feat: Support output schema with tools for LlmAgent

core/src/main/java/com/google/adk/agents/LlmAgent.java

@@ -48,6 +48,7 @@
 import com.google.adk.models.Model;
 import com.google.adk.tools.BaseTool;
 import com.google.adk.tools.BaseToolset;
+import com.google.adk.tools.SetModelResponseTool;
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
@@ -135,7 +136,17 @@ protected LlmAgent(Builder builder) {
     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();
+    ImmutableList<Object> userTools =
+        builder.toolsUnion != null ? builder.toolsUnion : ImmutableList.of();
+    if (builder.outputSchema != null && !userTools.isEmpty()) {
+      this.toolsUnion =
+          ImmutableList.builder()
+              .addAll(userTools)
+              .add(new SetModelResponseTool(builder.outputSchema))
+              .build();
+    } else {
+      this.toolsUnion = userTools;
+    }
     this.toolsets = extractToolsets(this.toolsUnion);
     this.codeExecutor = Optional.ofNullable(builder.codeExecutor);
 
@@ -541,12 +552,6 @@ protected void validate() {
                   + ": if outputSchema is set, subAgents must be empty to disable agent"
                   + " transfer.");
         }
-        if (this.toolsUnion != null && !this.toolsUnion.isEmpty()) {
-          throw new IllegalArgumentException(
-              "Invalid config for agent "
-                  + this.name
-                  + ": if outputSchema is set, tools must be empty.");
-        }
       }
     }
 

core/src/main/java/com/google/adk/flows/llmflows/Basic.java

@@ -57,7 +57,9 @@ public Single<RequestProcessor.RequestProcessingResult> processRequest(
             .config(agent.generateContentConfig().orElse(GenerateContentConfig.builder().build()))
             .liveConnectConfig(liveConnectConfigBuilder.build());
 
-    agent.outputSchema().ifPresent(builder::outputSchema);
+    if (agent.outputSchema().isPresent() && agent.toolsUnion().isEmpty()) {
+      builder.outputSchema(agent.outputSchema().get());
+    }
     return Single.just(
         RequestProcessor.RequestProcessingResult.create(builder.build(), ImmutableList.of()));
   }

core/src/main/java/com/google/adk/tools/SetModelResponseTool.java

@@ -0,0 +1,58 @@
+/*
+ * Copyright 2025 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;
+
+import com.google.adk.SchemaUtils;
+import com.google.genai.types.FunctionDeclaration;
+import com.google.genai.types.Schema;
+import io.reactivex.rxjava3.core.Single;
+import java.util.Map;
+import java.util.Optional;
+
+/** Tool for setting model response when using output_schema with other tools. */
+public class SetModelResponseTool extends BaseTool {
+
+  private final Schema outputSchema;
+
+  public SetModelResponseTool(Schema outputSchema) {
+    super(
+        "set_model_response",
+        "Set your final response using the required output schema. Use this tool to provide your"
+            + " final structured answer instead of outputting text directly.");
+    this.outputSchema = outputSchema;
+  }
+
+  @Override
+  public Optional<FunctionDeclaration> declaration() {
+    return Optional.of(
+        FunctionDeclaration.builder()
+            .name(name())
+            .description(description())
+            .parameters(outputSchema)
+            .build());
+  }
+
+  @Override
+  public Single<Map<String, Object>> runAsync(Map<String, Object> args, ToolContext toolContext) {
+    try {
+      SchemaUtils.validateMapOnSchema(args, outputSchema, /* isInput= */ false);
+    } catch (IllegalArgumentException e) {
+      return Single.error(e);
+    }
+    return Single.just(args);
+  }
+}

core/src/test/java/com/google/adk/agents/LlmAgentTest.java

@@ -140,7 +140,7 @@ public void run_withToolsAndMaxSteps_stopsAfterMaxSteps() {
   }
 
   @Test
-  public void build_withOutputSchemaAndTools_throwsIllegalArgumentException() {
+  public void build_withOutputSchemaAndTools_addsSetModelResponseTool() {
     BaseTool tool =
         new BaseTool("test_tool", "test_description") {
           @Override
@@ -156,22 +156,18 @@ public Optional<FunctionDeclaration> declaration() {
             .required(ImmutableList.of("status"))
             .build();
 
-    // Expecting an IllegalArgumentException when building the agent
-    IllegalArgumentException exception =
-        assertThrows(
-            IllegalArgumentException.class,
-            () ->
-                LlmAgent.builder() // Use the agent builder directly
-                    .name("agent with invalid tool config")
-                    .outputSchema(outputSchema) // Set the output schema
-                    .tools(ImmutableList.of(tool)) // Set tools (this should cause the error)
-                    .build()); // Attempt to build the agent
+    LlmAgent agent =
+        LlmAgent.builder()
+            .name("agent with valid tool config")
+            .outputSchema(outputSchema)
+            .tools(ImmutableList.of(tool))
+            .build();
 
-    assertThat(exception)
-        .hasMessageThat()
-        .contains(
-            "Invalid config for agent agent with invalid tool config: if outputSchema is set, tools"
-                + " must be empty");
+    List<BaseTool> tools = agent.tools();
+    assertThat(tools).hasSize(2);
+    assertThat(tools.stream().anyMatch(t -> t instanceof com.google.adk.tools.SetModelResponseTool))
+        .isTrue();
+    assertThat(tools.stream().anyMatch(t -> t.name().equals("test_tool"))).isTrue();
   }
 
   @Test

core/src/test/java/com/google/adk/tools/SetModelResponseToolTest.java

@@ -0,0 +1,123 @@
+/*
+ * Copyright 2025 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;
+
+import static com.google.common.truth.Truth.assertThat;
+import static org.junit.Assert.assertThrows;
+
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
+import com.google.genai.types.FunctionDeclaration;
+import com.google.genai.types.Schema;
+import java.util.Map;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+@RunWith(JUnit4.class)
+public final class SetModelResponseToolTest {
+
+  @Test
+  public void declaration_returnsCorrectFunctionDeclaration() {
+    Schema outputSchema =
+        Schema.builder()
+            .type("OBJECT")
+            .properties(ImmutableMap.of("field1", Schema.builder().type("STRING").build()))
+            .required(ImmutableList.of("field1"))
+            .build();
+
+    SetModelResponseTool tool = new SetModelResponseTool(outputSchema);
+    FunctionDeclaration declaration = tool.declaration().get();
+
+    assertThat(declaration.name()).hasValue("set_model_response");
+    assertThat(declaration.description()).isPresent();
+    assertThat(declaration.description().get()).contains("Set your final response");
+    assertThat(declaration.parameters()).hasValue(outputSchema);
+  }
+
+  @Test
+  public void runAsync_returnsArgs() {
+    Schema outputSchema =
+        Schema.builder()
+            .type("OBJECT")
+            .properties(ImmutableMap.of("field1", Schema.builder().type("STRING").build()))
+            .build();
+
+    SetModelResponseTool tool = new SetModelResponseTool(outputSchema);
+    Map<String, Object> args = ImmutableMap.of("field1", "value1");
+
+    Map<String, Object> result = tool.runAsync(args, null).blockingGet();
+
+    assertThat(result).isEqualTo(args);
+  }
+
+  @Test
+  public void runAsync_validatesArgs() {
+    Schema outputSchema =
+        Schema.builder()
+            .type("OBJECT")
+            .properties(ImmutableMap.of("field1", Schema.builder().type("STRING").build()))
+            .required(ImmutableList.of("field1"))
+            .build();
+
+    SetModelResponseTool tool = new SetModelResponseTool(outputSchema);
+    Map<String, Object> invalidArgs = ImmutableMap.of("field2", "value2");
+
+    // Should throw validation error
+    IllegalArgumentException exception =
+        assertThrows(
+            IllegalArgumentException.class, () -> tool.runAsync(invalidArgs, null).blockingGet());
+
+    assertThat(exception).hasMessageThat().contains("does not match agent output schema");
+  }
+
+  @Test
+  public void runAsync_validatesComplexArgs() {
+    Schema complexSchema =
+        Schema.builder()
+            .type("OBJECT")
+            .properties(
+                ImmutableMap.of(
+                    "id", Schema.builder().type("INTEGER").build(),
+                    "tags",
+                        Schema.builder()
+                            .type("ARRAY")
+                            .items(Schema.builder().type("STRING").build())
+                            .build(),
+                    "metadata",
+                        Schema.builder()
+                            .type("OBJECT")
+                            .properties(
+                                ImmutableMap.of("key", Schema.builder().type("STRING").build()))
+                            .build()))
+            .required(ImmutableList.of("id", "tags", "metadata"))
+            .build();
+
+    SetModelResponseTool tool = new SetModelResponseTool(complexSchema);
+    Map<String, Object> complexArgs =
+        ImmutableMap.of(
+            "id", 123,
+            "tags", ImmutableList.of("tag1", "tag2"),
+            "metadata", ImmutableMap.of("key", "value"));
+
+    Map<String, Object> result = tool.runAsync(complexArgs, null).blockingGet();
+
+    assertThat(result).containsEntry("id", 123);
+    assertThat(result).containsEntry("tags", ImmutableList.of("tag1", "tag2"));
+    assertThat(result).containsEntry("metadata", ImmutableMap.of("key", "value"));
+  }
+}