Fix: return String result directly instead of JSON in DefaultToolCallResultConverter

Signed-off-by: Jemin Huh <[email protected]>

Author: JM-Lab

spring-ai-model/src/main/java/org/springframework/ai/tool/execution/DefaultToolCallResultConverter.java

@@ -35,6 +35,7 @@
  * A default implementation of {@link ToolCallResultConverter}.
  *
  * @author Thomas Vitale
+ * @author Jemin Huh
  * @since 1.0.0
  */
 public final class DefaultToolCallResultConverter implements ToolCallResultConverter {
@@ -45,7 +46,11 @@ public final class DefaultToolCallResultConverter implements ToolCallResultConve
 	public String convert(@Nullable Object result, @Nullable Type returnType) {
 		if (returnType == Void.TYPE) {
 			logger.debug("The tool has no return type. Converting to conventional response.");
-			return JsonParser.toJson("Done");
+			return "Done";
+		}
+		if (returnType instanceof Class<?> cls && cls.equals(String.class)) {
+			logger.debug("Tool return type is String. Returning result as is.");
+			return String.valueOf(result);
 		}
 		if (result instanceof RenderedImage) {
 			final var buf = new ByteArrayOutputStream(1024 * 4);

spring-ai-model/src/test/java/org/springframework/ai/tool/execution/DefaultToolCallResultConverterTests.java

@@ -38,6 +38,7 @@
  * Unit tests for {@link DefaultToolCallResultConverter}.
  *
  * @author Thomas Vitale
+ * @author Jemin Huh
  */
 class DefaultToolCallResultConverterTests {
 
@@ -52,13 +53,13 @@ void convertWithNullReturnTypeShouldReturn() {
 	@Test
 	void convertVoidReturnTypeShouldReturnDoneJson() {
 		String result = this.converter.convert(null, void.class);
-		assertThat(result).isEqualTo("\"Done\"");
+		assertThat(result).isEqualTo("Done");
 	}
 
 	@Test
 	void convertStringReturnTypeShouldReturnJson() {
 		String result = this.converter.convert("test", String.class);
-		assertThat(result).isEqualTo("\"test\"");
+		assertThat(result).isEqualTo("test");
 	}
 
 	@Test