fix: prevent duplicate LLM stream snapshots

tests/test_llm_stream.py

@@ -0,0 +1,35 @@
+from __future__ import annotations
+
+import unittest
+
+from llm_stream import OpenAIStreamAccumulator
+
+
+class OpenAIStreamAccumulatorTests(unittest.TestCase):
+    def test_repeated_delta_chunks_are_preserved_as_model_output(self) -> None:
+        accumulator = OpenAIStreamAccumulator()
+        self.assertEqual(accumulator.feed({"delta": {"content": "yes"}}), "yes")
+        self.assertEqual(accumulator.feed({"delta": {"content": "yes"}}), "yes")
+        self.assertEqual(accumulator.text, "yesyes")
+
+    def test_final_snapshot_can_add_a_missing_suffix(self) -> None:
+        accumulator = OpenAIStreamAccumulator()
+        self.assertEqual(accumulator.feed({"delta": {"content": "first"}}), "first")
+        self.assertEqual(
+            accumulator.feed({"message": {"content": "first second"}}),
+            " second",
+        )
+        self.assertEqual(accumulator.text, "first second")
+
+    def test_incompatible_final_snapshot_is_not_appended_twice(self) -> None:
+        accumulator = OpenAIStreamAccumulator()
+        accumulator.feed({"delta": {"content": "streamed answer"}})
+        self.assertEqual(
+            accumulator.feed({"message": {"content": "rewritten final answer"}}),
+            "",
+        )
+        self.assertEqual(accumulator.text, "streamed answer")
+
+
+if __name__ == "__main__":
+    unittest.main()

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()

tests/test_review_assistant.py

@@ -45,6 +45,23 @@ class ReviewAssistantStreamingTests(unittest.TestCase):
                     stream_review_assistant({}, "question", [], "key", "https://example.test/v1", "model")
                 )
 
+    def test_final_full_message_does_not_duplicate_streamed_deltas(self):
+        response = StreamingResponse(
+            [
+                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("assistant_agent.urllib.request.urlopen", return_value=response):
+            chunks = list(
+                stream_review_assistant(
+                    {}, "question", [], "key", "https://example.test/v1", "model"
+                )
+            )
+        self.assertEqual(chunks, ["first", " second"])
+
     def test_ndjson_event_writer_flushes_complete_line(self):
         handler = RequestHandler.__new__(RequestHandler)
         handler.wfile = io.BytesIO()