fix: prevent duplicate LLM stream snapshots

tests/test_mentor_stream.py

@@ -66,6 +66,43 @@ class MentorStreamTests(unittest.TestCase):
             )
         self.assertEqual(result["answer"], "first second")
 
+    def test_final_full_message_does_not_duplicate_streamed_deltas(self):
+        lines = [
+            b'data: {"choices":[{"delta":{"content":"first"}}]}\n',
+            b'data: {"choices":[{"delta":{"content":" second"}}]}\n',
+            b'data: {"choices":[{"message":{"content":"first second"}}]}\n',
+            b"data: [DONE]\n",
+        ]
+        with patch(
+            "mentor_agent.urllib.request.urlopen",
+            return_value=FakeStreamResponse(lines),
+        ):
+            chunks = list(
+                stream_with_mentor(
+                    self.skill, {}, "question", [], "key",
+                    "https://example.test/v1", "model",
+                )
+            )
+        self.assertEqual(chunks, ["first", " second"])
+
+    def test_snapshot_only_stream_emits_only_new_suffix(self):
+        lines = [
+            b'data: {"choices":[{"message":{"content":"first"}}]}\n',
+            b'data: {"choices":[{"message":{"content":"first second"}}]}\n',
+            b"data: [DONE]\n",
+        ]
+        with patch(
+            "mentor_agent.urllib.request.urlopen",
+            return_value=FakeStreamResponse(lines),
+        ):
+            chunks = list(
+                stream_with_mentor(
+                    self.skill, {}, "question", [], "key",
+                    "https://example.test/v1", "model",
+                )
+            )
+        self.assertEqual(chunks, ["first", " second"])
+
 
 if __name__ == "__main__":
     unittest.main()