diff --git a/codex-rs/core/src/lib.rs b/codex-rs/core/src/lib.rs
index f73bef4051..e4f5709345 100644
--- a/codex-rs/core/src/lib.rs
+++ b/codex-rs/core/src/lib.rs
@@ -75,6 +75,7 @@ pub use rollout::find_conversation_path_by_id_str;
 pub use rollout::list::ConversationItem;
 pub use rollout::list::ConversationsPage;
 pub use rollout::list::Cursor;
+mod state;
 mod user_notification;
 pub mod util;
 
diff --git a/codex-rs/core/src/state/mod.rs b/codex-rs/core/src/state/mod.rs
new file mode 100644
index 0000000000..d88b674ef0
--- /dev/null
+++ b/codex-rs/core/src/state/mod.rs
@@ -0,0 +1,9 @@
+//! Session/turn state module scaffolding.
+//!
+//! This module will encapsulate all mutable state for a Codex session.
+//! It starts with lightweight placeholders to enable incremental refactors
+//! without changing behaviour.
+
+mod session;
+mod turn;
+
diff --git a/codex-rs/core/src/state/session.rs b/codex-rs/core/src/state/session.rs
new file mode 100644
index 0000000000..7765728363
--- /dev/null
+++ b/codex-rs/core/src/state/session.rs
@@ -0,0 +1,12 @@
+//! Session-wide mutable state scaffolding.
+
+/// Placeholder for session-persistent state.
+#[derive(Debug, Default)]
+pub(crate) struct SessionState;
+
+impl SessionState {
+    /// Create a new, empty session state.
+    pub(crate) fn new() -> Self {
+        Self
+    }
+}
diff --git a/codex-rs/core/src/state/turn.rs b/codex-rs/core/src/state/turn.rs
new file mode 100644
index 0000000000..5497e62ca6
--- /dev/null
+++ b/codex-rs/core/src/state/turn.rs
@@ -0,0 +1,9 @@
+//! Turn-scoped state and active turn metadata scaffolding.
+
+/// Metadata about the currently running turn.
+#[derive(Debug, Default)]
+pub(crate) struct ActiveTurn;
+
+/// Mutable state for a single turn.
+#[derive(Debug, Default)]
+pub(crate) struct TurnState;