fork conversation from a previous message (#2575)

This can be the underlying logic in order to start a conversation from a
previous message. will need some love in the UI.

Base for building this: #2588

codex-rs/core/src/codex.rs

@@ -14,6 +14,7 @@ use codex_apply_patch::ApplyPatchAction;
 use codex_apply_patch::MaybeApplyPatchVerified;
 use codex_apply_patch::maybe_parse_apply_patch_verified;
 use codex_login::AuthManager;
+use codex_protocol::protocol::ConversationHistoryResponseEvent;
 use codex_protocol::protocol::TurnAbortReason;
 use codex_protocol::protocol::TurnAbortedEvent;
 use futures::prelude::*;
@@ -147,6 +148,7 @@ impl Codex {
     pub async fn spawn(
         config: Config,
         auth_manager: Arc<AuthManager>,
+        initial_history: Option<Vec<ResponseItem>>,
     ) -> CodexResult<CodexSpawnOk> {
         let (tx_sub, rx_sub) = async_channel::bounded(64);
         let (tx_event, rx_event) = async_channel::unbounded();
@@ -177,6 +179,7 @@ impl Codex {
             config.clone(),
             auth_manager.clone(),
             tx_event.clone(),
+            initial_history,
         )
         .await
         .map_err(|e| {
@@ -186,7 +189,12 @@ impl Codex {
         let session_id = session.session_id;
 
         // This task will run until Op::Shutdown is received.
-        tokio::spawn(submission_loop(session, turn_context, config, rx_sub));
+        tokio::spawn(submission_loop(
+            session.clone(),
+            turn_context,
+            config,
+            rx_sub,
+        ));
         let codex = Codex {
             next_id: AtomicU64::new(0),
             tx_sub,
@@ -332,6 +340,7 @@ impl Session {
         config: Arc<Config>,
         auth_manager: Arc<AuthManager>,
         tx_event: Sender<Event>,
+        initial_history: Option<Vec<ResponseItem>>,
     ) -> anyhow::Result<(Arc<Self>, TurnContext)> {
         let ConfigureSession {
             provider,
@@ -391,14 +400,15 @@ impl Session {
         }
         let rollout_result = match rollout_res {
             Ok((session_id, maybe_saved, recorder)) => {
-                let restored_items: Option<Vec<ResponseItem>> =
+                let restored_items: Option<Vec<ResponseItem>> = initial_history.or_else(|| {
                     maybe_saved.and_then(|saved_session| {
                         if saved_session.items.is_empty() {
                             None
                         } else {
                             Some(saved_session.items)
                         }
-                    });
+                    })
+                });
                 RolloutResult {
                     session_id,
                     rollout_recorder: Some(recorder),
@@ -1286,6 +1296,21 @@ async fn submission_loop(
                 }
                 break;
             }
+            Op::GetHistory => {
+                let tx_event = sess.tx_event.clone();
+                let sub_id = sub.id.clone();
+
+                let event = Event {
+                    id: sub_id.clone(),
+                    msg: EventMsg::ConversationHistory(ConversationHistoryResponseEvent {
+                        conversation_id: sess.session_id,
+                        entries: sess.state.lock_unchecked().history.contents(),
+                    }),
+                };
+                if let Err(e) = tx_event.send(event).await {
+                    warn!("failed to send ConversationHistory event: {e}");
+                }
+            }
             _ => {
                 // Ignore unknown ops; enum is non_exhaustive to allow extensions.
             }

codex-rs/core/src/conversation_manager.rs

@@ -16,6 +16,7 @@ use crate::error::Result as CodexResult;
 use crate::protocol::Event;
 use crate::protocol::EventMsg;
 use crate::protocol::SessionConfiguredEvent;
+use codex_protocol::models::ResponseItem;
 
 /// Represents a newly created Codex conversation, including the first event
 /// (which is [`EventMsg::SessionConfigured`]).
@@ -59,8 +60,18 @@ impl ConversationManager {
         let CodexSpawnOk {
             codex,
             session_id: conversation_id,
-        } = Codex::spawn(config, auth_manager).await?;
+        } = {
+            let initial_history = None;
+            Codex::spawn(config, auth_manager, initial_history).await?
+        };
+        self.finalize_spawn(codex, conversation_id).await
+    }
 
+    async fn finalize_spawn(
+        &self,
+        codex: Codex,
+        conversation_id: Uuid,
+    ) -> CodexResult<NewConversation> {
         // The first event must be `SessionInitialized`. Validate and forward it
         // to the caller so that they can display it in the conversation
         // history.
@@ -98,4 +109,120 @@ impl ConversationManager {
             .cloned()
             .ok_or_else(|| CodexErr::ConversationNotFound(conversation_id))
     }
+
+    /// Fork an existing conversation by dropping the last `drop_last_messages`
+    /// user/assistant messages from its transcript and starting a new
+    /// conversation with identical configuration (unless overridden by the
+    /// caller's `config`). The new conversation will have a fresh id.
+    pub async fn fork_conversation(
+        &self,
+        conversation_history: Vec<ResponseItem>,
+        num_messages_to_drop: usize,
+        config: Config,
+    ) -> CodexResult<NewConversation> {
+        // Compute the prefix up to the cut point.
+        let truncated_history =
+            truncate_after_dropping_last_messages(conversation_history, num_messages_to_drop);
+
+        // Spawn a new conversation with the computed initial history.
+        let auth_manager = self.auth_manager.clone();
+        let CodexSpawnOk {
+            codex,
+            session_id: conversation_id,
+        } = Codex::spawn(config, auth_manager, Some(truncated_history)).await?;
+
+        self.finalize_spawn(codex, conversation_id).await
+    }
+}
+
+/// Return a prefix of `items` obtained by dropping the last `n` user messages
+/// and all items that follow them.
+fn truncate_after_dropping_last_messages(items: Vec<ResponseItem>, n: usize) -> Vec<ResponseItem> {
+    if n == 0 || items.is_empty() {
+        return items;
+    }
+
+    // Walk backwards counting only `user` Message items, find cut index.
+    let mut count = 0usize;
+    let mut cut_index = 0usize;
+    for (idx, item) in items.iter().enumerate().rev() {
+        if let ResponseItem::Message { role, .. } = item
+            && role == "user"
+        {
+            count += 1;
+            if count == n {
+                // Cut everything from this user message to the end.
+                cut_index = idx;
+                break;
+            }
+        }
+    }
+    if count < n {
+        // If fewer than n messages exist, drop everything.
+        Vec::new()
+    } else {
+        items.into_iter().take(cut_index).collect()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use codex_protocol::models::ContentItem;
+    use codex_protocol::models::ReasoningItemReasoningSummary;
+    use codex_protocol::models::ResponseItem;
+
+    fn user_msg(text: &str) -> ResponseItem {
+        ResponseItem::Message {
+            id: None,
+            role: "user".to_string(),
+            content: vec![ContentItem::OutputText {
+                text: text.to_string(),
+            }],
+        }
+    }
+    fn assistant_msg(text: &str) -> ResponseItem {
+        ResponseItem::Message {
+            id: None,
+            role: "assistant".to_string(),
+            content: vec![ContentItem::OutputText {
+                text: text.to_string(),
+            }],
+        }
+    }
+
+    #[test]
+    fn drops_from_last_user_only() {
+        let items = vec![
+            user_msg("u1"),
+            assistant_msg("a1"),
+            assistant_msg("a2"),
+            user_msg("u2"),
+            assistant_msg("a3"),
+            ResponseItem::Reasoning {
+                id: "r1".to_string(),
+                summary: vec![ReasoningItemReasoningSummary::SummaryText {
+                    text: "s".to_string(),
+                }],
+                content: None,
+                encrypted_content: None,
+            },
+            ResponseItem::FunctionCall {
+                id: None,
+                name: "tool".to_string(),
+                arguments: "{}".to_string(),
+                call_id: "c1".to_string(),
+            },
+            assistant_msg("a4"),
+        ];
+
+        let truncated = truncate_after_dropping_last_messages(items.clone(), 1);
+        assert_eq!(
+            truncated,
+            vec![items[0].clone(), items[1].clone(), items[2].clone()]
+        );
+
+        let truncated2 = truncate_after_dropping_last_messages(items, 2);
+        assert!(truncated2.is_empty());
+    }
 }

codex-rs/exec/src/event_processor_with_human_output.rs

@@ -540,6 +540,7 @@ impl EventProcessor for EventProcessorWithHumanOutput {
                 }
             },
             EventMsg::ShutdownComplete => return CodexStatus::Shutdown,
+            EventMsg::ConversationHistory(_) => {}
         }
         CodexStatus::Running
     }

codex-rs/mcp-server/src/codex_tool_runner.rs

@@ -275,6 +275,7 @@ async fn run_codex_tool_session_inner(
                     | EventMsg::GetHistoryEntryResponse(_)
                     | EventMsg::PlanUpdate(_)
                     | EventMsg::TurnAborted(_)
+                    | EventMsg::ConversationHistory(_)
                     | EventMsg::ShutdownComplete => {
                         // For now, we do not do anything extra for these
                         // events. Note that

codex-rs/protocol/src/protocol.rs

@@ -22,6 +22,7 @@ use uuid::Uuid;
 use crate::config_types::ReasoningEffort as ReasoningEffortConfig;
 use crate::config_types::ReasoningSummary as ReasoningSummaryConfig;
 use crate::message_history::HistoryEntry;
+use crate::models::ResponseItem;
 use crate::parse_command::ParsedCommand;
 use crate::plan_tool::UpdatePlanArgs;
 
@@ -137,6 +138,10 @@ pub enum Op {
     /// Request a single history entry identified by `log_id` + `offset`.
     GetHistoryEntryRequest { offset: usize, log_id: u64 },
 
+    /// Request the full in-memory conversation transcript for the current session.
+    /// Reply is delivered via `EventMsg::ConversationHistory`.
+    GetHistory,
+
     /// Request the list of MCP tools available across all configured servers.
     /// Reply is delivered via `EventMsg::McpListToolsResponse`.
     ListMcpTools,
@@ -471,6 +476,8 @@ pub enum EventMsg {
 
     /// Notification that the agent is shutting down.
     ShutdownComplete,
+
+    ConversationHistory(ConversationHistoryResponseEvent),
 }
 
 // Individual event payload types matching each `EventMsg` variant.
@@ -651,6 +658,14 @@ impl McpToolCallEndEvent {
     }
 }
 
+/// Response payload for `Op::GetHistory` containing the current session's
+/// in-memory transcript.
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct ConversationHistoryResponseEvent {
+    pub conversation_id: Uuid,
+    pub entries: Vec<ResponseItem>,
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize)]
 pub struct ExecCommandBeginEvent {
     /// Identifier so this can be paired with the ExecCommandEnd event.

codex-rs/tui/src/chatwidget.rs

@@ -839,6 +839,7 @@ impl ChatWidget {
                 self.on_background_event(message)
             }
             EventMsg::StreamError(StreamErrorEvent { message }) => self.on_stream_error(message),
+            EventMsg::ConversationHistory(_) => {}
         }
         // Coalesce redraws: issue at most one after handling the event
         if self.needs_redraw {