fix(json): prevent double-serialization of already valid JSON strings in JsonParser.toJson

If the input to JsonParser.toJson is a String that is already a valid JSON (e.g. a JSON array or object),
the method now returns the string as-is, instead of re-serializing it into a quoted string.

This avoids issues with deserialization errors, especially when tools expect structured types like List<T>.

test(json): add unit test to ensure toJson skips serialization of valid JSON strings

Signed-off-by: Ahmed Maruf <81756975+ohMaruf@users.noreply.github.com>

spring-ai-model/src/main/java/org/springframework/ai/util/json/JsonParser.java

@@ -100,9 +100,25 @@ public final class JsonParser {
 	}
 
 	/**
-	 * Converts a Java object to a JSON string.
+	 * Checks if a string is a valid JSON string.
+	 */
+	private static boolean isValidJson(String input) {
+		try {
+			OBJECT_MAPPER.readTree(input);
+			return true;
+		}
+		catch (JsonProcessingException e) {
+			return false;
+		}
+	}
+
+	/**
+	 * Converts a Java object to a JSON string if it's not already a valid JSON string.
 	 */
 	public static String toJson(@Nullable Object object) {
+		if (object instanceof String && isValidJson((String) object)) {
+			return (String) object;
+		}
 		try {
 			return OBJECT_MAPPER.writeValueAsString(object);
 		}

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

@@ -63,7 +63,7 @@ class MethodToolCallbackGenericTypesTest {
 		String result = callback.call(toolInput);
 
 		// Verify the result
-		assertThat(result).isEqualTo("\"3 strings processed: [one, two, three]\"");
+		assertThat(result).isEqualTo("3 strings processed: [one, two, three]");
 	}
 
 	@Test
@@ -97,7 +97,7 @@ class MethodToolCallbackGenericTypesTest {
 		String result = callback.call(toolInput);
 
 		// Verify the result
-		assertThat(result).isEqualTo("\"3 entries processed: {one=1, two=2, three=3}\"");
+		assertThat(result).isEqualTo("3 entries processed: {one=1, two=2, three=3}");
 	}
 
 	@Test
@@ -134,7 +134,7 @@ class MethodToolCallbackGenericTypesTest {
 		String result = callback.call(toolInput);
 
 		// Verify the result
-		assertThat(result).isEqualTo("\"2 maps processed: [{a=1, b=2}, {c=3, d=4}]\"");
+		assertThat(result).isEqualTo("2 maps processed: [{a=1, b=2}, {c=3, d=4}]");
 	}
 
 	/**

spring-ai-model/src/test/java/org/springframework/ai/util/json/JsonParserTests.java

@@ -255,6 +255,13 @@ class JsonParserTests {
 		assertThat(value).isEqualTo(1_500_000_000_000L);
 	}
 
+	@Test
+	void doesNotDoubleSerializeValidJsonString() {
+		String input = "[1,2,3]";
+		String result = JsonParser.toJson(input);
+		assertThat(input).isEqualTo(result);
+	}
+
 	record TestRecord(String name, Integer age) {
 	}