# `A2A.Extension.Timestamp`
[🔗](https://github.com/actioncard/a2a-elixir/blob/main/lib/a2a/extension/timestamp.ex#L1)

Reference `A2A.Extension` that timestamps requests and responses.

Stamps `Message.metadata` on the way in and `Task.metadata` on the way
out under the extension's URI:

    "https://a2a-protocol.org/extensions/timestamp/v1" => %{
      received_at: 1_700_000_000_000,
      completed_at: 1_700_000_000_042
    }

Times are wall-clock milliseconds via `System.system_time(:millisecond)`.

## Usage

Configure on the server and (optionally) on the client. Both must declare
the URI for the activation to round-trip.

    # Server
    Bandit.start_link(
      plug: {A2A.Plug,
        agent: MyAgent,
        base_url: "http://localhost:4000",
        extensions: [A2A.Extension.Timestamp]}
    )

    # Client
    client = A2A.Client.new("http://localhost:4000",
      extensions: [A2A.Extension.Timestamp])

    {:ok, task} = A2A.Client.send_message(client, "hi")
    task.metadata["https://a2a-protocol.org/extensions/timestamp/v1"]
    #=> %{"received_at" => 1700000000000, "completed_at" => 1700000000042}

Map keys are atoms on the server (before encoding) and strings on the
client (after JSON decode).

## What this exercises

This module implements every optional callback of `A2A.Extension`:

  * `c:A2A.Extension.declaration/1` — non-required profile extension.
  * `c:A2A.Extension.activate/3` — captures the per-request start time.
  * `c:A2A.Extension.handle_request/3` — stamps the inbound message metadata.
  * `c:A2A.Extension.handle_response/3` — stamps the outbound task metadata.

Copy it as a starting template for your own profile-style extension.

# `activation`

```elixir
@type activation() :: %{received_at: integer()}
```

Activation state: the wall-clock millisecond when the request was first
observed by the server.

# `uri`

```elixir
@spec uri() :: String.t()
```

The stable URI advertised by this extension.

---

*Consult [api-reference.md](api-reference.md) for complete listing*