fix(memory): keep history cursor monotonic

nanobot/agent/memory.py

@@ -336,18 +336,32 @@ class MemoryStore:
         session_key = entry.get("session_key")
         return session_key is None or isinstance(session_key, str)
 
+    def _read_cursor_counter(self) -> int | None:
+        """Return the persisted cursor counter when it is usable."""
+        if not self._cursor_file.exists():
+            return None
+        with suppress(ValueError, OSError):
+            cursor = int(self._cursor_file.read_text(encoding="utf-8").strip())
+            if cursor >= 0:
+                return cursor
+        return None
+
     def _next_cursor(self) -> int:
         """Read the current cursor counter and return the next value."""
-        if self._cursor_file.exists():
-            with suppress(ValueError, OSError):
-                return int(self._cursor_file.read_text(encoding="utf-8").strip()) + 1
+        cursor_counter = self._read_cursor_counter()
+        last = self._read_last_entry() or {}
+        last_cursor = self._valid_cursor(last.get("cursor"))
+        if cursor_counter is not None:
+            if last_cursor is not None:
+                return max(cursor_counter, last_cursor) + 1
+            max_history_cursor = max((c for _, c in self._iter_valid_entries()), default=0)
+            return max(cursor_counter, max_history_cursor) + 1
+
         # Fast path: trust the tail when intact.  Otherwise scan the whole
         # file and take ``max`` — that stays correct even if the monotonic
         # invariant was broken by external writes.
-        last = self._read_last_entry() or {}
-        cursor = self._valid_cursor(last.get("cursor"))
-        if cursor is not None:
-            return cursor + 1
+        if last_cursor is not None:
+            return last_cursor + 1
         return max((c for _, c in self._iter_valid_entries()), default=0) + 1
 
     def read_unprocessed_history(self, since_cursor: int) -> list[dict[str, Any]]:

tests/agent/test_cursor_recovery.py

@@ -6,8 +6,6 @@ history.jsonl (e.g. ``"cursor": "abc"``).  The original ``_next_cursor`` and
 ``TypeError`` / ``ValueError``, blocking all subsequent history appends.
 """
 
-import json
-
 import pytest
 
 from nanobot.agent.memory import MemoryStore
@@ -70,6 +68,40 @@ class TestNextCursorRecovery:
         cursor = store.append_history("after bad cursor file")
         assert cursor == 11
 
+    def test_stale_cursor_file_does_not_reuse_history_cursor(self, store):
+        """A stale .cursor file must not allocate a duplicate cursor."""
+        store.history_file.write_text(
+            '{"cursor": 10, "timestamp": "2026-04-01 10:00", "content": "valid"}\n',
+            encoding="utf-8",
+        )
+        store._cursor_file.write_text("2", encoding="utf-8")
+
+        cursor = store.append_history("after stale cursor file")
+
+        assert cursor == 11
+        entries = store.read_unprocessed_history(since_cursor=0)
+        assert [e["cursor"] for e in entries] == [10, 11]
+
+    def test_cursor_file_stays_ahead_after_history_compaction(self, store):
+        """A cursor counter ahead of the tail preserves monotonic allocation."""
+        store.history_file.write_text(
+            '{"cursor": 10, "timestamp": "2026-04-01 10:00", "content": "valid"}\n',
+            encoding="utf-8",
+        )
+        store._cursor_file.write_text("100", encoding="utf-8")
+
+        cursor = store.append_history("after compacted history")
+
+        assert cursor == 101
+
+    def test_negative_cursor_file_content_falls_back(self, store):
+        """A negative .cursor value is corrupt and should not produce negative IDs."""
+        store._cursor_file.write_text("-5", encoding="utf-8")
+
+        cursor = store.append_history("after negative cursor file")
+
+        assert cursor == 1
+
 
 class TestReadUnprocessedWithCorruption:
     """``read_unprocessed_history`` must skip entries with non-int cursors
@@ -159,6 +191,7 @@ class TestCursorValidationInvariant:
         warning, subsequent reads on the same store stay quiet.  Without
         this, a poisoned file produces one warning per agent turn."""
         import logging
+
         from loguru import logger as loguru_logger
 
         store.history_file.write_text(