Reinject canonical context for model-switch pre-turn compaction

codex-rs/core/src/codex.rs

@@ -4681,7 +4681,11 @@ async fn maybe_run_previous_model_inline_compact(
             sess,
             &previous_turn_context,
             AutoCompactCallsite::PreTurnExcludingIncomingUserMessage,
-            TurnContextReinjection::Skip,
+            // Even though incoming turn items are excluded here, this pass can be the only
+            // compaction run before submission. Reinject canonical context so unchanged
+            // model-visible instructions remain present if no follow-up pre-turn compaction
+            // is needed.
+            TurnContextReinjection::ReinjectAboveLastRealUser,
             None,
         )
         .await?;

codex-rs/core/tests/suite/compact.rs

@@ -132,11 +132,20 @@ fn assert_pre_sampling_switch_compaction_requests(
         !compact_body.contains("<model_switch>"),
         "pre-sampling compact request should strip trailing model-switch update item"
     );
+    let first_body = first.to_string();
+    assert!(
+        body_contains_text(&first_body, "<environment_context>"),
+        "first request should include canonical environment context"
+    );
     let follow_up_body = follow_up.to_string();
     assert!(
         follow_up_body.contains("<model_switch>"),
         "follow-up request after successful model-switch compaction should include model-switch update item"
     );
+    assert!(
+        body_contains_text(&follow_up_body, "<environment_context>"),
+        "follow-up request should preserve canonical environment context after pre-sampling compaction"
+    );
 }
 
 async fn assert_compaction_uses_turn_lifecycle_id(codex: &std::sync::Arc<codex_core::CodexThread>) {