diff --git a/src/ai/agents/agent.py b/src/ai/agents/agent.py
index f6ecc06..06bea9e 100644
--- a/src/ai/agents/agent.py
+++ b/src/ai/agents/agent.py
@@ -225,6 +225,7 @@ class MessageAggregator(
 ):
     def __init__(self) -> None:
         self._messages: list[types.messages.Message] = []
+        self._index_by_id: dict[str, int] = {}
 
     def feed(self, item: events_.AgentEvent) -> None:
         if isinstance(item, events_.PartialToolCallResult):
@@ -232,10 +233,17 @@ def feed(self, item: events_.AgentEvent) -> None:
         msg = item.message
         if msg is None:
             return
-        if self._messages and self._messages[-1].id == msg.id:
-            self._messages[-1] = msg
-        else:
+        # Later snapshots of a message replace earlier ones in place.
+        # Snapshots of the same message are not always consecutive —
+        # e.g. a tool-result message can land between two snapshots of
+        # the assistant message that called it — so dedupe by id, not
+        # by checking the tail.
+        index = self._index_by_id.get(msg.id)
+        if index is None:
+            self._index_by_id[msg.id] = len(self._messages)
             self._messages.append(msg)
+        else:
+            self._messages[index] = msg
 
     def snapshot(self) -> MessageBundle:
         return MessageBundle(messages=tuple(self._messages))
diff --git a/tests/agents/test_message_aggregator.py b/tests/agents/test_message_aggregator.py
new file mode 100644
index 0000000..f4a081c
--- /dev/null
+++ b/tests/agents/test_message_aggregator.py
@@ -0,0 +1,47 @@
+"""MessageAggregator — deduping message snapshots by id."""
+
+from __future__ import annotations
+
+import ai
+from ai.types import events as events_
+
+from ..conftest import text_msg, tool_result_msg
+
+
+def test_consecutive_snapshots_replace() -> None:
+    """Later snapshots of the same message replace the earlier ones."""
+    agg = ai.agents.MessageAggregator()
+    agg.feed(events_.StreamEnd(message=text_msg("partial", id="msg-a")))
+    agg.feed(events_.StreamEnd(message=text_msg("complete", id="msg-a")))
+
+    bundle = agg.snapshot()
+    assert [m.text for m in bundle.messages] == ["complete"]
+
+
+def test_interleaved_snapshots_replace() -> None:
+    """Snapshots of the same message dedupe even when another message
+    lands in between (e.g. a tool-result message mid-stream)."""
+    agg = ai.agents.MessageAggregator()
+    tool = tool_result_msg(tc_id="tc-1", result="r")
+    agg.feed(events_.StreamEnd(message=text_msg("partial", id="msg-a")))
+    agg.feed(events_.StreamEnd(message=tool))
+    agg.feed(events_.StreamEnd(message=text_msg("complete", id="msg-a")))
+
+    bundle = agg.snapshot()
+    assert [m.id for m in bundle.messages] == ["msg-a", tool.id]
+    assert bundle.messages[0].text == "complete"
+
+
+def test_first_occurrence_position_is_kept() -> None:
+    """Replacement keeps the message at its original position."""
+    agg = ai.agents.MessageAggregator()
+    tool = tool_result_msg(tc_id="tc-1", result="r")
+    agg.feed(events_.StreamEnd(message=text_msg("a1", id="msg-a")))
+    agg.feed(events_.StreamEnd(message=tool))
+    agg.feed(events_.StreamEnd(message=text_msg("b1", id="msg-b")))
+    agg.feed(events_.StreamEnd(message=text_msg("a2", id="msg-a")))
+    agg.feed(events_.StreamEnd(message=text_msg("b2", id="msg-b")))
+
+    bundle = agg.snapshot()
+    assert [m.id for m in bundle.messages] == ["msg-a", tool.id, "msg-b"]
+    assert [m.text for m in bundle.messages] == ["a2", "", "b2"]