fix: assistant prefill prevents malformed harvest/refresh responses

memsync/sync.py

@@ -65,14 +65,18 @@ def harvest_memory_content(transcript: str, current_memory: str, config: Config)
 SESSION TRANSCRIPT:
 {transcript}"""
 
+    prefill = _build_prefill(current_memory)
     response = client.messages.create(
         model=config.model,
         max_tokens=4096,
         system=HARVEST_SYSTEM_PROMPT,
-        messages=[{"role": "user", "content": user_prompt}],
+        messages=[
+            {"role": "user", "content": user_prompt},
+            {"role": "assistant", "content": prefill},
+        ],
     )
 
-    updated_content = _strip_model_wrapper(response.content[0].text)
+    updated_content = _strip_model_wrapper(prefill + response.content[0].text)
 
     if not _looks_like_memory_file(updated_content):
         return {
@@ -98,6 +102,20 @@ def harvest_memory_content(transcript: str, current_memory: str, config: Config)
     }
 
 
+def _build_prefill(current_memory: str) -> str:
+    """
+    Build an assistant prefill string that forces the model to start outputting
+    the memory file rather than a narrative summary.
+
+    Uses the first line of the current memory if it looks like a valid start
+    (heading or comment marker), otherwise falls back to the memsync comment.
+    """
+    first_line = current_memory.strip().splitlines()[0] if current_memory.strip() else ""
+    if first_line.startswith("#") or first_line.startswith("<!--"):
+        return first_line
+    return "<!-- memsync v0.2 -->"
+
+
 def _strip_model_wrapper(content: str) -> str:
     """
     Strip wrapper artifacts the model sometimes adds around the memory file:
@@ -154,14 +172,18 @@ def refresh_memory_content(notes: str, current_memory: str, config: Config) -> d
 SESSION NOTES:
 {notes}"""
 
+    prefill = _build_prefill(current_memory)
     response = client.messages.create(
         model=config.model,
         max_tokens=4096,
         system=SYSTEM_PROMPT,
-        messages=[{"role": "user", "content": user_prompt}],
+        messages=[
+            {"role": "user", "content": user_prompt},
+            {"role": "assistant", "content": prefill},
+        ],
     )
 
-    updated_content = _strip_model_wrapper(response.content[0].text)
+    updated_content = _strip_model_wrapper(prefill + response.content[0].text)
 
     # Reject responses that look like narrative explanations rather than a memory file.
     # The model occasionally ignores "no preamble" and returns prose — writing that

tests/test_harvest.py

@@ -308,7 +308,19 @@ def test_ignores_invalid_dict_entries(self, tmp_path):
 # ---------------------------------------------------------------------------
 
 class TestHarvestMemoryContent:
-    def _make_mock_response(self, text: str, stop_reason: str = "end_turn") -> MagicMock:
+    def _make_mock_response(self, text: str, stop_reason: str = "end_turn",
+                            current_memory: str = SAMPLE_MEMORY) -> MagicMock:
+        """
+        Simulate the API returning a continuation after the prefill.
+
+        With assistant prefill, the API only returns the text *after* the prefill.
+        The code then combines: prefill + response.content[0].text.
+        So the mock must strip the prefill line from the expected output.
+        """
+        from memsync.sync import _build_prefill
+        prefill = _build_prefill(current_memory)
+        if text.startswith(prefill):
+            text = text[len(prefill):]
         mock = MagicMock()
         mock.content = [MagicMock(text=text)]
         mock.stop_reason = stop_reason

tests/test_sync.py

@@ -136,7 +136,19 @@ def test_content_includes_notes(self, tmp_path):
 
 
 class TestRefreshMemoryContent:
-    def _make_mock_response(self, text: str, stop_reason: str = "end_turn") -> MagicMock:
+    def _make_mock_response(self, text: str, stop_reason: str = "end_turn",
+                            current_memory: str = SAMPLE_MEMORY) -> MagicMock:
+        """
+        Simulate the API returning a continuation after the prefill.
+
+        With assistant prefill, the API only returns the text *after* the prefill.
+        The code then combines: prefill + response.content[0].text.
+        So the mock must strip the prefill line from the expected output.
+        """
+        from memsync.sync import _build_prefill
+        prefill = _build_prefill(current_memory)
+        if text.startswith(prefill):
+            text = text[len(prefill):]
         mock_response = MagicMock()
         mock_response.content = [MagicMock(text=text)]
         mock_response.stop_reason = stop_reason