Flatten prior compaction highlights directly into the top-level list (prefixed with -), while keeping the current round’s highlights under - Newly compacted context:

[joshbowyer]

if !previous_highlights.is_empty() {
lines.extend(previous_highlights.into_iter().map(|line| format!("- {line}")));
}

This makes compaction summary size O(depth) rather than O(depth²) and nesting depth stays at 1 regardless of how many compactions run.Testing

• Compaction runs cleanly in a live session
• Session file nesting depth confirmed at 1 after multiple compaction cycles
• No regression in compact_session unit testsCloses #3050

Bug: Compaction summary nesting causes context window inflation

Issue

See #3050

When Claw Code performs context compaction, the build_compact_summary function in rust/crates/runtime/src/compact.rs prepends the prior compaction summary under a - Previously compacted context: header:

if !previous_highlights.is_empty() {
    lines.push("- Previously compacted context:".to_string());
    lines.extend(
        previous_highlights
            .into_iter()
            .map(|line| format!("  {line}")),
    );
}

The problem: those previous_highlights were themselves generated by a prior call to build_compact_summary, which already contained a - Previously compacted context: entry from the round before that. Each compaction re-wraps the entire prior summary under a new nesting layer, so the summary grows as:

• O(depth²) in raw content — each level adds all prior levels plus overhead
• O(depth) in nesting depth — after N compactions the <summary> tag is N levels deep

After 20–30 compaction cycles in a long session, the session file contained summaries that were 30+ levels deep and 500KB+, causing error 2013 context_window_blocked crashes.

Root Cause

previous_highlights already contains prior-compaction highlights that should not be re-nested — they were already incorporated into the flat summary structure. Re-wrapping them under a new - Previously compacted context: heading each cycle is the bug.

Fix

Flatten prior compaction highlights directly into the top-level list (prefixed with -), while keeping the current round's highlights under - Newly compacted context::

if !previous_highlights.is_empty() {
    lines.extend(previous_highlights.into_iter().map(|line| format!("- {line}")));
}

This makes compaction summary size O(depth) rather than O(depth²) and nesting depth stays at 1 regardless of how many compactions run.

Testing

• Compaction runs cleanly in a live session
• Session file nesting depth confirmed at 1 after multiple compaction cycles
• No regression in compact_session unit tests

Closes #26

Disclaimer: This was written with the assistance of an LLM (Claw-Code/MiniMax-M2.7)