Class Message

spec/src/main/java/org/a2aproject/sdk/spec/Message.java:37–299

Represents a single message in the conversation between a user and an agent in the A2A Protocol. A Message encapsulates communication content exchanged during agent interactions. It contains the message role (user or agent), content parts (text, files, or data), and contextual metadata for messa

 * @see <a href="https://a2a-protocol.org/latest/">A2A Protocol Specification</a>
 */
public record Message(Role role, List<Part<?>> parts,
        String messageId,
        @Nullable String contextId,
        @Nullable String taskId,
        @Nullable List<String> referenceTaskIds,
        @Nullable Map<String, Object> metadata,
        @Nullable List<String> extensions
        ) implements EventKind, StreamingEventKind {

    /**
     * The identifier when used in streaming responses
     */
    public static final String STREAMING_EVENT_ID = "message";

    /**
     * Compact constructor with validation and defensive copying.
     *
     * @param role the role of the message sender (user or agent)
     * @param parts the content parts of the message (text, file, or data)
     * @param messageId the unique identifier for this message
     * @param contextId the conversation context identifier
     * @param taskId the task identifier this message is associated with
     * @param referenceTaskIds list of reference task identifiers
     * @param metadata additional metadata for the message
     * @param extensions list of protocol extensions used in this message
     * @throws IllegalArgumentException if role, parts, or messageId is null, or if parts is empty
     */
    public Message {
        Assert.checkNotNullParam("role", role);
        Assert.checkNotNullParam("parts", parts);
        Assert.checkNotNullParam("messageId", messageId);
        if (parts.isEmpty()) {
            throw new IllegalArgumentException("Parts cannot be empty");
        }
        parts = List.copyOf(parts);
        referenceTaskIds = referenceTaskIds != null ? List.copyOf(referenceTaskIds) : null;
        metadata = (metadata != null) ? Map.copyOf(metadata) : null;
        extensions = extensions != null ? List.copyOf(extensions) : null;
    }

    @Override
    public String kind() {
        return STREAMING_EVENT_ID;
    }

    /**
     * Creates a new Builder for constructing Message instances.
     *
     * @return a Message.builder instance
     */
    public static Builder builder() {
        return new Builder();
    }

    /**
     * Creates a new Builder initialized with values from an existing Message.
     *
     * @param message the Message to copy values from