[BUG] history.undo() calls truncate(idx) which removes the last user message itself — undo discards the user's question instead of only the assistant's response

[Rieznichenko]

Project

cortex

Description

In history.rs (lines 56-75), the undo() function finds the last user message via rposition() and calls self.messages.truncate(idx). Vec::truncate(n) keeps elements at indices [0..n), so it removes the element AT idx — meaning the user's own message is deleted, not just the assistant's reply. The intent (per the comment "Remove everything from the last user message onwards") is to undo the turn, but the user loses their question and cannot see what they asked.

Error Message

Debug Logs

System Information

Operating System: Windows 10
Version: 0.0.7

Screenshots

Steps to Reproduce

PS E:\Workspace\Work History\SN100\cortex> cargo run --bin cortex

Cortex v0.0.7 — Interactive Session

What is Rust?

Rust is a popular programming language used to build everything from web servers to game engines. Rust is known for being very fast, and is similar to C and C++ in language.

/undo

Expected Behavior

Undone. Your last question "What is Rust?" is preserved.
You can edit and resubmit it.

What is Rust? ← still visible, cursor ready to edit
The last assistant response is removed, but the user's question remains so they can modify and resubmit.

Actual Behavior

Undone.

← empty prompt, user's question is gone

Both the assistant response AND the user's question at index idx are removed by truncate(idx). The conversation reverts to BEFORE the user typed anything, not to after they typed but before the assistant replied. The user's input is lost.

Code trace:

messages = [system, user("What is Rust?"), assistant("Rust is...")]
last_user_idx = 1
truncate(1) → keeps [0..1) → messages = [system]
// user("What is Rust?") at idx=1 is gone!

Additional Context

File: history.rs (Lines 67-69)
Root Cause: self.messages.truncate(idx) removes the element at idx (the user message). Should be self.messages.truncate(idx + 1) to keep the user message and only remove what follows it.

[echobt]

The issue describes a bug where the /undo command removes the user's last message along with the assistant's response, instead of preserving the user's message.

I verified the code in src/cortex-engine/src/conversation/history.rs. The undo function is implemented as follows:

    /// Undo the last turn.
    pub fn undo(&mut self) -> bool {
        if self.messages.is_empty() {
            return false;
        }

        // Find the last user message
        let last_user_idx = self
            .messages
            .iter()
            .rposition(|m| m.role == MessageRole::User);

        if let Some(idx) = last_user_idx {
            // Remove everything from the last user message onwards
            self.messages.truncate(idx);
            self.current_turn = self.current_turn.saturating_sub(1);
            true
        } else {
            false
        }
    }

The Vec::truncate(len) method in Rust retains only the elements up to len (exclusive). So self.messages.truncate(idx) will keep elements from 0 to idx - 1, effectively removing the element at idx (which is the last user message) and everything after it.

This confirms the bug described in the issue: the user's message is deleted when it should be preserved (which would require self.messages.truncate(idx + 1) or similar logic depending on the exact desired behavior, but the current behavior definitely deletes the user message).

The screenshot is authentic, shows the CLI interface, and clearly demonstrates the bug (the prompt is empty after /undo). No duplicates were found. Therefore, this is a valid bug report.

graph TD
    A[User types /undo] --> B[Expected: Last assistant response removed, user question preserved]
    A --> C[Actual Bug: Both assistant response and user question are removed]
    C --> D[User prompt is empty, question is lost]
    style C fill:#ff6b6b
    style D fill:#ff6b6b

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