Reinject previous-turn context for pre-sampling model-switch compaction

codex-rs/core/src/codex.rs

@@ -4670,7 +4670,7 @@ async fn maybe_run_previous_model_inline_compact(
         sess,
         // We use previous turn context here because we compact with the previous model
         &previous_turn_context,
-        AutoCompactCallsite::PreTurnExcludingIncomingUserMessage,
+        AutoCompactCallsite::PreSamplingModelSwitch,
         None,
     )
     .await

codex-rs/core/src/compact.rs

@@ -44,6 +44,9 @@ pub(crate) enum AutoCompactCallsite {
     /// default pre-turn flow and retained for future model-specific strategies.
     #[allow(dead_code)]
     PreTurnExcludingIncomingUserMessage,
+    /// Pre-sampling compaction triggered by model switch to a smaller context window.
+    /// This compacts prior-turn history only and should reinsert previous-turn canonical context.
+    PreSamplingModelSwitch,
     /// Mid-turn compaction between assistant responses in a follow-up loop.
     MidTurnContinuation,
 }
@@ -284,7 +287,9 @@ async fn run_compact_task_inner(
     let mut new_history = process_compacted_history(compacted_history);
     if matches!(
         auto_compact_callsite,
-        Some(AutoCompactCallsite::MidTurnContinuation)
+        Some(
+            AutoCompactCallsite::MidTurnContinuation | AutoCompactCallsite::PreSamplingModelSwitch
+        )
     ) {
         let initial_context = sess.build_initial_context(turn_context.as_ref()).await;
         insert_initial_context_before_last_real_user(&mut new_history, initial_context);

codex-rs/core/src/compact_remote.rs

@@ -168,7 +168,10 @@ async fn run_remote_compact_task_inner_impl(
         })
         .await?;
     new_history = process_compacted_history(new_history);
-    if auto_compact_callsite == AutoCompactCallsite::MidTurnContinuation {
+    if matches!(
+        auto_compact_callsite,
+        AutoCompactCallsite::MidTurnContinuation | AutoCompactCallsite::PreSamplingModelSwitch
+    ) {
         let initial_context = sess.build_initial_context(turn_context.as_ref()).await;
         insert_initial_context_before_last_real_user(&mut new_history, initial_context);
     }

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

@@ -142,8 +142,8 @@ fn assert_pre_sampling_switch_compaction_requests(
         "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 not reinsert canonical environment context after pre-sampling compaction"
+        body_contains_text(&follow_up_body, "<environment_context>"),
+        "follow-up request should include canonical environment context from previous-turn context reinjection"
     );
 }
 

codex-rs/core/tests/suite/snapshots/all__suite__compact__pre_sampling_model_switch_compaction_shapes.snap

@@ -1,6 +1,5 @@
 ---
 source: core/tests/suite/compact.rs
-assertion_line: 1460
 expression: "format_labeled_requests_snapshot(\"Pre-sampling compaction on model switch to a smaller context window: current behavior compacts using prior-turn history only (incoming user message excluded), and the follow-up request carries compacted history plus the new user message.\",\n&[(\"Initial Request (Previous Model)\", &requests[0]),\n(\"Pre-sampling Compaction Request\", &requests[1]),\n(\"Post-Compaction Follow-up Request (Next Model)\", &requests[2]),])"
 ---
 Scenario: Pre-sampling compaction on model switch to a smaller context window: current behavior compacts using prior-turn history only (incoming user message excluded), and the follow-up request carries compacted history plus the new user message.
@@ -22,7 +21,10 @@ Scenario: Pre-sampling compaction on model switch to a smaller context window: c
 06:message/user:<SUMMARIZATION_PROMPT>
 
 ## Post-Compaction Follow-up Request (Next Model)
-00:message/user:before switch
-01:message/user:<COMPACTION_SUMMARY>\nPRE_SAMPLING_SUMMARY
-02:message/developer:<model_switch>\nThe user was previously using a different model....
-03:message/user:after switch
+00:message/developer:<PERMISSIONS_INSTRUCTIONS>
+01:message/user:<AGENTS_MD>
+02:message/user:<ENVIRONMENT_CONTEXT:cwd=<CWD>>
+03:message/user:before switch
+04:message/user:<COMPACTION_SUMMARY>\nPRE_SAMPLING_SUMMARY
+05:message/developer:<model_switch>\nThe user was previously using a different model....
+06:message/user:after switch