diff --git a/nanobot/session/manager.py b/nanobot/session/manager.py
index 1febf10c9..efb5b475c 100644
--- a/nanobot/session/manager.py
+++ b/nanobot/session/manager.py
@@ -99,6 +99,11 @@ class Session:
     metadata: dict[str, Any] = field(default_factory=dict)
     last_consolidated: int = 0  # Number of messages already consolidated to files
 
+    def __post_init__(self) -> None:
+        # An out-of-range offset (corrupt metadata) would hide all history; reset it.
+        if not 0 <= self.last_consolidated <= len(self.messages):
+            self.last_consolidated = 0
+
     @staticmethod
     def _annotate_message_time(message: dict[str, Any], content: Any) -> Any:
         """Expose persisted turn timestamps to the model for relative-date reasoning.
diff --git a/tests/agent/test_session_atomic.py b/tests/agent/test_session_atomic.py
index 4720c028a..1fe5b9caa 100644
--- a/tests/agent/test_session_atomic.py
+++ b/tests/agent/test_session_atomic.py
@@ -205,7 +205,8 @@ class TestRepairCorruptFile:
 
         session = mgr._load("test:badts")
         assert session is not None
-        assert session.last_consolidated == 5
+        # offset 5 exceeds the single loaded message; reset to avoid hiding history (#4066)
+        assert session.last_consolidated == 0
         assert isinstance(session.created_at, datetime)
 
     def test_read_session_file_repairs_corrupt_jsonl(self, tmp_path: Path):
diff --git a/tests/session/test_consolidated_offset_clamp.py b/tests/session/test_consolidated_offset_clamp.py
new file mode 100644
index 000000000..6f9358ffc
--- /dev/null
+++ b/tests/session/test_consolidated_offset_clamp.py
@@ -0,0 +1,19 @@
+"""Reset a corrupt last_consolidated offset instead of hiding history (#4066)."""
+
+from nanobot.session.manager import Session
+
+
+def _session(count: int, last_consolidated: int) -> Session:
+    msgs = [{"role": "user", "content": f"msg{i}"} for i in range(count)]
+    return Session(key="chan:chat", messages=msgs, last_consolidated=last_consolidated)
+
+
+def test_out_of_range_offset_is_reset():
+    assert _session(10, 999).last_consolidated == 0
+    assert _session(3, -5).last_consolidated == 0
+
+
+def test_valid_offset_is_preserved():
+    session = _session(10, 4)
+    assert session.last_consolidated == 4
+    assert len(session.get_history()) == 6