Fix MCP toolcall returning list values

mcp-annotations/src/main/java/org/springaicommunity/mcp/method/tool/AbstractSyncMcpToolMethodCallback.java

@@ -19,7 +19,9 @@
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
 import java.lang.reflect.Type;
+import java.util.List;
 import java.util.Map;
+import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
 import org.springaicommunity.mcp.annotation.McpMeta;
@@ -41,6 +43,7 @@
  * @param <T> The type of the context parameter (e.g., McpTransportContext or
  * McpSyncServerExchange)
  * @author Christian Tzolov
+ * @author Ilayaperumal Gopinathan
  */
 public abstract class AbstractSyncMcpToolMethodCallback<T> {
 
@@ -156,9 +159,15 @@ protected CallToolResult processResult(Object result) {
 			return CallToolResult.builder().addTextContent(JsonParser.toJson("Done")).build();
 		}
 		else if (this.returnMode == ReturnMode.STRUCTURED) {
-			String jsonOutput = JsonParser.toJson(result);
-			Map<String, Object> structuredOutput = JsonParser.fromJson(jsonOutput, MAP_TYPE_REFERENCE);
-			return CallToolResult.builder().structuredContent(structuredOutput).build();
+			if (result instanceof List<?>) {
+				List<String> texts = ((List<?>) result).stream().map(String::valueOf).collect(Collectors.toList());
+				return CallToolResult.builder().textContent(texts).build();
+			}
+			else {
+				String jsonOutput = JsonParser.toJson(result);
+				Map<String, Object> structuredOutput = JsonParser.fromJson(jsonOutput, MAP_TYPE_REFERENCE);
+				return CallToolResult.builder().structuredContent(structuredOutput).build();
+			}
 		}
 
 		// Default to text output

mcp-annotations/src/test/java/org/springaicommunity/mcp/method/tool/SyncMcpToolMethodCallbackTests.java

@@ -106,6 +106,11 @@ public TestObject returnObjectTool(String name, int value) {
 			return new TestObject(name, value);
 		}
 
+		@McpTool(name = "return-list-tool", description = "Tool that returns a list")
+		public List<String> returnListTool() {
+			return List.of("this", "is", "a", "test");
+		}
+
 	}
 
 	public static class TestObject {
@@ -507,4 +512,23 @@ public void testToolReturningComplexObject() throws Exception {
 		assertThat(result.structuredContent()).containsEntry("value", 42);
 	}
 
+	@Test
+	public void testToolReturningList() throws Exception {
+		TestToolProvider provider = new TestToolProvider();
+		Method method = TestToolProvider.class.getMethod("returnListTool", null);
+		SyncMcpToolMethodCallback callback = new SyncMcpToolMethodCallback(ReturnMode.STRUCTURED, method, provider);
+
+		McpSyncServerExchange exchange = mock(McpSyncServerExchange.class);
+		CallToolRequest request = new CallToolRequest("return-list-tool", Map.of());
+
+		CallToolResult result = callback.apply(exchange, request);
+
+		assertThat(result).isNotNull();
+		assertThat(result.isError()).isFalse();
+		assertThat(result.content()).isNotEmpty();
+		result.content().forEach(textContent -> {
+			assertThat(((TextContent) textContent).text()).containsAnyOf("this", "is", "a", "test");
+		});
+	}
+
 }