ListRecord.Field.message` deserialization fails when Slack returns an array

[bkyryliuk]

Issue summary

When the Slack API returns a conversations.replies (or conversations.history) response that contains a Slack Lists item attachment, the SDK throws JsonSyntaxException whenever any ListRecord.Field.message value is a JSON
array instead of a single object. The whole API call aborts and no Message data is returned.

This is the same class of bug as #1450 (open) and #1454 (closed as dup of #1450). Both are still observable against the latest release v1.48.1 — reproduced below with a minimal payload.

Affected versions

• v1.46.0 through v1.48.1 (verified)
• Likely earlier versions back to when the field was introduced

Root cause

com.slack.api.model.list.ListRecord.Field declares message as a single Message:

// slack-api-model/src/main/java/com/slack/api/model/list/ListRecord.java
public static class Field {
    ...
    private Message message;
    ...
}

Slack's API can return this value as an array of Message objects, and Gson then throws:

com.google.gson.JsonSyntaxException: java.lang.IllegalStateException:
  Expected BEGIN_OBJECT but was BEGIN_ARRAY at line 1 column N path
  $.messages[I].attachments[J].list_record.record.fields[K].message

Every other field on ListRecord.Field is modeled as a List<X> (number, select, date, user, attachment, email, phone, channel, rating, timestamp, link, reference) — only message is a scalar. That
asymmetry is what makes this fail.

Minimal reproduction

import com.google.gson.Gson;
import com.slack.api.methods.response.conversations.ConversationsRepliesResponse;
import com.slack.api.util.json.GsonFactory;

public class Repro {
    public static void main(String[] args) {
        // Mirrors the shape Slack returns when a Lists item's `message` field
        // has multiple values (array form).
        String payload =
            "{\"ok\":true,\"messages\":[{\"type\":\"message\",\"user\":\"U1\",\"ts\":\"1.0\","
            + "\"text\":\"hi\",\"attachments\":[{\"list_record\":{\"record\":{\"fields\":["
            + "{\"key\":\"name\",\"message\":{\"text\":\"ok\"}},"
            + "{\"key\":\"details\",\"message\":[{\"text\":\"a\"},{\"text\":\"b\"}]}"
            + "]}}}]}]}";

        Gson sdkGson = GsonFactory.createSnakeCase();
        // Throws JsonSyntaxException at
        // $.messages[0].attachments[0].list_record.record.fields[1].message
        ConversationsRepliesResponse parsed =
            sdkGson.fromJson(payload, ConversationsRepliesResponse.class);
    }
}

Expected behavior

The SDK should accept both single-object and array shapes for ListRecord.Field.message, matching Slack's actual API behavior.

[hello-ashleyintech]

Hi, @bkyryliuk! Thanks for submitting this issue.

Could you send me the logged payload output you're getting from your specific API call? I'm asking because typically when we have a property named message, that indicates an object and no array, vs. messages, like in your example payload, which is an array of messages. I noticed that typing is the same in the Node SDK as well; will also dig into if anything changed with this field recently on the API end.

[hello-ashleyintech]

Hi @bkyryliuk! I have checked internally and it does look like internally the type for message is expected to be a vector of messages rather than a single object, like you pointed out. I will dive into this tomorrow and also file issues in Node and potentially Python SDKs as well since this typing bug is duplicated there.

[bkyryliuk]

Great, ty!