Drop orphan reasoning items before replay

Trim trailing reasoning items from prompt history before the next user message or prompt end so interrupted turns do not replay invalid Responses API input.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/context_manager/history.rs

@@ -333,7 +333,8 @@ impl ContextManager {
     /// This function enforces a couple of invariants on the in-memory history:
     /// 1. every call (function/custom) has a corresponding output entry
     /// 2. every output has a corresponding call entry
-    /// 3. when images are unsupported, image content is stripped from messages and tool outputs
+    /// 3. trailing reasoning items are dropped before the next user message and at prompt end
+    /// 4. when images are unsupported, image content is stripped from messages and tool outputs
     fn normalize_history(&mut self, input_modalities: &[InputModality]) {
         // all function/tool calls must have a corresponding output
         normalize::ensure_call_outputs_present(&mut self.items);
@@ -341,6 +342,9 @@ impl ContextManager {
         // all outputs must have a corresponding function/tool call
         normalize::remove_orphan_outputs(&mut self.items);
 
+        // interrupted turns can leave replay-unsafe trailing reasoning before the next user item
+        normalize::remove_trailing_reasoning_before_user_messages(&mut self.items);
+
         // strip images when model does not support them
         normalize::strip_images_when_unsupported(input_modalities, &mut self.items);
     }

codex-rs/core/src/context_manager/history_tests.rs

@@ -442,6 +442,59 @@ fn for_prompt_preserves_image_generation_calls_when_images_are_supported() {
     );
 }
 
+#[test]
+fn for_prompt_drops_trailing_reasoning_before_next_user_message() {
+    let history = create_history_with_items(vec![
+        user_input_text_msg("first"),
+        assistant_msg("done"),
+        reasoning_msg("orphaned"),
+        user_input_text_msg("second"),
+    ]);
+
+    assert_eq!(
+        history.for_prompt(&default_input_modalities()),
+        vec![
+            user_input_text_msg("first"),
+            assistant_msg("done"),
+            user_input_text_msg("second"),
+        ]
+    );
+}
+
+#[test]
+fn for_prompt_drops_trailing_reasoning_at_prompt_end() {
+    let history = create_history_with_items(vec![
+        user_input_text_msg("first"),
+        assistant_msg("done"),
+        reasoning_msg("orphaned"),
+    ]);
+
+    assert_eq!(
+        history.for_prompt(&default_input_modalities()),
+        vec![user_input_text_msg("first"), assistant_msg("done"),]
+    );
+}
+
+#[test]
+fn for_prompt_keeps_reasoning_when_followed_by_assistant_message() {
+    let history = create_history_with_items(vec![
+        user_input_text_msg("first"),
+        reasoning_msg("kept"),
+        assistant_msg("done"),
+        user_input_text_msg("second"),
+    ]);
+
+    assert_eq!(
+        history.for_prompt(&default_input_modalities()),
+        vec![
+            user_input_text_msg("first"),
+            reasoning_msg("kept"),
+            assistant_msg("done"),
+            user_input_text_msg("second"),
+        ]
+    );
+}
+
 #[test]
 fn for_prompt_clears_image_generation_result_when_images_are_unsupported() {
     let history = create_history_with_items(vec![

codex-rs/core/src/context_manager/normalize.rs

@@ -194,6 +194,33 @@ pub(crate) fn remove_orphan_outputs(items: &mut Vec<ResponseItem>) {
     });
 }
 
+pub(crate) fn remove_trailing_reasoning_before_user_messages(items: &mut Vec<ResponseItem>) {
+    let mut normalized = Vec::with_capacity(items.len());
+    let mut removed_count = 0usize;
+
+    for item in items.drain(..) {
+        if matches!(&item, ResponseItem::Message { role, .. } if role == "user") {
+            while matches!(normalized.last(), Some(ResponseItem::Reasoning { .. })) {
+                normalized.pop();
+                removed_count += 1;
+            }
+        }
+
+        normalized.push(item);
+    }
+
+    while matches!(normalized.last(), Some(ResponseItem::Reasoning { .. })) {
+        normalized.pop();
+        removed_count += 1;
+    }
+
+    if removed_count > 0 {
+        info!("Removed {removed_count} trailing reasoning item(s) before user input or prompt end");
+    }
+
+    *items = normalized;
+}
+
 pub(crate) fn remove_corresponding_for(items: &mut Vec<ResponseItem>, item: &ResponseItem) {
     match item {
         ResponseItem::FunctionCall { call_id, .. } => {