diff --git a/core/src/main/java/com/google/adk/sessions/InMemorySessionService.java b/core/src/main/java/com/google/adk/sessions/InMemorySessionService.java
index 72a14cc4d..c11e3d1db 100644
--- a/core/src/main/java/com/google/adk/sessions/InMemorySessionService.java
+++ b/core/src/main/java/com/google/adk/sessions/InMemorySessionService.java
@@ -20,6 +20,7 @@
 
 import com.google.adk.events.Event;
 import com.google.adk.events.EventActions;
+import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.ImmutableList;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import io.reactivex.rxjava3.core.Completable;
@@ -32,10 +33,13 @@
 import java.util.Map;
 import java.util.Objects;
 import java.util.Optional;
+import java.util.Set;
 import java.util.UUID;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
 import org.jspecify.annotations.Nullable;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * An in-memory implementation of {@link BaseSessionService} assuming {@link Session} objects are
@@ -49,6 +53,23 @@
  * during retrieval operations ({@code getSession}, {@code createSession}).
  */
 public final class InMemorySessionService implements BaseSessionService {
+
+  private static final Logger log = LoggerFactory.getLogger(InMemorySessionService.class);
+
+  /**
+   * Reserved session-state keys that are managed internally by the ADK framework. Callers are not
+   * permitted to set or override these keys through the public API (initial session state or
+   * per-run stateDelta). Allowing external writes to these keys would let an untrusted caller steer
+   * internal framework behaviour, such as hijacking the code-execution session identifier used by
+   * {@code VertexAiCodeExecutor}.
+   */
+  private static final Set<String> RESERVED_STATE_KEYS =
+      ImmutableSet.of(
+          "_code_execution_context",
+          "_code_executor_input_files",
+          "_code_executor_error_counts",
+          "_code_execution_results");
+
   // Structure: appName -> userId -> sessionId -> Session
   private final ConcurrentMap<String, ConcurrentMap<String, ConcurrentMap<String, Session>>>
       sessions;
@@ -65,6 +86,31 @@ public InMemorySessionService() {
     this.appState = new ConcurrentHashMap<>();
   }
 
+  /**
+   * Removes reserved internal keys from a caller-supplied state map before it is persisted.
+   * Logs a warning for each key that is dropped.
+   *
+   * @param state The caller-supplied state map (may be null).
+   * @return A new {@link ConcurrentHashMap} containing only the non-reserved entries.
+   */
+  private static ConcurrentMap<String, Object> sanitizeCallerState(
+      @Nullable Map<String, Object> state) {
+    if (state == null) {
+      return new ConcurrentHashMap<>();
+    }
+    ConcurrentMap<String, Object> sanitized = new ConcurrentHashMap<>();
+    state.forEach(
+        (key, value) -> {
+          if (RESERVED_STATE_KEYS.contains(key)) {
+            log.warn(
+                "Caller attempted to set reserved internal state key '{}'; ignoring.", key);
+          } else {
+            sanitized.put(key, value);
+          }
+        });
+    return sanitized;
+  }
+
   @Override
   public Single<Session> createSession(
       String appName,
@@ -89,9 +135,8 @@ public Single<Session> createSession(
             .filter(s -> !s.isEmpty())
             .orElseGet(() -> UUID.randomUUID().toString());
 
-    // Ensure state map and events list are mutable for the new session
-    ConcurrentMap<String, Object> initialState =
-        (state == null) ? new ConcurrentHashMap<>() : new ConcurrentHashMap<>(state);
+    // Sanitize caller-supplied state: strip reserved internal keys before persisting.
+    ConcurrentMap<String, Object> initialState = sanitizeCallerState(state);
 
     // Assuming Session constructor or setters allow setting these mutable collections
     Session newSession =
@@ -268,6 +313,12 @@ public Single<Event> appendEvent(Session session, Event event) {
                       .put(userStateKey, value);
                 }
               } else {
+                // Reject writes to reserved internal keys from any external stateDelta.
+                if (RESERVED_STATE_KEYS.contains(key)) {
+                  log.warn(
+                      "stateDelta contains reserved internal key '{}'; ignoring write.", key);
+                  return;
+                }
                 if (value == State.REMOVED) {
                   session.state().remove(key);
                 } else {