fix(mcp): guard empty tool parameters in adkToMcpToolType

core/src/main/java/com/google/adk/tools/mcp/ConversionUtils.java

@@ -29,16 +29,15 @@ public final class ConversionUtils {
 
   private static final McpJsonMapper jsonMapper = McpJsonDefaults.getMapper();
 
-  public McpSchema.Tool adkToMcpToolType(BaseTool tool) {
-    Optional<FunctionDeclaration> toolDeclaration = tool.declaration();
-    if (toolDeclaration.isEmpty()) {
+  public static McpSchema.Tool adkToMcpToolType(BaseTool tool) {
+    Optional<Schema> parameters = tool.declaration().flatMap(FunctionDeclaration::parameters);
+    if (parameters.isEmpty()) {
       return McpSchema.Tool.builder().name(tool.name()).description(tool.description()).build();
     }
-    Schema geminiSchema = toolDeclaration.get().parameters().get();
     return McpSchema.Tool.builder()
         .name(tool.name())
         .description(tool.description())
-        .inputSchema(jsonMapper, geminiSchema.toJson())
+        .inputSchema(jsonMapper, parameters.get().toJson())
         .build();
   }
 

core/src/test/java/com/google/adk/tools/mcp/ConversionUtilsTest.java

@@ -0,0 +1,88 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.adk.tools.mcp;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.adk.tools.BaseTool;
+import com.google.genai.types.FunctionDeclaration;
+import com.google.genai.types.Schema;
+import io.modelcontextprotocol.spec.McpSchema;
+import java.util.Optional;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+@RunWith(JUnit4.class)
+public final class ConversionUtilsTest {
+
+  /** Minimal {@link BaseTool} whose declaration is supplied by the test. */
+  private static final class FakeTool extends BaseTool {
+    private final Optional<FunctionDeclaration> declaration;
+
+    FakeTool(String name, String description, Optional<FunctionDeclaration> declaration) {
+      super(name, description);
+      this.declaration = declaration;
+    }
+
+    @Override
+    public Optional<FunctionDeclaration> declaration() {
+      return declaration;
+    }
+  }
+
+  @Test
+  public void adkToMcpToolType_declarationWithParameters_setsInputSchema() {
+    FunctionDeclaration declaration =
+        FunctionDeclaration.builder()
+            .name("withParams")
+            .parameters(Schema.builder().type("OBJECT").build())
+            .build();
+    BaseTool tool = new FakeTool("withParams", "has params", Optional.of(declaration));
+
+    McpSchema.Tool result = ConversionUtils.adkToMcpToolType(tool);
+
+    assertThat(result.name()).isEqualTo("withParams");
+    assertThat(result.description()).isEqualTo("has params");
+    assertThat(result.inputSchema()).isNotNull();
+  }
+
+  @Test
+  public void adkToMcpToolType_declarationWithoutParameters_omitsInputSchema() {
+    // A present declaration with no parameters is a valid no-argument tool. Before the fix this
+    // threw NoSuchElementException from an unguarded Optional.get() on parameters().
+    FunctionDeclaration declaration = FunctionDeclaration.builder().name("noParams").build();
+    BaseTool tool = new FakeTool("noParams", "no params", Optional.of(declaration));
+
+    McpSchema.Tool result = ConversionUtils.adkToMcpToolType(tool);
+
+    assertThat(result.name()).isEqualTo("noParams");
+    assertThat(result.description()).isEqualTo("no params");
+    assertThat(result.inputSchema()).isNull();
+  }
+
+  @Test
+  public void adkToMcpToolType_noDeclaration_omitsInputSchema() {
+    BaseTool tool = new FakeTool("bare", "no declaration", Optional.empty());
+
+    McpSchema.Tool result = ConversionUtils.adkToMcpToolType(tool);
+
+    assertThat(result.name()).isEqualTo("bare");
+    assertThat(result.description()).isEqualTo("no declaration");
+    assertThat(result.inputSchema()).isNull();
+  }
+}