diff --git a/cueapi/resources/messages.py b/cueapi/resources/messages.py
index a7ea2e8..4d565f3 100644
--- a/cueapi/resources/messages.py
+++ b/cueapi/resources/messages.py
@@ -2,7 +2,8 @@
 
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, Any, Dict, Optional
+from datetime import datetime
+from typing import TYPE_CHECKING, Any, Dict, Optional, Union
 
 if TYPE_CHECKING:
     from cueapi.client import CueAPI
@@ -33,6 +34,7 @@ def send(
         reply_to_agent: Optional[str] = None,
         metadata: Optional[Dict[str, Any]] = None,
         idempotency_key: Optional[str] = None,
+        send_at: Optional[Union[str, datetime]] = None,
     ) -> dict:
         """Send a message.
 
@@ -69,6 +71,11 @@ def send(
             metadata: Optional JSON metadata blob.
             idempotency_key: Optional ``Idempotency-Key`` header
                 (≤255 chars).
+            send_at: Optional ISO 8601 timestamp (or ``datetime``) to
+                delay this message's delivery. If omitted, the message
+                is delivered immediately. Per-message scheduling landed
+                in cueapi #623 — server stores ``send_at`` on the
+                message row and the worker picks it up when due.
 
         Returns:
             Dict matching the server's ``MessageResponse`` shape.
@@ -95,6 +102,10 @@ def send(
             payload["reply_to_agent"] = reply_to_agent
         if metadata is not None:
             payload["metadata"] = metadata
+        if send_at is not None:
+            payload["send_at"] = (
+                send_at.isoformat() if isinstance(send_at, datetime) else send_at
+            )
 
         headers: Dict[str, str] = {"X-Cueapi-From-Agent": from_agent}
         if idempotency_key is not None:
diff --git a/tests/test_messages_resource.py b/tests/test_messages_resource.py
index d5c5ede..02f54c7 100644
--- a/tests/test_messages_resource.py
+++ b/tests/test_messages_resource.py
@@ -137,3 +137,64 @@ def test_ack(self):
         mock_client._post.assert_called_once_with(
             "/v1/messages/msg_x/ack", json={},
         )
+
+
+class TestSendAt:
+    """Per-message scheduling via send_at (cueapi #623 parity port).
+
+    Mock-based following the existing pattern in this file. Asserts on
+    the request body shape — that's the SDK contract; what the server
+    does with it (delay then deliver) is exercised by the server suite.
+    """
+
+    def test_send_with_send_at_iso_string(self):
+        """send_at as ISO 8601 string flows into the request body verbatim."""
+        mock_client = MagicMock()
+        mock_client._post.return_value = {"id": "msg_x", "delivery_state": "queued"}
+        r = MessagesResource(mock_client)
+
+        r.send(
+            from_agent="sender@x",
+            to="recipient@y",
+            body="hi",
+            send_at="2030-01-01T12:00:00Z",
+        )
+
+        mock_client._post.assert_called_once_with(
+            "/v1/messages",
+            json={
+                "to": "recipient@y",
+                "body": "hi",
+                "send_at": "2030-01-01T12:00:00Z",
+            },
+            headers={"X-Cueapi-From-Agent": "sender@x"},
+        )
+
+    def test_send_with_send_at_datetime_auto_isoformats(self):
+        """send_at as datetime auto-serializes via .isoformat()."""
+        from datetime import datetime, timezone
+
+        mock_client = MagicMock()
+        mock_client._post.return_value = {"id": "msg_x", "delivery_state": "queued"}
+        r = MessagesResource(mock_client)
+
+        r.send(
+            from_agent="sender@x",
+            to="recipient@y",
+            body="hi",
+            send_at=datetime(2030, 1, 1, 12, 0, 0, tzinfo=timezone.utc),
+        )
+
+        call = mock_client._post.call_args
+        assert call.kwargs["json"]["send_at"] == "2030-01-01T12:00:00+00:00"
+
+    def test_send_without_send_at_omits_field(self):
+        """send_at unset → field NOT present in request body."""
+        mock_client = MagicMock()
+        mock_client._post.return_value = {"id": "msg_x", "delivery_state": "queued"}
+        r = MessagesResource(mock_client)
+
+        r.send(from_agent="sender@x", to="recipient@y", body="hi")
+
+        call = mock_client._post.call_args
+        assert "send_at" not in call.kwargs["json"]