diff --git a/codex-rs/core/src/codex.rs b/codex-rs/core/src/codex.rs
index 4c9951a96c..30794012f5 100644
--- a/codex-rs/core/src/codex.rs
+++ b/codex-rs/core/src/codex.rs
@@ -5966,12 +5966,25 @@ fn build_server_side_compaction_replacement_history(
     let checkpoint_turn_items = history_at_checkpoint
         .strip_prefix(history_before_turn)
         .unwrap_or(history_at_checkpoint);
-    let checkpoint_turn_items = checkpoint_turn_items
-        .strip_prefix(turn_start_context_items)
-        .unwrap_or(checkpoint_turn_items);
-    let checkpoint_turn_items = checkpoint_turn_items
-        .strip_prefix(compaction_initial_context)
-        .unwrap_or(checkpoint_turn_items);
+    let stripped_compaction_initial_context =
+        checkpoint_turn_items.strip_prefix(compaction_initial_context);
+    let stripped_turn_start_context_items =
+        checkpoint_turn_items.strip_prefix(turn_start_context_items);
+    let checkpoint_turn_items = match (
+        stripped_compaction_initial_context,
+        stripped_turn_start_context_items,
+    ) {
+        (Some(after_compaction_initial_context), Some(after_turn_start_context_items)) => {
+            if compaction_initial_context.len() >= turn_start_context_items.len() {
+                after_compaction_initial_context
+            } else {
+                after_turn_start_context_items
+            }
+        }
+        (Some(after_compaction_initial_context), None) => after_compaction_initial_context,
+        (None, Some(after_turn_start_context_items)) => after_turn_start_context_items,
+        (None, None) => checkpoint_turn_items,
+    };
     let post_checkpoint_turn_items = current_history
         .strip_prefix(history_at_checkpoint)
         .unwrap_or_default();
diff --git a/codex-rs/core/src/codex_tests.rs b/codex-rs/core/src/codex_tests.rs
index ce97e48604..d95c7774c6 100644
--- a/codex-rs/core/src/codex_tests.rs
+++ b/codex-rs/core/src/codex_tests.rs
@@ -549,6 +549,60 @@ fn build_server_side_compaction_replacement_history_reuses_existing_initial_cont
     );
 }
 
+#[test]
+fn build_server_side_compaction_replacement_history_prefers_longer_initial_context_prefix() {
+    let history_before_turn = vec![user_message("earlier")];
+    let turn_start_context_items = vec![developer_message("<model_switch>\nuse the new model")];
+    let compaction_initial_context = vec![
+        turn_start_context_items[0].clone(),
+        environment_context_message("/fresh"),
+    ];
+    let current_turn_user = user_message("current turn");
+    let prior_compaction = ResponseItem::Compaction {
+        encrypted_content: "INLINE_SUMMARY_1".to_string(),
+    };
+    let new_compaction = ResponseItem::Compaction {
+        encrypted_content: "INLINE_SUMMARY_2".to_string(),
+    };
+    let current_turn_tool_output = ResponseItem::FunctionCallOutput {
+        call_id: "call-1".to_string(),
+        output: FunctionCallOutputPayload::from_text("tool result".to_string()),
+    };
+    let history_at_checkpoint = vec![
+        compaction_initial_context[0].clone(),
+        compaction_initial_context[1].clone(),
+        current_turn_user.clone(),
+        prior_compaction,
+    ];
+    let current_history = vec![
+        history_at_checkpoint[0].clone(),
+        history_at_checkpoint[1].clone(),
+        history_at_checkpoint[2].clone(),
+        history_at_checkpoint[3].clone(),
+        current_turn_tool_output.clone(),
+    ];
+
+    let replacement_history = build_server_side_compaction_replacement_history(
+        new_compaction.clone(),
+        &compaction_initial_context,
+        &turn_start_context_items,
+        &history_before_turn,
+        &history_at_checkpoint,
+        &current_history,
+    );
+
+    assert_eq!(
+        replacement_history,
+        vec![
+            compaction_initial_context[0].clone(),
+            compaction_initial_context[1].clone(),
+            current_turn_user,
+            new_compaction,
+            current_turn_tool_output,
+        ]
+    );
+}
+
 #[test]
 fn build_server_side_compaction_replacement_history_keeps_checkpoint_before_post_compaction_items()
 {