1", (Callbacks.BeforeModelCallback) (ctx, req) -> Maybe.empty());
+        pfx + "after_agent_1", (Callbacks.AfterAgentCallback) (unusedCtx) -> Maybe.empty());
     registry.register(
-        pfx + "after_model_1", (Callbacks.AfterModelCallback) (ctx, resp) -> Maybe.empty());
+        pfx + "before_model_1",
+        (Callbacks.BeforeModelCallback) (unusedCtx, unusedReq) -> Maybe.empty());
+    registry.register(
+        pfx + "after_model_1",
+        (Callbacks.AfterModelCallback) (unusedCtx, unusedResp) -> Maybe.empty());
     registry.register(
         pfx + "before_tool_1",
-        (Callbacks.BeforeToolCallback) (inv, tool, args, toolCtx) -> Maybe.empty());
+        (Callbacks.BeforeToolCallback)
+            (unusedInv, unusedTool, unusedArgs, unusedToolCtx) -> Maybe.empty());
     registry.register(
         pfx + "after_tool_1",
-        (Callbacks.AfterToolCallback) (inv, tool, args, toolCtx, resp) -> Maybe.empty());
+        (Callbacks.AfterToolCallback)
+            (unusedInv, unusedTool, unusedArgs, unusedToolCtx, unusedResp) -> Maybe.empty());
 
     File configFile = tempFolder.newFile("with_callbacks.yaml");
     Files.writeString(
@@ -1209,15 +1214,11 @@ public void fromConfig_withConfiguredCallbacks_resolvesCallbacks()
     assertThat(agent.afterAgentCallback()).isPresent();
     assertThat(agent.afterAgentCallback().get()).hasSize(1);
 
-    assertThat(llm.beforeModelCallback()).isPresent();
-    assertThat(llm.beforeModelCallback().get()).hasSize(1);
-    assertThat(llm.afterModelCallback()).isPresent();
-    assertThat(llm.afterModelCallback().get()).hasSize(1);
+    assertThat(llm.beforeModelCallback()).hasSize(1);
+    assertThat(llm.afterModelCallback()).hasSize(1);
 
-    assertThat(llm.beforeToolCallback()).isPresent();
-    assertThat(llm.beforeToolCallback().get()).hasSize(1);
-    assertThat(llm.afterToolCallback()).isPresent();
-    assertThat(llm.afterToolCallback().get()).hasSize(1);
+    assertThat(llm.beforeToolCallback()).hasSize(1);
+    assertThat(llm.afterToolCallback()).hasSize(1);
   }
 
   @Test
diff --git a/core/src/test/java/com/google/adk/agents/LlmAgentTest.java b/core/src/test/java/com/google/adk/agents/LlmAgentTest.java
index 519c9055..8a2ff6df 100644
--- a/core/src/test/java/com/google/adk/agents/LlmAgentTest.java
+++ b/core/src/test/java/com/google/adk/agents/LlmAgentTest.java
@@ -341,6 +341,13 @@ public void canonicalCallbacks_returnsEmptyListWhenNull() {
     assertThat(agent.canonicalBeforeToolCallbacks()).isEmpty();
     assertThat(agent.canonicalAfterToolCallbacks()).isEmpty();
     assertThat(agent.canonicalOnToolErrorCallbacks()).isEmpty();
+
+    assertThat(agent.beforeModelCallback()).isEmpty();
+    assertThat(agent.afterModelCallback()).isEmpty();
+    assertThat(agent.onModelErrorCallback()).isEmpty();
+    assertThat(agent.beforeToolCallback()).isEmpty();
+    assertThat(agent.afterToolCallback()).isEmpty();
+    assertThat(agent.onToolErrorCallback()).isEmpty();
   }
 
   @Test
@@ -371,5 +378,12 @@ public void canonicalCallbacks_returnsListWhenPresent() {
     assertThat(agent.canonicalBeforeToolCallbacks()).containsExactly(btc);
     assertThat(agent.canonicalAfterToolCallbacks()).containsExactly(atc);
     assertThat(agent.canonicalOnToolErrorCallbacks()).containsExactly(otec);
+
+    assertThat(agent.beforeModelCallback()).containsExactly(bmc);
+    assertThat(agent.afterModelCallback()).containsExactly(amc);
+    assertThat(agent.onModelErrorCallback()).containsExactly(omec);
+    assertThat(agent.beforeToolCallback()).containsExactly(btc);
+    assertThat(agent.afterToolCallback()).containsExactly(atc);
+    assertThat(agent.onToolErrorCallback()).containsExactly(otec);
   }
 }