feat: Improve validation in MethodToolCallbackProvider

This ensures that validation errors are caught early during object construction
rather than later when methods are called, providing better error feedback.

- Add validation for tool-annotated methods during construction
- Validate duplicate tool names in constructor instead of only at getToolCallbacks() time
- Add comprehensive test suite for MethodToolCallbackProvider

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>

spring-ai-model/src/main/java/org/springframework/ai/tool/method/MethodToolCallbackProvider.java

@@ -19,6 +19,7 @@ package org.springframework.ai.tool.method;
 import java.lang.reflect.Method;
 import java.util.Arrays;
 import java.util.List;
+import java.util.Optional;
 import java.util.function.Consumer;
 import java.util.function.Function;
 import java.util.function.Supplier;
@@ -44,6 +45,7 @@ import org.springframework.util.ReflectionUtils;
  * {@link Tool}-annotated methods.
  *
  * @author Thomas Vitale
+ * @author Christian Tzolov
  * @since 1.0.0
  */
 public final class MethodToolCallbackProvider implements ToolCallbackProvider {
@@ -55,7 +57,26 @@ public final class MethodToolCallbackProvider implements ToolCallbackProvider {
 	private MethodToolCallbackProvider(List<Object> toolObjects) {
 		Assert.notNull(toolObjects, "toolObjects cannot be null");
 		Assert.noNullElements(toolObjects, "toolObjects cannot contain null elements");
+		assertToolAnnotatedMethodsPresent(toolObjects);
 		this.toolObjects = toolObjects;
+		validateToolCallbacks(getToolCallbacks());
+	}
+
+	private void assertToolAnnotatedMethodsPresent(List<Object> toolObjects) {
+
+		for (Object toolObject : toolObjects) {
+			List<Method> toolMethods = Stream
+				.of(ReflectionUtils.getDeclaredMethods(
+						AopUtils.isAopProxy(toolObject) ? AopUtils.getTargetClass(toolObject) : toolObject.getClass()))
+				.filter(toolMethod -> toolMethod.isAnnotationPresent(Tool.class))
+				.filter(toolMethod -> !isFunctionalType(toolMethod))
+				.toList();
+
+			if (toolMethods.isEmpty()) {
+				throw new IllegalStateException("No @Tool annotated methods found in " + toolObject + "."
+						+ "Did you mean to pass a ToolCallback or ToolCallbackProvider? If so, you have to use .toolCallbacks() instead of .tool()");
+			}
+		}
 	}
 
 	@Override

spring-ai-model/src/test/java/org/springframework/ai/tool/method/MethodToolCallbackProviderTests.java

@@ -0,0 +1,140 @@
+/*
+ * Copyright 2025-2025 the original author or authors.
+ *
+ * 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
+ *
+ *      https://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 org.springframework.ai.tool.method;
+
+import java.util.function.Consumer;
+import java.util.function.Function;
+import java.util.function.Supplier;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.ai.tool.annotation.Tool;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+/**
+ * Unit tests for {@link MethodToolCallbackProvider}.
+ *
+ * @author Christian Tzolov
+ */
+class MethodToolCallbackProviderTests {
+
+	@Test
+	void whenToolObjectHasToolAnnotatedMethodThenSucceed() {
+		MethodToolCallbackProvider provider = MethodToolCallbackProvider.builder()
+			.toolObjects(new ValidToolObject())
+			.build();
+
+		assertThat(provider.getToolCallbacks()).hasSize(1);
+		assertThat(provider.getToolCallbacks()[0].getToolDefinition().name()).isEqualTo("validTool");
+	}
+
+	@Test
+	void whenToolObjectHasNoToolAnnotatedMethodThenThrow() {
+		assertThatThrownBy(
+				() -> MethodToolCallbackProvider.builder().toolObjects(new NoToolAnnotatedMethodObject()).build())
+			.isInstanceOf(IllegalStateException.class)
+			.hasMessageContaining("No @Tool annotated methods found in");
+	}
+
+	@Test
+	void whenToolObjectHasOnlyFunctionalTypeToolMethodsThenThrow() {
+		assertThatThrownBy(() -> MethodToolCallbackProvider.builder()
+			.toolObjects(new OnlyFunctionalTypeToolMethodsObject())
+			.build()).isInstanceOf(IllegalStateException.class)
+			.hasMessageContaining("No @Tool annotated methods found in");
+	}
+
+	@Test
+	void whenToolObjectHasMixOfValidAndFunctionalTypeToolMethodsThenSucceed() {
+		MethodToolCallbackProvider provider = MethodToolCallbackProvider.builder()
+			.toolObjects(new MixedToolMethodsObject())
+			.build();
+
+		assertThat(provider.getToolCallbacks()).hasSize(1);
+		assertThat(provider.getToolCallbacks()[0].getToolDefinition().name()).isEqualTo("validTool");
+	}
+
+	@Test
+	void whenMultipleToolObjectsWithSameToolNameThenThrow() {
+		assertThatThrownBy(() -> MethodToolCallbackProvider.builder()
+			.toolObjects(new ValidToolObject(), new DuplicateToolNameObject())
+			.build()).isInstanceOf(IllegalStateException.class)
+			.hasMessageContaining("Multiple tools with the same name (validTool) found in sources");
+	}
+
+	static class ValidToolObject {
+
+		@Tool
+		public String validTool() {
+			return "Valid tool result";
+		}
+
+	}
+
+	static class NoToolAnnotatedMethodObject {
+
+		public String notATool() {
+			return "Not a tool";
+		}
+
+	}
+
+	static class OnlyFunctionalTypeToolMethodsObject {
+
+		@Tool
+		public Function<String, String> functionTool() {
+			return input -> "Function result: " + input;
+		}
+
+		@Tool
+		public Supplier<String> supplierTool() {
+			return () -> "Supplier result";
+		}
+
+		@Tool
+		public Consumer<String> consumerTool() {
+			return input -> System.out.println("Consumer received: " + input);
+		}
+
+	}
+
+	static class MixedToolMethodsObject {
+
+		@Tool
+		public String validTool() {
+			return "Valid tool result";
+		}
+
+		@Tool
+		public Function<String, String> functionTool() {
+			return input -> "Function result: " + input;
+		}
+
+	}
+
+	static class DuplicateToolNameObject {
+
+		@Tool
+		public String validTool() {
+			return "Duplicate tool result";
+		}
+
+	}
+
+}