0:00Every question we have put to the analyst carried a stock and a period. This time we leave the period out.
0:06There is no brief to write without one, so the work never starts. The task comes back with a question on it instead.
0:13That is a state of its own. The task is still open, and it is waiting on us.
0:17So we take the analyst's side first, and the check that stops the task. Then ours, and the answer going back.
0:25This is the file that serves the analyst. Its card is at the top, the executor below it. Twenty lines are new, and they are all about the period.
0:32A brief needs a period, so the server keeps a small pattern of the wordings it accepts.
0:36This is where every message arrives. The first one opens a task, and a later one joins the task that is already open.
0:44Then the executor collects every turn sent on this task, adds the one that just arrived, and joins them.
0:51Now the check. With no period anywhere in that text, the analyst puts its question on the task and returns.
0:58The task goes to input required, and the model was never called.
1:01With a period, the work goes ahead, and the brief is written from the whole joined text.
1:06So the analyst can ask for what it needs. Answering is our side.
1:11This is the client. It sends our question, then sends the answer to whatever comes back.
1:16Every message it sends can name a task and a context. Empty, and the agent opens a new task. Filled in, the message joins one that already exists.
1:27The rest of the file is the exchange itself.
1:30Send the question, and if the task comes back needing input, send the reply on the task we already have. Let's run it.
1:37The analyst is serving in another terminal. This one is ours.
1:41Here is our question, with no period in it. The second argument is the reply to send if the analyst asks for more.
1:47The task comes straight back, and it has not started.
1:50Input required, and the analyst's question is on the task itself.
1:54So the client answers. Two words, and they never mention Apple. They do not have to, because the server joins this turn to the first one before it reads anything.
2:03The task that completed is the one that stopped. Same id, same context.
2:08And the brief is about Apple, across the month we named. The stock came from our first message and the period from our second.
2:15So the brief we asked for took two messages instead of one.
2:19The analyst stopped in the middle of a task, asked for what it was missing, and finished that same task once we answered.
2:25None of that needed a second piece of work. The id that came back with the question is the id that carried the brief.
2:32That holds while the task is open. Once one has ended it cannot restart, and a follow-up then is a new task under the same context.
2:40Next time we put a tap between the two sides and read what actually crosses.

When the Agent Asks Back

Created by Abhyuday DesaiUpdated 4 Oct, v2Runs 2 min 50 sec

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About this video

In Episode 07 of A2A Explained, the analyst is asked how Apple did, with no period in the question. It cannot write that brief, so the task stops in input-required and the analyst asks which period to cover. The reply carries the task's own id, the analyst reads both turns together, and the same task completes with its brief.

You'll learn how to:

Report input-required from an agent, with the question the caller has to answer
Run that check before any model call, so an incomplete question costs nothing
Join a second message to a task that is already open instead of starting a new one
Read every turn of a task's history, so a reply does not have to repeat what the first message said
Send a reply that lands on the same task, by carrying its task id and context id
Know what changes once a task has ended: a follow-up is a new task under the same context id

This video is for developers who know MCP's vocabulary. The code is in the series repo, pinned to the commit the video shows.

What changed

v204 Oct 2026abhydesai2 min 50 secre-render of the same sourcebff3009

Re-recorded with the closing attribution card, which the first cut left out.

Same chapters and transcript. 5 sec longer.

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v104 Oct 2026abhydesai2 min 45 secbff3009

First published cut

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