Render whole-file patches from change metadata

server/src/session/codex/translate.rs

@@ -9,7 +9,7 @@ use std::sync::Arc;
 
 use serde_json::{Value, json};
 
-use super::super::driver::{Event, SessionStatus, patch_start};
+use super::super::driver::{Event, SessionStatus, patch_start, prefixed_lines};
 use super::super::subagent::Subagents;
 
 #[derive(Default)]
@@ -741,12 +741,17 @@ fn file_change_diff(item: &Value) -> String {
         Some(Value::Object(changes)) => changes
             .iter()
             .map(|(path, change)| {
+                let kind = change.get("type").and_then(Value::as_str);
                 render_file_diff(
                     path,
-                    change.get("type").and_then(Value::as_str),
+                    kind,
                     change.get("move_path").and_then(Value::as_str),
                     change
-                        .get("unified_diff")
+                        .get(if matches!(kind, Some("add" | "delete")) {
+                            "content"
+                        } else {
+                            "unified_diff"
+                        })
                         .and_then(Value::as_str)
                         .unwrap_or_default(),
                 )
@@ -759,7 +764,9 @@ fn file_change_diff(item: &Value) -> String {
 
 fn render_file_diff(path: &str, kind: Option<&str>, moved: Option<&str>, body: &str) -> String {
     // Some protocol revisions carry complete unified diffs and some carry only their hunks.
-    if body.starts_with("--- ") || body.starts_with("diff --git ") {
+    if !matches!(kind, Some("add" | "delete"))
+        && (body.starts_with("--- ") || body.starts_with("diff --git "))
+    {
         return body.to_string();
     }
     let from = if kind == Some("add") {
@@ -772,6 +779,14 @@ fn render_file_diff(path: &str, kind: Option<&str>, moved: Option<&str>, body: &
     } else {
         moved.unwrap_or(path)
     };
+    let body = match kind {
+        // Add and delete carry file content, not a unified diff. Prefixing it here makes the
+        // protocol's change kind authoritative: a '-' that belongs to a Markdown bullet can never
+        // be mistaken for diff syntax.
+        Some("add") => prefixed_lines('+', body),
+        Some("delete") => prefixed_lines('-', body),
+        _ => body.to_string(),
+    };
     format!("--- {from}\n+++ {to}\n{body}")
 }
 
@@ -1069,6 +1084,28 @@ mod tests {
         );
     }
 
+    #[test]
+    fn whole_file_changes_get_markers_from_their_kind_not_their_content() {
+        let item = line(
+            r#"{"changes":[{"path":"added.md","kind":{"type":"add"},"diff":"--- a rule\n- a bullet\nplain\n"},{"path":"deleted.md","kind":{"type":"delete"},"diff":"- another bullet\nplain\n"}]}"#,
+        );
+        assert_eq!(
+            file_change_diff(&item),
+            "--- /dev/null\n+++ added.md\n+--- a rule\n+- a bullet\n+plain\n\n--- deleted.md\n+++ /dev/null\n-- another bullet\n-plain\n"
+        );
+    }
+
+    #[test]
+    fn legacy_whole_file_changes_show_the_content_they_changed() {
+        let item = line(
+            r#"{"changes":{"added.md":{"type":"add","content":"new\n"},"deleted.md":{"type":"delete","content":"old\n"}}}"#,
+        );
+        assert_eq!(
+            file_change_diff(&item),
+            "--- /dev/null\n+++ added.md\n+new\n\n--- deleted.md\n+++ /dev/null\n-old\n"
+        );
+    }
+
     #[test]
     fn native_app_server_completion_corrects_provisional_streaming() {
         let mut translator = Translator::default();