Function prune_tool_outputs

core/src/compaction.rs:157–224

Prune large tool outputs from messages to reclaim context space. Iterates backward from recent messages, protecting the first `TOOL_OUTPUT_PROTECT_TOKENS` worth of tool output tokens. Only prunes outputs that would save at least `MIN_PRUNE_SAVINGS_TOKENS`. Returns `Some(pruned_messages)` if any outputs were pruned, or `None` if no pruning was needed.

(messages: &[Message])

/// Returns `Some(pruned_messages)` if any outputs were pruned, or `None`
/// if no pruning was needed.
pub(crate) fn prune_tool_outputs(messages: &[Message]) -> Option<Vec<Message>> {
    // First pass: estimate total tool output tokens (backward)
    let mut tool_outputs: Vec<(usize, usize, usize)> = Vec::new(); // (msg_idx, block_idx, token_count)

    for (msg_idx, msg) in messages.iter().enumerate() {
        for (block_idx, block) in msg.content.iter().enumerate() {
            if let ContentBlock::ToolResult { content, .. } = block {
                let text = content.as_text();
                let token_count = estimate_tokens(&text);
                if token_count > 0 {
                    tool_outputs.push((msg_idx, block_idx, token_count));
                }
            }
        }
    }

    if tool_outputs.is_empty() {
        return None;
    }

    // Calculate total tool output tokens
    let total_tool_tokens: usize = tool_outputs.iter().map(|(_, _, t)| *t).sum();

    // If total is small, no pruning needed
    if total_tool_tokens <= TOOL_OUTPUT_PROTECT_TOKENS {
        return None;
    }

    // Iterate from oldest to newest, protecting the most recent outputs
    // We prune old outputs first, keeping recent ones intact
    let mut protected_tokens = 0usize;
    let mut to_prune: Vec<(usize, usize)> = Vec::new();
    let mut savings = 0usize;

    // Walk backward (newest first) to protect recent outputs
    for &(msg_idx, block_idx, token_count) in tool_outputs.iter().rev() {
        if protected_tokens < TOOL_OUTPUT_PROTECT_TOKENS {
            protected_tokens += token_count;
        } else {
            to_prune.push((msg_idx, block_idx));
            savings += token_count;
        }
    }

    // Only prune if savings are significant
    if savings < MIN_PRUNE_SAVINGS_TOKENS {
        return None;
    }

    // Apply pruning
    let mut pruned = messages.to_vec();
    for (msg_idx, block_idx) in &to_prune {
        if let Some(msg) = pruned.get_mut(*msg_idx) {
            if let Some(ContentBlock::ToolResult { content, .. }) = msg.content.get_mut(*block_idx)
            {
                *content = ToolResultContentField::Text(PRUNED_MARKER.to_string());
            }
        }