Child agent results carry a machine-built receipt (diff, out-of-scope files, commands+exit codes, test counts, time, spend)

[Hmbown]

Gap. A child's final report is prose written by the child itself. Parents, human or model, have to re-verify every claim with git and CI, and files the child changed without declaring them go unnoticed.

Change. When a child finishes, whether completed, cancelled, out of budget or failed, the harness attaches a receipt built from its own records, never from the child's text:

• Files: git diff --stat against the child's start commit (worktree or shared tree), plus a list of files changed outside the declared write scope, flagged.
• Commits: the ones the child created.
• Commands: every shell or command run, with exit code and duration. Truncate to the last N, keeping failures.
• Tests: counts parsed from known runner outputs (cargo test, nextest, node --test, npm test, pytest). Each count is marked with the command it came from. Never inferred.
• Cost: wall time, steps, tokens, and spend where priced.

Where it goes

• The model sees it in the subagent_completion runtime event and in agent status/wait.
• The person sees it in the agent card.
• It is written to the child's record.

Size limits. Keep it compact. The model can page the full list with retrieve_tool_result.

Tests

• receipt_lists_out_of_scope_file_changes
• receipt_commands_include_exit_codes
• receipt_test_counts_parsed_from_cargo_output
• receipt_built_from_harness_records_not_child_text
• receipt_present_on_cancel_and_budget_end

Size: M/L.