Customizable Serialization for Activity Return Values in Durable Functions

What’s the problem?

Hey👋,

I'm working with Durable Functions in Java, and I’ve encountered an issue with how activity return values are serialized. Specifically:

• Activity functions return POJOs annotated with Jackson and Gson and having subTypes, which is deserialized using Gson when entering the activity but none is used for serializing return value.
• This creates inconsistencies.
• There's no way to customize the serialization mechanism for Durable Functions activities.
• Raises an Exception of InvalidTypeIdException because the type is missing at com.microsoft.durabletask.JacksonDataConverter#deserialize.

How to reproduce it?

Here’s a minimal Durable Function example where an Activity returns a POJO, but the Azure Functions Java Worker does not deserialize it properly:

1️⃣ POJO with Jackson & Gson annotations and Subtypes

import com.fasterxml.jackson.annotation.JsonSubTypes;
import com.fasterxml.jackson.annotation.JsonTypeInfo;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.google.gson.annotations.JsonAdapter;
import com.google.gson.annotations.SerializedName;
import com.google.gson.*;
import java.io.IOException;

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "type")
@JsonSubTypes({
    @JsonSubTypes.Type(value = AdminUser.class, name = "admin"),
    @JsonSubTypes.Type(value = RegularUser.class, name = "regular")
})
@JsonAdapter(UserTypeAdapter.class)
public abstract class User {
    private String id;

    private String name;

    public User() {}

    public User(String id, String name) {
        this.id = id;
        this.name = name;
    }

    public String getId() { return id; }
    public String getName() { return name; }
}

public class AdminUser extends User {
    private int adminLevel;

    public AdminUser() {}
    public AdminUser(String id, String name, int adminLevel) {
        super(id, name);
        this.adminLevel = adminLevel;
    }
}

public class RegularUser extends User {
    private String membershipStatus;

    public RegularUser() {}
    public RegularUser(String id, String name, String membershipStatus) {
        super(id, name);
        this.membershipStatus = membershipStatus;
    }
}

public class UserTypeAdapter extends TypeAdapter<User> {
    @Override
    public void write(JsonWriter out, User user) throws IOException {
        out.beginObject();
        out.name("type").value(user instanceof AdminUser ? "admin" : "regular");
        out.name("id").value(user.getId());
        out.name("name").value(user.getName());
        if (user instanceof AdminUser) {
            out.name("adminLevel").value(((AdminUser) user).adminLevel);
        } else if (user instanceof RegularUser) {
            out.name("membershipStatus").value(((RegularUser) user).membershipStatus);
        }
        out.endObject();
    }

    @Override
    public User read(JsonReader in) throws IOException {
        in.beginObject();
        String type = "";
        String id = "";
        String name = "";
        int adminLevel = 0;
        String membershipStatus = "";
        
        while (in.hasNext()) {
            String fieldName = in.nextName();
            switch (fieldName) {
                case "type":
                    type = in.nextString();
                    break;
                case "id":
                    id = in.nextString();
                    break;
                case "name":
                    name = in.nextString();
                    break;
                case "adminLevel":
                    adminLevel = in.nextInt();
                    break;
                case "membershipStatus":
                    membershipStatus = in.nextString();
                    break;
                default:
                    in.skipValue();
                    break;
            }
        }
        in.endObject();
        
        return "admin".equals(type) ? new AdminUser(id, name, adminLevel) : new RegularUser(id, name, membershipStatus);
    }
}

What do you want to happen?

✅ Supporting GSON for serialization.

Start with com.microsoft.durabletask.JacksonDataConverter and trace how activity return values are serialized and later deserialized. Use the provided polymorphic User example to understand the failure, then determine how a configurable Gson serializer would integrate. Done means activity POJOs using Gson annotations can round-trip without the missing-type InvalidTypeIdException.