core/agent+cli: fix compact session restore and persist cursor

After /compact, reload seed from DB so _persist_from matches stored rows.
rebuild_client preserves the checkpoint via replace_messages. Clarify that
the active session is summary-only and full history stays on the old id.

src/plyngent/agent/chat.py

@@ -200,14 +200,38 @@ class ChatAgent:
         if self._persist_from == 0:
             self._persist_from = 1
 
+    def replace_messages(
+        self,
+        messages: Sequence[AnyChatMessage],
+        *,
+        persisted: bool = True,
+        persist_from: int | None = None,
+    ) -> None:
+        """Replace in-memory history and align the persistence checkpoint.
+
+        When *persist_from* is set, it is used as the checkpoint cursor
+        (clamped to ``[0, len(messages)]``). Otherwise *persisted* true means
+        all messages are already stored; false means none are.
+        """
+        self.messages = list(messages)
+        if persist_from is not None:
+            self._persist_from = max(0, min(persist_from, len(self.messages)))
+        else:
+            self._persist_from = len(self.messages) if persisted else 0
+        self._ensure_system_prompt()
+
+    @property
+    def persist_from(self) -> int:
+        """Index of the first unpersisted message (checkpoint cursor)."""
+        return self._persist_from
+
     async def load_history(self) -> None:
         """Replace in-memory messages from the bound memory session."""
         if self.memory is None or self.session_id is None:
             msg = "load_history requires memory and session_id"
             raise RuntimeError(msg)
-        self.messages = await self.memory.list_messages(self.session_id)
-        self._persist_from = len(self.messages)
-        self._ensure_system_prompt()
+        loaded = await self.memory.list_messages(self.session_id)
+        self.replace_messages(loaded, persisted=True)
 
     async def bind_session(self, session_id: int, *, load: bool = True) -> None:
         """Attach a memory session id; optionally load existing messages."""

src/plyngent/cli/slash.py

@@ -576,6 +576,11 @@ def compact_cmd(state: ReplState, name: str | None) -> None:
         return
     preview = summary if len(summary) <= _COMPACT_PREVIEW else summary[:_COMPACT_PREVIEW] + "…"
     click.echo(f"compacted session {old_id} -> new session {new_id}")
+    click.secho(
+        f"active session is summary-only ({len(state.agent.messages)} messages); "
+        f"full history remains on /resume {old_id}",
+        fg="bright_black",
+    )
     click.secho(preview, fg="bright_black")
 
 

src/plyngent/cli/state.py

@@ -145,12 +145,14 @@ class ReplState:
     def rebuild_client(self) -> None:
         """Recreate client and agent after provider/model/tools change."""
         messages = list(self.agent.messages)
+        persist_from = self.agent.persist_from
         # Preserve live stream toggle if agent already exists.
         if hasattr(self, "agent"):
             self.stream_enabled = self.agent.stream
         self.client = cast("ChatClient", cast("object", create_client(self.provider)))
         self.agent = self._make_agent()
-        self.agent.messages = messages
+        # Restore history without re-marking already-stored messages as dirty.
+        self.agent.replace_messages(messages, persist_from=persist_from)
         self.sync_display_flags()
         # Drop remote catalog when provider identity/url changed (not on model-only switch).
         if self._remote_models_key is not None and self._remote_models_key != self._models_cache_key():
@@ -474,7 +476,12 @@ class ReplState:
             source_session_id=old_id,
             seed_text=self.config.agent_config.compact_seed_text or None,
         )
-        self.agent.messages = list(seed)
         for message in seed:
             _ = await self.memory.append_message(new_id, message)
+        # Reload from DB so RAM matches stored rows and the persist cursor is correct
+        # (assigning messages alone left _persist_from at 0 and broke later checkpoints).
+        await self.agent.load_history()
+        if not self.agent.messages:
+            msg = f"compact session {new_id} has no messages after seed"
+            raise RuntimeError(msg)
         return old_id, new_id, summary