fix(agent): address review feedback on session state persistence

- Validate version field on load; bail on unsupported versions
- Refresh stale system prompt from history[0] on resume
- Use write-to-temp-then-rename for atomic saves
- Add tests for nonexistent file, malformed JSON, unsupported version,
  and stale system prompt refresh

src/agent/loop_.rs

@@ -259,14 +259,23 @@ fn load_interactive_session_history(path: &Path, system_prompt: &str) -> Result<
     }
 
     let raw = std::fs::read_to_string(path)?;
-    let mut state: InteractiveSessionState = serde_json::from_str(&raw)?;
-    if state.history.is_empty() {
-        state.history.push(ChatMessage::system(system_prompt));
-    } else if state.history.first().map(|msg| msg.role.as_str()) != Some("system") {
-        state.history.insert(0, ChatMessage::system(system_prompt));
+    let state: InteractiveSessionState = serde_json::from_str(&raw)?;
+
+    if state.version != 1 {
+        anyhow::bail!("unsupported session state version: {}", state.version);
     }
 
-    Ok(state.history)
+    let mut history = state.history;
+    if history.is_empty() {
+        history.push(ChatMessage::system(system_prompt));
+    } else if history.first().map(|msg| msg.role.as_str()) == Some("system") {
+        // Always refresh with the current system prompt so config changes take effect.
+        history[0] = ChatMessage::system(system_prompt);
+    } else {
+        history.insert(0, ChatMessage::system(system_prompt));
+    }
+
+    Ok(history)
 }
 
 fn save_interactive_session_history(path: &Path, history: &[ChatMessage]) -> Result<()> {
@@ -275,7 +284,11 @@ fn save_interactive_session_history(path: &Path, history: &[ChatMessage]) -> Res
     }
 
     let payload = serde_json::to_string_pretty(&InteractiveSessionState::from_history(history))?;
-    std::fs::write(path, payload)?;
+
+    // Write to a temporary file then rename for atomicity.
+    let tmp_path = path.with_extension("tmp");
+    std::fs::write(&tmp_path, payload)?;
+    std::fs::rename(&tmp_path, path)?;
     Ok(())
 }
 
@@ -3566,6 +3579,66 @@ mod tests {
         assert_eq!(restored[1].content, "orphan");
     }
 
+    #[test]
+    fn interactive_session_state_nonexistent_file_returns_fresh_history() {
+        let dir = tempdir().unwrap();
+        let path = dir.path().join("does_not_exist.json");
+
+        let history = load_interactive_session_history(&path, "fresh prompt").unwrap();
+
+        assert_eq!(history.len(), 1);
+        assert_eq!(history[0].role, "system");
+        assert_eq!(history[0].content, "fresh prompt");
+    }
+
+    #[test]
+    fn interactive_session_state_malformed_json_returns_error() {
+        let dir = tempdir().unwrap();
+        let path = dir.path().join("bad.json");
+        std::fs::write(&path, "NOT VALID JSON {{{").unwrap();
+
+        let result = load_interactive_session_history(&path, "prompt");
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn interactive_session_state_unsupported_version_returns_error() {
+        let dir = tempdir().unwrap();
+        let path = dir.path().join("v99.json");
+        let payload = serde_json::to_string_pretty(&InteractiveSessionState {
+            version: 99,
+            history: vec![ChatMessage::user("msg")],
+        })
+        .unwrap();
+        std::fs::write(&path, payload).unwrap();
+
+        let result = load_interactive_session_history(&path, "prompt");
+        assert!(result.is_err());
+        let err_msg = result.unwrap_err().to_string();
+        assert!(
+            err_msg.contains("unsupported session state version"),
+            "unexpected error: {err_msg}"
+        );
+    }
+
+    #[test]
+    fn interactive_session_state_refreshes_stale_system_prompt() {
+        let dir = tempdir().unwrap();
+        let path = dir.path().join("session.json");
+        let history = vec![
+            ChatMessage::system("old system prompt"),
+            ChatMessage::user("hello"),
+        ];
+
+        save_interactive_session_history(&path, &history).unwrap();
+        let restored = load_interactive_session_history(&path, "new system prompt").unwrap();
+
+        assert_eq!(restored.len(), 2);
+        assert_eq!(restored[0].role, "system");
+        assert_eq!(restored[0].content, "new system prompt");
+        assert_eq!(restored[1].content, "hello");
+    }
+
     use super::*;
     use async_trait::async_trait;
     use base64::{engine::general_purpose::STANDARD, Engine as _};