fix: Type fix and doc clarification (#877)

Release-As: 0.2.6

---------

Co-authored-by: a2a-bot <[email protected]>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: 3 people

docs/specification.md

@@ -287,7 +287,7 @@ These objects define the structure of data exchanged within the JSON-RPC methods
 
 ### 6.1. `Task` Object
 
-Represents the stateful unit of work being processed by the A2A Server for an A2A Client. A task encapsulates the entire interaction related to a specific goal or request. A task which has reached a terminal state (completed, canceled, rejected, or failed) can't be restarted. For more information, refer to the [Life of a Task guide](./topics/life-of-a-task.md).
+Represents the stateful unit of work being processed by the A2A Server for an A2A Client. A task encapsulates the entire interaction related to a specific goal or request. A task which has reached a terminal state (completed, canceled, rejected, or failed) can't be restarted. Tasks in completed state SHOULD use artifacts for returning the generated output to the clients. For more information, refer to the [Life of a Task guide](./topics/life-of-a-task.md).
 
 ```ts { .no-copy }
 --8<-- "types/src/types.ts:Task"

docs/topics/key-concepts.md

@@ -31,7 +31,7 @@ The Agent2Agent (A2A) protocol is built around a set of core concepts that defin
 
     - Represents a single turn or unit of communication between a client and an agent.
     - Messages have a `role` (either `"user"` for client-sent messages or `"agent"` for server-sent messages) and contain one or more `Part` objects that carry the actual content. `messageId` part of the Message object is a unique identifier for each message set by the sender of the message.
-    - Used for conveying instructions, context, questions, answers, or status updates that are not necessarily formal `Artifacts`.
+    - Used for conveying instructions, context, questions, answers, or status updates that are not necessarily formal `Artifacts` that are part of a `Task`.
     - See details in the [Protocol Specification: Message Object](../specification.md#64-message-object).
 
 - **Part:**
@@ -45,6 +45,7 @@ The Agent2Agent (A2A) protocol is built around a set of core concepts that defin
 - **Artifact:**
     - Represents a tangible output or result generated by the remote agent during the processing of a task.
     - Examples include generated documents, images, spreadsheets, structured data results, or any other self-contained piece of information that is a direct result of the task.
+    - Tasks in completed state SHOULD use artifact objects for returning the generated output to the clients.
     - Artifacts are composed of one or more `Part` objects and can be streamed incrementally.
     - See details in the [Protocol Specification: Artifact Object](../specification.md#67-artifact-object).
 

specification/json/a2a.json

@@ -291,7 +291,7 @@ export interface DeleteTaskPushNotificationConfigParams extends TaskIdParams {
 /**Configuration for the send message request. */
 export interface MessageSendConfiguration {
   /** Accepted output modalities by the client. */
-  acceptedOutputModes: string[];
+  acceptedOutputModes?: string[];
   /** Number of recent messages to be retrieved. */
   historyLength?: number;
   /** Where the server should send notifications when disconnected. */