diff --git a/dev/src/main/java/com/google/adk/web/AdkWebServer.java b/dev/src/main/java/com/google/adk/web/AdkWebServer.java
index 0d7c7d0e1..b321cef27 100644
--- a/dev/src/main/java/com/google/adk/web/AdkWebServer.java
+++ b/dev/src/main/java/com/google/adk/web/AdkWebServer.java
@@ -35,6 +35,8 @@
 import org.springframework.context.ApplicationContextInitializer;
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Primary;
+import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
 import org.springframework.web.servlet.config.annotation.ResourceHandlerRegistry;
 import org.springframework.web.servlet.config.annotation.ViewControllerRegistry;
 import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
@@ -87,10 +89,25 @@ public BaseMemoryService memoryService() {
    * @return Configured ObjectMapper instance
    */
   @Bean
+  @Primary
   public ObjectMapper objectMapper() {
     return JsonBaseModel.getMapper();
   }
 
+  /**
+   * Configures the message converter to use the custom ADK ObjectMapper. This ensures that Spring
+   * Web uses the correct JSON serialization settings (like omitting absent optional fields) and
+   * prevents double-serialization issues, particularly for Server-Sent Events (SSE).
+   *
+   * @param objectMapper The primary ObjectMapper configured for the ADK.
+   * @return A configured MappingJackson2HttpMessageConverter.
+   */
+  @Bean
+  public MappingJackson2HttpMessageConverter mappingJackson2HttpMessageConverter(
+      ObjectMapper objectMapper) {
+    return new MappingJackson2HttpMessageConverter(objectMapper);
+  }
+
   /**
    * Configures resource handlers for serving static content (like the Dev UI). Maps requests
    * starting with "/dev-ui/" to the directory specified by the 'adk.web.ui.dir' system property.
diff --git a/dev/src/main/java/com/google/adk/web/controller/ExecutionController.java b/dev/src/main/java/com/google/adk/web/controller/ExecutionController.java
index 6d5a2764c..e88a83cef 100644
--- a/dev/src/main/java/com/google/adk/web/controller/ExecutionController.java
+++ b/dev/src/main/java/com/google/adk/web/controller/ExecutionController.java
@@ -81,7 +81,11 @@ public List<Event> agentRun(@RequestBody AgentRunRequest request) {
       RunConfig runConfig = RunConfig.builder().setStreamingMode(StreamingMode.NONE).build();
       Flowable<Event> eventStream =
           runner.runAsync(
-              request.userId, request.sessionId, request.newMessage, runConfig, request.stateDelta);
+              request.userId,
+              request.sessionId,
+              request.getNewMessage(),
+              runConfig,
+              request.stateDelta);
 
       List<Event> events = Lists.newArrayList(eventStream.blockingIterable());
       log.info("Agent run for session {} generated {} events.", request.sessionId, events.size());
@@ -155,7 +159,7 @@ public SseEmitter agentRunSse(@RequestBody AgentRunRequest request) {
               runner.runAsync(
                   request.userId,
                   request.sessionId,
-                  request.newMessage,
+                  request.getNewMessage(),
                   runConfig,
                   request.stateDelta);
 
@@ -167,7 +171,7 @@ public SseEmitter agentRunSse(@RequestBody AgentRunRequest request) {
                         try {
                           log.debug(
                               "SseEmitter: Sending event {} for session {}", event.id(), sessionId);
-                          emitter.send(SseEmitter.event().data(event.toJson()));
+                          emitter.send(SseEmitter.event().data(event));
                         } catch (IOException e) {
                           log.error(
                               "SseEmitter: IOException sending event for session {}: {}",
diff --git a/dev/src/main/java/com/google/adk/web/dto/AgentRunRequest.java b/dev/src/main/java/com/google/adk/web/dto/AgentRunRequest.java
index 652de5aac..025aa64eb 100644
--- a/dev/src/main/java/com/google/adk/web/dto/AgentRunRequest.java
+++ b/dev/src/main/java/com/google/adk/web/dto/AgentRunRequest.java
@@ -17,6 +17,7 @@
 package com.google.adk.web.dto;
 
 import com.fasterxml.jackson.annotation.JsonProperty;
+import com.google.adk.JsonBaseModel;
 import com.google.genai.types.Content;
 import java.util.Map;
 import javax.annotation.Nullable;
@@ -36,7 +37,7 @@ public class AgentRunRequest {
   public String sessionId;
 
   @JsonProperty("newMessage")
-  public Content newMessage;
+  public Object newMessage;
 
   @JsonProperty("streaming")
   public boolean streaming = false;
@@ -65,7 +66,17 @@ public String getSessionId() {
   }
 
   public Content getNewMessage() {
-    return newMessage;
+    if (newMessage instanceof Content) {
+      return (Content) newMessage;
+    }
+    if (newMessage != null) {
+      try {
+        return JsonBaseModel.getMapper().convertValue(newMessage, Content.class);
+      } catch (IllegalArgumentException e) {
+        throw new IllegalStateException("Failed to parse newMessage into Content", e);
+      }
+    }
+    return null;
   }
 
   public boolean getStreaming() {