codex/codex-rs/core/src/conversation_history.rs

use crate::models::ResponseItem;

/// Transcript of conversation history
#[derive(Debug, Clone, Default)]
pub(crate) struct ConversationHistory {
    /// The oldest items are at the beginning of the vector.
    items: Vec<ResponseItem>,
}

impl ConversationHistory {
    pub(crate) fn new() -> Self {
        Self { items: Vec::new() }
    }

    /// Returns a clone of the contents in the transcript.
    pub(crate) fn contents(&self) -> Vec<ResponseItem> {
        self.items.clone()
    }

    /// `items` is ordered from oldest to newest.
    pub(crate) fn record_items<I>(&mut self, items: I)
    where
        I: IntoIterator,
        I::Item: std::ops::Deref<Target = ResponseItem>,
    {
        for item in items {
            if is_api_message(&item) {
                // Note agent-loop.ts also does filtering on some of the fields.
                self.items.push(item.clone());
            }
        }
    }

    pub(crate) fn keep_last_messages(&mut self, n: usize) {
        if n == 0 {
            self.items.clear();
            return;
        }

        // Collect the last N message items (assistant/user), newest to oldest.
        let mut kept: Vec<ResponseItem> = Vec::with_capacity(n);
        for item in self.items.iter().rev() {
            if let ResponseItem::Message { role, content, .. } = item {
                kept.push(ResponseItem::Message {
                    // we need to remove the id or the model will complain that messages are sent without
                    // their reasonings
                    id: None,
                    role: role.clone(),
                    content: content.clone(),
                });
                if kept.len() == n {
                    break;
                }
            }
        }

        // Preserve chronological order (oldest to newest) within the kept slice.
        kept.reverse();
        self.items = kept;
    }
}

/// Anything that is not a system message or "reasoning" message is considered
/// an API message.
fn is_api_message(message: &ResponseItem) -> bool {
    match message {
        ResponseItem::Message { role, .. } => role.as_str() != "system",
        ResponseItem::FunctionCallOutput { .. }
        | ResponseItem::FunctionCall { .. }
        | ResponseItem::LocalShellCall { .. }
        | ResponseItem::Reasoning { .. } => true,
        ResponseItem::Other => false,
    }
}