Background-task completions delivered via sessions_send arrive as bare "Background task done" with no payload. The terminalSummary field is null for non-error completions, so callers see only the boilerplate label and have to go look up the task to read its output.

TL;DR

Update the resolveBackgroundTaskTerminalResult function to return progressSummary as terminalSummary when no other summary is available and consider increasing ACP_BACKGROUND_TASK_PROGRESS_MAX_LENGTH to improve summary usefulness.

Guidance

• Review the resolveBackgroundTaskTerminalResult function to ensure it correctly handles non-blocked completions by returning progressSummary as terminalSummary when terminalSummary is not set.
• Verify that increasing ACP_BACKGROUND_TASK_PROGRESS_MAX_LENGTH to a higher value (e.g., 1200) improves the usefulness of task summaries without causing other issues.
• Test the updated function with various task completion scenarios to ensure that summaries are correctly populated and not truncated excessively.
• Consider applying the suggested fix to all affected versions (2026.4.26, 4.21, 4.15) to ensure consistency across different environments.

Example

function resolveBackgroundTaskTerminalResult(...) {
    ...
    const cleaned = normalized.replace(/\s*ANNOUNCE_SKIP\s*$/i, "").trim();
    if (!cleaned) {
        // Fall back to progressSummary when terminalSummary is not available
        return { terminalSummary: progressSummary };
    }
    return { terminalSummary: cleaned };
}

Notes

The suggested fix assumes that progressSummary is a suitable fallback for terminalSummary when the latter is not available. However, this might not always be the case, and additional logic might be needed to handle different task completion scenarios.

Recommendation

Apply the workaround by updating the resolveBackgroundTaskTerminalResult function and increasing ACP_BACKGROUND_TASK_PROGRESS_MAX_LENGTH to improve task summary usefulness, as this approach directly addresses the identified issues and has been patched locally with positive results.