diff --git a/nanobot/channels/manager.py b/nanobot/channels/manager.py
index 3294afc0..23a342f1 100644
--- a/nanobot/channels/manager.py
+++ b/nanobot/channels/manager.py
@@ -404,7 +404,8 @@ class ChannelManager:
         Returns:
             tuple of (merged_message, list_of_non_matching_messages)
         """
-        target_key = (first_msg.channel, first_msg.chat_id, first_msg.metadata.get("_stream_id"))
+        first_metadata = first_msg.metadata or {}
+        target_key = (first_msg.channel, first_msg.chat_id, first_metadata.get("_stream_id"))
         combined_content = first_msg.content
         final_metadata = dict(first_msg.metadata or {})
         non_matching: list[OutboundMessage] = []
@@ -418,9 +419,14 @@ class ChannelManager:
                 break
 
             # Check if this message belongs to the same stream
-            same_target = (next_msg.channel, next_msg.chat_id, next_msg.metadata.get("_stream_id") if next_msg.metadata else None) == target_key
-            is_delta = next_msg.metadata and next_msg.metadata.get("_stream_delta")
-            is_end = next_msg.metadata and next_msg.metadata.get("_stream_end")
+            next_metadata = next_msg.metadata or {}
+            same_target = (
+                next_msg.channel,
+                next_msg.chat_id,
+                next_metadata.get("_stream_id"),
+            ) == target_key
+            is_delta = next_metadata.get("_stream_delta")
+            is_end = next_metadata.get("_stream_end")
 
             if same_target and is_delta and not final_metadata.get("_stream_end"):
                 # Accumulate content
diff --git a/tests/channels/test_channel_manager_delta_coalescing.py b/tests/channels/test_channel_manager_delta_coalescing.py
index adec72e7..e3ed8347 100644
--- a/tests/channels/test_channel_manager_delta_coalescing.py
+++ b/tests/channels/test_channel_manager_delta_coalescing.py
@@ -142,6 +142,31 @@ class TestDeltaCoalescing:
         assert pending[0].chat_id == "chat2"
         assert pending[0].content == "World"
 
+    @pytest.mark.asyncio
+    async def test_deltas_different_stream_ids_not_coalesced(self, manager, bus):
+        """Deltas for the same chat but different streams should not be merged."""
+        await bus.publish_outbound(OutboundMessage(
+            channel="mock",
+            chat_id="chat1",
+            content="A1",
+            metadata={"_stream_delta": True, "_stream_id": "stream-a"},
+        ))
+        await bus.publish_outbound(OutboundMessage(
+            channel="mock",
+            chat_id="chat1",
+            content="B1",
+            metadata={"_stream_delta": True, "_stream_id": "stream-b"},
+        ))
+
+        first_msg = await bus.consume_outbound()
+        merged, pending = manager._coalesce_stream_deltas(first_msg)
+
+        assert merged.content == "A1"
+        assert merged.metadata.get("_stream_id") == "stream-a"
+        assert len(pending) == 1
+        assert pending[0].content == "B1"
+        assert pending[0].metadata.get("_stream_id") == "stream-b"
+
     @pytest.mark.asyncio
     async def test_stream_end_terminates_coalescing(self, manager, bus):
         """_stream_end should stop coalescing and be included in final message."""