diff --git a/core/src/main/java/com/google/adk/tools/AgentTool.java b/core/src/main/java/com/google/adk/tools/AgentTool.java
index 66d4a2700..5e8798d0e 100644
--- a/core/src/main/java/com/google/adk/tools/AgentTool.java
+++ b/core/src/main/java/com/google/adk/tools/AgentTool.java
@@ -31,6 +31,7 @@
 import com.google.adk.runner.Runner;
 import com.google.adk.sessions.State;
 import com.google.common.annotations.VisibleForTesting;
+import com.google.common.base.Strings;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.google.genai.types.Content;
@@ -131,8 +132,14 @@ private Optional<Schema> getOutputSchema(BaseAgent agent) {
 
   @Override
   public Optional<FunctionDeclaration> declaration() {
+
+    // The genai FunctionDeclaration builder uses Optional.of() internally, which NPEs on a null
+    // description. Coerce null to an empty string. This matches the Python, Go, and Kotlin ports,
+    // which send an empty description when the agent has none.
+    String desc = Strings.nullToEmpty(this.description());
+
     FunctionDeclaration.Builder builder =
-        FunctionDeclaration.builder().description(this.description()).name(this.name());
+        FunctionDeclaration.builder().description(desc).name(this.name());
 
     Optional<Schema> agentInputSchema = getInputSchema(agent);
 
diff --git a/core/src/test/java/com/google/adk/tools/AgentToolTest.java b/core/src/test/java/com/google/adk/tools/AgentToolTest.java
index b37db6611..755cebc46 100644
--- a/core/src/test/java/com/google/adk/tools/AgentToolTest.java
+++ b/core/src/test/java/com/google/adk/tools/AgentToolTest.java
@@ -43,6 +43,7 @@
 import io.reactivex.rxjava3.core.Flowable;
 import io.reactivex.rxjava3.core.Maybe;
 import java.util.Map;
+import java.util.Optional;
 import java.util.concurrent.atomic.AtomicBoolean;
 import org.junit.Before;
 import org.junit.Test;
@@ -882,4 +883,21 @@ private ToolContext createToolContext(BaseAgent agent) {
                 .build())
         .build();
   }
+
+  @Test
+  public void declaration_withNullDescription_usesEmptyDescription() {
+    // Create an agent with a name but NO description
+    LlmAgent testAgent =
+        LlmAgent.builder()
+            .name("TestAgent")
+            // .description() is intentionally left out
+            .build();
+
+    AgentTool tool = AgentTool.create(testAgent);
+    Optional<FunctionDeclaration> declaration = tool.declaration();
+    assertThat(declaration).isPresent();
+    assertThat(declaration.get().name()).hasValue("TestAgent");
+    // Matches the Python, Go, and Kotlin ports: a missing description becomes an empty string.
+    assertThat(declaration.get().description()).hasValue("");
+  }
 }