custom_java_impl示例中发现 返回中history-->parts中 "kind": null, 是null有问题啊。 正确的应该是"kind": "text"

[yanyinxi]

示例：

client请求：

{
  "id": "7981a512-3096-433e-a2c9-63fdfba1522a",
  "jsonrpc": "2.0",
  "method": "message/send",
  "params": {
    "id": "german-cancel-task",
    "sessionId": null,
    "message": {
      "messageId": "b46b3722-8cd7-47d0-9914-d86895a707bf",
      "kind": "message",
      "role": "user",
      "parts": [
        {
          "kind": "text",
          "text": "美丽 翻译成日语",
          "metadata": null
        },
        {
          "kind": "text",
          "text": "美丽 翻译成韩语",
          "metadata": null
        },
        {
          "kind": "text",
          "text": "美丽 翻译成英文",
          "metadata": null
        }
      ],
      "contextId": null,
      "taskId": null,
      "referenceTaskIds": null,
      "metadata": null
    },
    "pushNotification": null,
    "historyLength": null,
    "metadata": {}
  }
}

server返回：

{
  "id": "7981a512-3096-433e-a2c9-63fdfba1522a",
  "jsonrpc": "2.0",
  "result": {
    "id": "german-cancel-task",
    "contextId": "62d125c0-d4ad-423c-8d8a-58af5773cc38",
    "kind": "task",
    "status": {
      "state": "COMPLETED",
      "message": null,
      "timestamp": "2025-11-15T07:43:28.953500Z"
    },
    "artifacts": null,
    "history": [
      {
        "messageId": "b46b3722-8cd7-47d0-9914-d86895a707bf",
        "kind": "message",
        "role": "user",
        "parts": [
          {
             "kind": null,
            "text": "美丽 翻译成日语",
            "metadata": null
          },
          {
            "kind": null,
            "text": "美丽 翻译成韩语",
            "metadata": null
          },
          {
            "kind": null,
            "text": "美丽 翻译成英文",
            "metadata": null
          }
        ],
        "contextId": null,
        "taskId": null,
        "referenceTaskIds": null,
        "metadata": null
      },
      {
        "messageId": "d9a1d1e0-88d6-4850-8d52-268b2ff2c464",
        "kind": "message",
        "role": "assistant",
        "parts": [
          {
            "kind": null,
            "text": "Translate 'beautiful' to Japanese\nTranslate 'beautiful' to Korean\nTranslate 'beautiful' to English",
            "metadata": null
          }
        ],
        "contextId": null,
        "taskId": "german-cancel-task",
        "referenceTaskIds": [
          "b46b3722-8cd7-47d0-9914-d86895a707bf"
        ],
        "metadata": null
      }
    ],
    "metadata": {}
  },
  "error": null
}

[yanyinxi]

Part is annotated with @JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "kind") and the only registered subtypes are text/file/data (custom_java_impl/model/src/main/java/com/google/a2a/model/Part.java (line 9)). When Jackson deserializes a Message, it first consumes the kind property to figure out which subtype it should instantiate.
Because the @JsonTypeInfo property is not marked as visible = true, that kind value is never forwarded to the actual field defined on TextPart (custom_java_impl/model/src/main/java/com/google/a2a/model/TextPart.java (lines 10-35)). So after deserialization every TextPart instance in your Message.parts() list has a kind field of null, even though the incoming JSON carried "kind": "text".
When the server (or the client when logging the response) later serializes those TextPart objects back to JSON—for example when sending the task history—you’re just seeing the value of that now-null field, hence the "kind": null entries you highlighted.
So the root cause is the way @JsonTypeInfo is configured: it consumes the kind discriminator but doesn’t keep it visible for the TextPart record, so nobody ever re-populates the kind field before the objects are serialized again.

[yanyinxi]

Solution ： visible = true Is it correct？ @zeroasterisk @DracoBlue @longfin @whitlockjc

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "kind", visible = true)

======================================

package com.google.a2a.model;

import com.fasterxml.jackson.annotation.JsonSubTypes;
import com.fasterxml.jackson.annotation.JsonTypeInfo;

/**
 * Part represents a part of a message, which can be text, a file, or structured data
 */
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "kind", visible = true)
@JsonSubTypes({
    @JsonSubTypes.Type(value = TextPart.class, name = "text"),
    @JsonSubTypes.Type(value = FilePart.class, name = "file"),
    @JsonSubTypes.Type(value = DataPart.class, name = "data")
})
public sealed interface Part permits TextPart, FilePart, DataPart {
}