core: add context window lineage IDs

codex-rs/app-server-protocol/src/protocol/thread_history.rs

@@ -3341,6 +3341,8 @@ mod tests {
                 message: String::new(),
                 replacement_history: None,
                 window_number: None,
+                first_window_id: None,
+                previous_window_id: None,
                 window_id: None,
             }),
             RolloutItem::EventMsg(EventMsg::TurnComplete(TurnCompleteEvent {

codex-rs/core/src/agent/control_tests.rs

@@ -1160,6 +1160,8 @@ async fn spawn_agent_fork_strips_parent_usage_hints_from_compacted_history() {
                 message: String::new(),
                 replacement_history: Some(replacement_history),
                 window_number: None,
+                first_window_id: None,
+                previous_window_id: None,
                 window_id: None,
             }),
             RolloutItem::TurnContext(turn_context.to_turn_context_item()),

codex-rs/core/src/compact.rs

@@ -302,7 +302,7 @@ async fn run_compact_task_inner_impl(
     let user_messages = collect_user_messages(history_items);
 
     let mut new_history = build_compacted_history(Vec::new(), &user_messages, &summary_text);
-    let (window_number, window_id) = sess.advance_auto_compact_window().await;
+    let (window_number, window_ids) = sess.advance_auto_compact_window().await;
 
     if matches!(
         initial_context_injection,
@@ -320,7 +320,9 @@ async fn run_compact_task_inner_impl(
         message: summary_text.clone(),
         replacement_history: Some(new_history.clone()),
         window_number: Some(window_number),
-        window_id: Some(window_id),
+        first_window_id: Some(window_ids.first_window_id.to_string()),
+        previous_window_id: window_ids.previous_window_id.map(|id| id.to_string()),
+        window_id: Some(window_ids.window_id.to_string()),
     };
     sess.replace_compacted_history(
         turn_context.as_ref(),

codex-rs/core/src/compact_remote.rs

@@ -258,7 +258,7 @@ async fn run_remote_compact_task_inner_impl(
             &responses_metadata,
         )
         .await?;
-    let (new_window_number, new_window_id) = sess.advance_auto_compact_window().await;
+    let (new_window_number, new_window_ids) = sess.advance_auto_compact_window().await;
     new_history = process_compacted_history(
         sess.as_ref(),
         turn_context.as_ref(),
@@ -275,7 +275,9 @@ async fn run_remote_compact_task_inner_impl(
         message: String::new(),
         replacement_history: Some(new_history.clone()),
         window_number: Some(new_window_number),
-        window_id: Some(new_window_id),
+        first_window_id: Some(new_window_ids.first_window_id.to_string()),
+        previous_window_id: new_window_ids.previous_window_id.map(|id| id.to_string()),
+        window_id: Some(new_window_ids.window_id.to_string()),
     };
     // Install is the semantic boundary where the compact endpoint's output becomes live
     // thread history. Keep it distinct from the later inference request so the reducer can

codex-rs/core/src/compact_remote_v2.rs

@@ -292,7 +292,7 @@ async fn run_remote_compact_task_inner_impl(
     let (compacted_history, retained_images) =
         build_v2_compacted_history(&prompt_input, compaction_output);
     analytics_details.retained_image_count = Some(retained_images);
-    let (new_window_number, new_window_id) = sess.advance_auto_compact_window().await;
+    let (new_window_number, new_window_ids) = sess.advance_auto_compact_window().await;
     let new_history = process_compacted_history(
         sess.as_ref(),
         turn_context.as_ref(),
@@ -309,7 +309,9 @@ async fn run_remote_compact_task_inner_impl(
         message: String::new(),
         replacement_history: Some(new_history.clone()),
         window_number: Some(new_window_number),
-        window_id: Some(new_window_id),
+        first_window_id: Some(new_window_ids.first_window_id.to_string()),
+        previous_window_id: new_window_ids.previous_window_id.map(|id| id.to_string()),
+        window_id: Some(new_window_ids.window_id.to_string()),
     };
     compaction_trace.record_installed(&CompactionCheckpointTracePayload {
         input_history: &trace_input_history,

codex-rs/core/src/context/token_budget_context.rs

@@ -5,14 +5,24 @@ use uuid::Uuid;
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub(crate) struct TokenBudgetContext {
     thread_id: ThreadId,
+    first_window_id: Uuid,
+    previous_window_id: Option<Uuid>,
     window_id: Uuid,
     tokens_left: i64,
 }
 
 impl TokenBudgetContext {
-    pub(crate) fn new(thread_id: ThreadId, window_id: Uuid, tokens_left: i64) -> Self {
+    pub(crate) fn new(
+        thread_id: ThreadId,
+        first_window_id: Uuid,
+        previous_window_id: Option<Uuid>,
+        window_id: Uuid,
+        tokens_left: i64,
+    ) -> Self {
         Self {
             thread_id,
+            first_window_id,
+            previous_window_id,
             window_id,
             tokens_left,
         }
@@ -34,10 +44,14 @@ impl ContextualUserFragment for TokenBudgetContext {
 
     fn body(&self) -> String {
         let thread_id = self.thread_id;
+        let first_window_id = self.first_window_id;
+        let previous_window_id = self
+            .previous_window_id
+            .map_or_else(|| "none".to_string(), |window_id| window_id.to_string());
         let window_id = self.window_id;
         let tokens_left = self.tokens_left;
         format!(
-            "Thread id {thread_id}.\nCurrent context window id {window_id}.\nYou have {tokens_left} tokens left in this context window."
+            "Thread id {thread_id}.\nFirst context window id {first_window_id}.\nPrevious context window id {previous_window_id}.\nCurrent context window id {window_id}.\nYou have {tokens_left} tokens left in this context window."
         )
     }
 }

codex-rs/core/src/session/mod.rs

@@ -311,6 +311,7 @@ use crate::session_startup_prewarm::SessionStartupPrewarmHandle;
 use crate::shell;
 #[cfg(test)]
 use crate::skills::SkillLoadOutcome;
+use crate::state::AutoCompactWindowIds;
 use crate::state::AutoCompactWindowSnapshot;
 use crate::state::PendingRequestPermissions;
 use crate::state::SessionServices;
@@ -1364,6 +1365,8 @@ impl Session {
             previous_turn_settings,
             reference_context_item,
             window_number,
+            first_window_id,
+            previous_window_id,
             window_id,
         } = self
             .reconstruct_history_from_rollout(turn_context, rollout_items)
@@ -1382,8 +1385,16 @@ impl Session {
         {
             let mut state = self.state.lock().await;
             state.replace_history(history, reference_context_item);
-            let window_id = window_id.unwrap_or_else(|| state.auto_compact_window_id());
-            state.restore_auto_compact_window(window_number, window_id);
+            let fallback_ids = state.auto_compact_window_ids();
+            let window_id = window_id.unwrap_or(fallback_ids.window_id);
+            state.restore_auto_compact_window(
+                window_number,
+                AutoCompactWindowIds {
+                    first_window_id: first_window_id.unwrap_or(window_id),
+                    previous_window_id,
+                    window_id,
+                },
+            );
             state.set_previous_turn_settings(previous_turn_settings.clone());
         }
         let prefix_tokens = if matches!(
@@ -3011,7 +3022,7 @@ impl Session {
             collaboration_mode,
             base_instructions,
             session_source,
-            auto_compact_window_id,
+            auto_compact_window_ids,
         ) = {
             let state = self.state.lock().await;
             (
@@ -3020,7 +3031,7 @@ impl Session {
                 state.session_configuration.collaboration_mode.clone(),
                 state.session_configuration.base_instructions.clone(),
                 state.session_configuration.session_source.clone(),
-                state.auto_compact_window_id(),
+                state.auto_compact_window_ids(),
             )
         };
         if let Some(model_switch_message) =
@@ -3210,7 +3221,9 @@ impl Session {
             developer_sections.push(
                 crate::context::TokenBudgetContext::new(
                     self.thread_id(),
-                    auto_compact_window_id,
+                    auto_compact_window_ids.first_window_id,
+                    auto_compact_window_ids.previous_window_id,
+                    auto_compact_window_ids.window_id,
                     model_context_window,
                 )
                 .render(),
@@ -3309,10 +3322,9 @@ impl Session {
         format!("{thread_id}:{window_number}")
     }
 
-    pub(crate) async fn advance_auto_compact_window(&self) -> (u64, String) {
+    pub(crate) async fn advance_auto_compact_window(&self) -> (u64, AutoCompactWindowIds) {
         let mut state = self.state.lock().await;
-        let (window_number, window_id) = state.advance_auto_compact_window();
-        (window_number, window_id.to_string())
+        state.advance_auto_compact_window()
     }
 
     pub(crate) async fn request_new_context_window(&self) {
@@ -3328,7 +3340,7 @@ impl Session {
             let mut state = self.state.lock().await;
             state.start_new_context_window_if_requested()
         };
-        let (window_number, window_id) = window?;
+        let (window_number, window_ids) = window?;
         let context_items = self.build_initial_context(turn_context).await;
         let turn_context_item = turn_context.to_turn_context_item();
         let replacement_history = context_items;
@@ -3341,7 +3353,9 @@ impl Session {
                 message: String::new(),
                 replacement_history: Some(replacement_history),
                 window_number: Some(window_number),
-                window_id: Some(window_id.to_string()),
+                first_window_id: Some(window_ids.first_window_id.to_string()),
+                previous_window_id: window_ids.previous_window_id.map(|id| id.to_string()),
+                window_id: Some(window_ids.window_id.to_string()),
             }),
             RolloutItem::TurnContext(turn_context_item),
         ])

codex-rs/core/src/session/rollout_reconstruction.rs

@@ -10,12 +10,16 @@ pub(super) struct RolloutReconstruction {
     pub(super) previous_turn_settings: Option<PreviousTurnSettings>,
     pub(super) reference_context_item: Option<TurnContextItem>,
     pub(super) window_number: u64,
+    pub(super) first_window_id: Option<Uuid>,
+    pub(super) previous_window_id: Option<Uuid>,
     pub(super) window_id: Option<Uuid>,
 }
 
 #[derive(Debug, Clone, Copy)]
 struct ReconstructedWindow {
     number: u64,
+    first_id: Option<Uuid>,
+    previous_id: Option<Uuid>,
     id: Option<Uuid>,
 }
 
@@ -133,6 +137,11 @@ impl Session {
                     {
                         active_segment.window = Some(ReconstructedWindow {
                             number: window_number,
+                            first_id: compacted.first_window_id.as_deref().and_then(parse_uuid_v7),
+                            previous_id: compacted
+                                .previous_window_id
+                                .as_deref()
+                                .and_then(parse_uuid_v7),
                             id: compacted.window_id.as_deref().and_then(parse_uuid_v7),
                         });
                     }
@@ -341,13 +350,17 @@ impl Session {
 
         let window = window.unwrap_or(ReconstructedWindow {
             number: fallback_window_number,
+            first_id: None,
+            previous_id: None,
             id: None,
         });
         RolloutReconstruction {
             history: history.raw_items().to_vec(),
             previous_turn_settings,
             reference_context_item,
             window_number: window.number,
+            first_window_id: window.first_id,
+            previous_window_id: window.previous_id,
             window_id: window.id,
         }
     }

codex-rs/core/src/session/rollout_reconstruction_tests.rs

@@ -829,6 +829,8 @@ async fn record_initial_history_resumed_rollback_drops_incomplete_user_turn_comp
             message: String::new(),
             replacement_history: Some(Vec::new()),
             window_number: None,
+            first_window_id: None,
+            previous_window_id: None,
             window_id: None,
         }),
         RolloutItem::EventMsg(EventMsg::ThreadRolledBack(
@@ -887,6 +889,8 @@ async fn record_initial_history_resumed_does_not_seed_reference_context_item_aft
             message: String::new(),
             replacement_history: Some(Vec::new()),
             window_number: None,
+            first_window_id: None,
+            previous_window_id: None,
             window_id: None,
         }),
     ];
@@ -914,6 +918,8 @@ async fn reconstruct_history_legacy_compaction_without_replacement_history_does_
             message: "legacy summary".to_string(),
             replacement_history: None,
             window_number: None,
+            first_window_id: None,
+            previous_window_id: None,
             window_id: None,
         }),
     ];
@@ -947,6 +953,8 @@ async fn reconstruct_history_legacy_compaction_without_replacement_history_clear
             message: "legacy summary".to_string(),
             replacement_history: None,
             window_number: None,
+            first_window_id: None,
+            previous_window_id: None,
             window_id: None,
         }),
         RolloutItem::EventMsg(EventMsg::TurnStarted(
@@ -1043,6 +1051,8 @@ async fn record_initial_history_resumed_turn_context_after_compaction_reestablis
             message: String::new(),
             replacement_history: Some(Vec::new()),
             window_number: None,
+            first_window_id: None,
+            previous_window_id: None,
             window_id: None,
         }),
         RolloutItem::TurnContext(previous_context_item),
@@ -1196,6 +1206,8 @@ async fn record_initial_history_resumed_aborted_turn_without_id_clears_active_tu
             message: String::new(),
             replacement_history: Some(Vec::new()),
             window_number: None,
+            first_window_id: None,
+            previous_window_id: None,
             window_id: None,
         }),
     ];
@@ -1433,6 +1445,8 @@ async fn record_initial_history_resumed_trailing_incomplete_turn_compaction_clea
             message: String::new(),
             replacement_history: Some(Vec::new()),
             window_number: None,
+            first_window_id: None,
+            previous_window_id: None,
             window_id: None,
         }),
     ];
@@ -1599,6 +1613,8 @@ async fn record_initial_history_resumed_replaced_incomplete_compacted_turn_clear
             message: String::new(),
             replacement_history: Some(Vec::new()),
             window_number: None,
+            first_window_id: None,
+            previous_window_id: None,
             window_id: None,
         }),
         // A newer TurnStarted replaces the incomplete compacted turn without a matching