Hmbown/CodeWhale · error

Compaction coverage floor: last-round assistant output was…

Error message

Compaction coverage floor: last-round assistant output was dropped; history was not replaced.

What it means

The survival contract matches the assistant's actual output text from the last round. An existential 'some assistant message survived' check was insufficient — a replacement whose only assistant message was the summary itself passed it. This check requires each last-round assistant text to appear in the replacement; otherwise the rewrite is rejected.

Solutions

  1. Keep the last-round assistant output text verbatim in the replacement history.
  2. Fix the rewrite so the summary does not displace the actual last assistant answer.
  3. Ensure `assistant_text_of` covers all text blocks so validation sees the real output.
  4. Run `coverage_floor_rejects_assistant_output_replaced_by_a_summary` to reproduce.

Example fix

// before: summary replaced the assistant answer
replacement = [summary_msg];
// after
replacement = [summary_msg, original_assistant_msg];
Defensive patterns

Strategy: validation

Validate before calling

fn keeps_assistant_text(last_round: &[Message], replacement: &[Message]) -> bool {
    last_round.iter().filter_map(assistant_text_of)
        .all(|t| replacement.iter().any(|m| assistant_text_of(m) == Some(t)))
}

Try / catch

match compaction::apply_replacement(&replacement) {
    Err(e) if e.to_string().contains("assistant output was dropped") => keep_original_history(),
    other => other,
}

Prevention

When it happens

Trigger: `validate_last_round_coverage` finds a last-round assistant text block whose content is not present in any message of the replacement history (per `survives`/`assistant_text_of`).

Common situations: A rewrite replaces the last assistant answer with the summary; a naive compaction keeps 'an assistant message' but it is the summary, not the real answer; regression in rewrite code after refactor.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

AI-assisted analysis of Hmbown/CodeWhale@73e0f67d83 (2026-09-22). Data as JSON: /api/errors/d622097122dd2cc1. Report an issue: GitHub.

Appendix: source

Thrown at crates/tui/src/compaction/last_round.rs:403

    }
    // The call, not just its result. Keeping a tool_result whose tool_use was
    // summarized away leaves an orphaned result that providers reject outright.
    for id in last_round.iter().copied().flat_map(tool_use_ids) {
        if !replacement
            .iter()
            .any(|message| has_tool_use_id(message, &id))
        {
            anyhow::bail!(
                "Compaction coverage floor: last-round tool call {id} was dropped; history was not replaced."
            );
        }
    }
    // Match the assistant's actual output. An existential "some assistant
    // message survived" check passed on a replacement whose only assistant
    // message was the summary the rewrite had just written.
    for text in last_round.iter().copied().filter_map(assistant_text_of) {
        if !survives(&text, replacement, assistant_text_of) {
            anyhow::bail!(
                "Compaction coverage floor: last-round assistant output was dropped; history was not replaced."
            );
        }
    }
    if last_round
        .iter()
        .any(|message| message.role.is_assistant_like())
        && !replacement
            .iter()
            .any(|message| message.role.is_assistant_like())
    {
        anyhow::bail!(
            "Compaction coverage floor: last-round assistant output was dropped; history was not replaced."
        );
    }
    Ok(())
}

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