fix(core): let JacksonJsonCodec read back every string it can write

agentscope-core/src/main/java/io/agentscope/core/util/JacksonJsonCodec.java

@@ -16,7 +16,9 @@
 
 package io.agentscope.core.util;
 
+import com.fasterxml.jackson.core.JsonFactory;
 import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.core.StreamReadConstraints;
 import com.fasterxml.jackson.core.type.TypeReference;
 import com.fasterxml.jackson.databind.DeserializationFeature;
 import com.fasterxml.jackson.databind.JavaType;
@@ -33,6 +35,9 @@
  * Jackson's ObjectMapper with the following configuration:
  * <ul>
  *   <li>{@code FAIL_ON_UNKNOWN_PROPERTIES = false} - allows unknown fields in JSON</li>
+ *   <li>{@code StreamReadConstraints.maxStringLength = Integer.MAX_VALUE} - reads back any
+ *       string value {@link #toJson} can emit; the other read constraints keep their
+ *       Jackson defaults</li>
  * </ul>
  *
  * <p>Users can access the underlying ObjectMapper via {@link #getObjectMapper()}
@@ -69,7 +74,20 @@ public JacksonJsonCodec(ObjectMapper objectMapper) {
      * @return configured ObjectMapper
      */
     private static ObjectMapper createDefaultObjectMapper() {
-        ObjectMapper mapper = new ObjectMapper();
+        // toJson puts no cap on string values, yet Jackson's default read constraints reject
+        // any single string above 20,000,000 characters. Every AgentStateStore persists
+        // through this codec and inline Base64 media routinely exceeds that, so the default
+        // turned a successful save into a state that could never be loaded again (#3465).
+        // The reader must accept whatever the writer emitted; nesting depth, number and name
+        // length keep their defaults.
+        JsonFactory factory =
+                JsonFactory.builder()
+                        .streamReadConstraints(
+                                StreamReadConstraints.builder()
+                                        .maxStringLength(Integer.MAX_VALUE)
+                                        .build())
+                        .build();
+        ObjectMapper mapper = new ObjectMapper(factory);
         mapper.registerModule(new JavaTimeModule());
         mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
         return mapper;

agentscope-core/src/test/java/io/agentscope/core/util/JacksonJsonCodecTest.java

@@ -273,4 +273,20 @@ void testRoundTripSerializationNested() {
         assertEquals(original.author.age, deserialized.author.age);
         assertEquals(original.tags, deserialized.tags);
     }
+
+    @Test
+    void roundTripsStringsLongerThanJacksonDefaultReadLimit() {
+        // Jackson's default StreamReadConstraints reject any single string value above
+        // 20,000,000 characters on read, while toJson imposes no cap. Every AgentStateStore
+        // persists through this codec and inline Base64 media routinely exceeds that, so a
+        // state that saved successfully must stay loadable: what the codec writes, it reads.
+        String value = "a".repeat(20_000_001);
+        SimpleModel original = new SimpleModel(value, 1);
+
+        String json = codec.toJson(original);
+        SimpleModel deserialized = codec.fromJson(json, SimpleModel.class);
+
+        assertEquals(value.length(), deserialized.name.length());
+        assertEquals(value, deserialized.name);
+    }
 }