MessageSerializer

This docs was updated at: 2026-03-21

com.paragon.responses.json.MessageSerializer  ·  Class

Extends StdSerializer<Message>

---

Custom Jackson serializer for Message and its subclasses.

The OpenAI Responses API (and OpenRouter) requires content to be a plain string rather than an array of content objects. Without this serializer, Jackson would serialize content as [{"type":"input_text","text":"..."]} which is rejected with "Invalid input: expected string, received array".

This serializer writes content as a plain string when the message has exactly one Text content item (the common case for conversation history), and falls back to the array format otherwise (for multi-content messages with images, files, etc.).

Methods

serializeWithType

public void serializeWithType(
      Message value,
      JsonGenerator gen,
      SerializationContext provider,
      TypeSerializer typeSer)
      throws tools.jackson.core.JacksonException

Called when serializing within a polymorphic context (e.g., List with @JsonTypeInfo). Since we embed "type" directly in the JSON object, we skip the external type wrapper and delegate to .serialize.