Fix Gemini Tool Calling for texts returned from MethodToolCallback

models/spring-ai-vertex-ai-gemini/src/main/java/org/springframework/ai/vertexai/gemini/VertexAiGeminiChatModel.java

@@ -340,7 +340,10 @@ private static Struct jsonToStruct(String json) {
 
 			Struct.Builder structBuilder = Struct.newBuilder();
 
-			if (rootNode.isArray()) {
+			if (rootNode.isTextual()) {
+				structBuilder.putFields("result", Value.newBuilder().setStringValue(json).build());
+			}
+			else if (rootNode.isArray()) {
 				// Handle JSON array
 				List<Value> values = new ArrayList<>();
 

models/spring-ai-vertex-ai-gemini/src/test/java/org/springframework/ai/vertexai/gemini/VertexAiGeminiChatModelIT.java

@@ -361,6 +361,38 @@ void jsonArrayToolCallingTest() {
 
 	}
 
+	@Test
+	void jsonTextToolCallingTest() {
+		// Test for the improved jsonToStruct method that handles JSON texts in tool
+		// calling
+
+		ToolCallingManager toolCallingManager = ToolCallingManager.builder()
+			.observationRegistry(ObservationRegistry.NOOP)
+			.build();
+
+		VertexAiGeminiChatModel chatModelWithTools = VertexAiGeminiChatModel.builder()
+			.vertexAI(vertexAiApi())
+			.toolCallingManager(toolCallingManager)
+			.defaultOptions(VertexAiGeminiChatOptions.builder()
+				.model(VertexAiGeminiChatModel.ChatModel.GEMINI_2_0_FLASH)
+				.temperature(0.1)
+				.build())
+			.build();
+
+		ChatClient chatClient = ChatClient.builder(chatModelWithTools).build();
+
+		// Create a prompt that will trigger the tool call with a specific request that
+		// should invoke the tool
+		String response = chatClient.prompt()
+			.tools(new CurrentTimeTools())
+			.user("Get the current time. Make sure to use the tool to get this information.")
+			.call()
+			.content();
+
+		assertThat(response).isNotEmpty();
+		assertThat(response).contains("2025-05-08T10:10:10+02:00[Europe/Berlin]");
+	}
+
 	/**
 	 * Tool class that returns a JSON array to test the jsonToStruct method's ability to
 	 * handle JSON arrays. This specifically tests the PR changes that improve the
@@ -378,6 +410,21 @@ public List<Map<String, String>> getScientists() {
 
 	}
 
+	/**
+	 * Tool class that returns a String to test the jsonToStruct method's ability to
+	 * handle JSON texts. This specifically tests the PR changes that improve the
+	 * jsonToStruct method to handle JSON texts in addition to JSON objects and JSON
+	 * arrays.
+	 */
+	public static class CurrentTimeTools {
+
+		@Tool(description = "Get the current date and time in the user's timezone")
+		String getCurrentDateTime() {
+			return "2025-05-08T10:10:10+02:00[Europe/Berlin]";
+		}
+
+	}
+
 	record ActorsFilmsRecord(String actor, List<String> movies) {
 
 	}

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

@@ -45,7 +45,7 @@ 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 "Done";
+			return JsonParser.toJson("Done");
 		}
 		if (result instanceof RenderedImage) {
 			final var buf = new ByteArrayOutputStream(1024 * 4);

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

@@ -50,9 +50,9 @@ void convertWithNullReturnTypeShouldReturn() {
 	}
 
 	@Test
-	void convertVoidReturnTypeShouldReturnDone() {
+	void convertVoidReturnTypeShouldReturnDoneJson() {
 		String result = this.converter.convert(null, void.class);
-		assertThat(result).isEqualTo("Done");
+		assertThat(result).isEqualTo("\"Done\"");
 	}
 
 	@Test