diff --git a/src/bub/message_store/__init__.py b/src/bub/message_store/__init__.py
new file mode 100644
index 0000000..5b54bda
--- /dev/null
+++ b/src/bub/message_store/__init__.py
@@ -0,0 +1,4 @@
+from bub.message_store.service import MessageStore as MessageStore
+from bub.message_store.service import StoredMessage as StoredMessage
+
+__all__ = ["MessageStore", "StoredMessage"]
diff --git a/src/bub/message_store/service.py b/src/bub/message_store/service.py
new file mode 100644
index 0000000..945e0e5
--- /dev/null
+++ b/src/bub/message_store/service.py
@@ -0,0 +1,178 @@
+"""Message store service for proactive interaction."""
+
+from __future__ import annotations
+
+import json
+import sqlite3
+import threading
+import time
+from dataclasses import dataclass
+from typing import Literal
+
+
+@dataclass
+class StoredMessage:
+    """Represents a stored message."""
+
+    id: str
+    chat_id: int
+    thread_id: int | None
+    role: str
+    name: str | None
+    content: str
+    tool_call_id: str | None
+    tool_calls: list | None
+    timestamp: float
+
+
+class MessageStore:
+    """SQLite-based message store with thread-safe access."""
+
+    def __init__(self, db_path: str = ":memory:"):
+        self.db_path = db_path
+        self._local = threading.local()
+
+    @property
+    def _conn(self) -> sqlite3.Connection:
+        if not hasattr(self._local, "conn") or self._local.conn is None:
+            conn = sqlite3.connect(self.db_path)
+            conn.row_factory = sqlite3.Row
+            self._init_db(conn)
+            self._local.conn = conn
+            return conn
+        return self._local.conn  # type: ignore[no-any-return]
+
+    def _init_db(self, conn: sqlite3.Connection) -> None:
+        conn.execute("""
+            CREATE TABLE IF NOT EXISTS messages (
+                id TEXT PRIMARY KEY,
+                chat_id INTEGER NOT NULL,
+                thread_id INTEGER,
+                role TEXT NOT NULL,
+                name TEXT,
+                content TEXT,
+                tool_call_id TEXT,
+                tool_calls TEXT,
+                timestamp REAL NOT NULL
+            )
+        """)
+        conn.commit()
+
+    def add_message(self, msg: StoredMessage) -> None:
+        self._conn.execute(
+            """INSERT OR REPLACE INTO messages
+            (id, chat_id, thread_id, role, name, content, tool_call_id, tool_calls, timestamp)
+            VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)""",
+            (
+                msg.id,
+                msg.chat_id,
+                msg.thread_id,
+                msg.role,
+                msg.name,
+                msg.content,
+                msg.tool_call_id,
+                json.dumps(msg.tool_calls) if msg.tool_calls else None,
+                msg.timestamp,
+            ),
+        )
+        self._conn.commit()
+
+    def get_messages(self, chat_id: int, thread_id: int | None = None, limit: int = 100) -> list[StoredMessage]:
+        if thread_id is not None:
+            rows = self._conn.execute(
+                """SELECT * FROM messages WHERE chat_id = ? AND thread_id = ?
+                ORDER BY timestamp DESC LIMIT ?""",
+                (chat_id, thread_id, limit),
+            ).fetchall()
+        else:
+            rows = self._conn.execute(
+                """SELECT * FROM messages WHERE chat_id = ? AND thread_id IS NULL
+                ORDER BY timestamp DESC LIMIT ?""",
+                (chat_id, limit),
+            ).fetchall()
+
+        messages = []
+        for row in reversed(rows):
+            messages.append(
+                StoredMessage(
+                    id=row["id"],
+                    chat_id=row["chat_id"],
+                    thread_id=row["thread_id"],
+                    role=row["role"],
+                    name=row["name"],
+                    content=row["content"],
+                    tool_call_id=row["tool_call_id"],
+                    tool_calls=json.loads(row["tool_calls"]) if row["tool_calls"] else None,
+                    timestamp=row["timestamp"],
+                )
+            )
+        return messages
+
+    def delete_messages(self, chat_id: int, thread_id: int | None = None) -> None:
+        if thread_id is not None:
+            self._conn.execute("DELETE FROM messages WHERE chat_id = ? AND thread_id = ?", (chat_id, thread_id))
+        else:
+            self._conn.execute("DELETE FROM messages WHERE chat_id = ? AND thread_id IS NULL", (chat_id,))
+        self._conn.commit()
+
+    def get_last_message_by_role(
+        self, chat_id: int, role: Literal["user", "assistant"], thread_id: int | None = None
+    ) -> StoredMessage | None:
+        """Get the last message by a specific role in a chat."""
+        if thread_id is not None:
+            row = self._conn.execute(
+                """SELECT * FROM messages
+                WHERE chat_id = ? AND thread_id = ? AND role = ?
+                ORDER BY timestamp DESC LIMIT 1""",
+                (chat_id, thread_id, role),
+            ).fetchone()
+        else:
+            row = self._conn.execute(
+                """SELECT * FROM messages
+                WHERE chat_id = ? AND thread_id IS NULL AND role = ?
+                ORDER BY timestamp DESC LIMIT 1""",
+                (chat_id, role),
+            ).fetchone()
+
+        if row is None:
+            return None
+
+        return StoredMessage(
+            id=row["id"],
+            chat_id=row["chat_id"],
+            thread_id=row["thread_id"],
+            role=row["role"],
+            name=row["name"],
+            content=row["content"],
+            tool_call_id=row["tool_call_id"],
+            tool_calls=json.loads(row["tool_calls"]) if row["tool_calls"] else None,
+            timestamp=row["timestamp"],
+        )
+
+    def has_unreplied_message(self, chat_id: int, min_age_seconds: float = 300) -> bool:
+        """Check if there is a user message that has not been replied to for at least min_age_seconds."""
+        last_user = self.get_last_message_by_role(chat_id, "user")
+        if last_user is None:
+            return False
+
+        last_assistant = self.get_last_message_by_role(chat_id, "assistant")
+        if last_assistant is None:
+            return (time.time() - last_user.timestamp) >= min_age_seconds
+
+        if last_user.timestamp > last_assistant.timestamp:
+            return (time.time() - last_user.timestamp) >= min_age_seconds
+
+        return False
+
+    def get_active_chats(self, since: float) -> list[int]:
+        """Get list of chat_ids that have messages since the given timestamp."""
+        rows = self._conn.execute(
+            "SELECT DISTINCT chat_id FROM messages WHERE timestamp >= ?",
+            (since,),
+        ).fetchall()
+        return [row["chat_id"] for row in rows]
+
+    def close(self) -> None:
+        if hasattr(self._local, "conn") and self._local.conn:
+            self._local.conn.close()
+            self._local.conn = None