Duct

Duct Run Capture

Use this skill when a command, build, test, verify step, or long-running job should be measured rather than run blind. It wraps the command with con/duct, a lightweight monitor that records resource usage for a process and all of its children.

The point is agent review: after a run, another agent (or a human reviewer) can read the captured logs and see how long the command took, how much memory and CPU it used, and whether it succeeded, without re-running anything.

When To Use

• Before claiming a change is "fast enough", "leaner", or "within budget": measure it.
• When a verify or benchmark command feeds a keep/discard decision in an experiment loop.
• When a long-running job is left unattended and a reviewer later needs to know what it consumed.
• When comparing two alternatives by a resource metric (time, peak memory, CPU).

If a run does not need to be reviewed for resource usage, do not add the wrapper; plain execution is fine.

Install

pip install con-duct          # provides `duct` and `con-duct`
pip install 'con-duct[all]'   # adds plotting, pretty-print, and .env support

duct is equivalent to con-duct run .

Basic Usage

Wrap the real command after duct and its options:

duct -p .duct/logs/{datetime}-{pid}_ -- pytest -q

• Everything after -- is the command being measured, run unchanged.
• -p/--output-prefix controls where logs go. The default is .duct/logs/{datetime}-{pid}_; {datetime} and {pid} are filled in so parallel runs never collide.
• duct exits with the wrapped command's own exit code and prints a one-line summary (exit code, wall-clock time, peak/average RSS, peak/average CPU).

Useful options:

• --sample-interval / --s-i: seconds between resource polls (default 1.0; lower for short runs).
• --report-interval / --r-i: seconds between rows written to the usage log.
• -c {all,none,stdout,stderr}: which streams to capture to files.
• -o {all,none,stdout,stderr}: which streams to also forward to your terminal.
• --message / -m: a free-text note stored with the run (e.g. the hypothesis or experiment id).
• --fail-time: delete logs if the command fails within N seconds (skip noise from instant failures).
• --clobber: overwrite existing log files at the prefix.

Many options also read from environment variables (DUCT_OUTPUT_PREFIX, DUCT_SAMPLE_INTERVAL, DUCT_REPORT_INTERVAL, DUCT_CAPTURE_OUTPUTS, DUCT_MESSAGE, ...) and from .env files under /etc/duct/, ${XDG_CONFIG_HOME:-~/.config}/duct/, and .duct/.

Output Files

For an output prefix P, duct writes:

• Pinfo.json: system info plus an execution_summary (exit code, wall-clock time, peak/average RSS and VSZ, peak/average CPU and memory percent, sample/report counts, start/end time, working directory) and the output_paths.
• Pusage.jsonl: JSON Lines, one resource sample row per report interval (per-process and aggregate stats over time).
• Pstdout and Pstderr: captured output streams.

info.json is the file a reviewer should read first; it is small and holds the headline numbers.

Reviewing A Run

Pretty-print, list, and plot helpers ship with con-duct[all]:

con-duct ls   .duct/logs/*info.json     # tabular index of past runs
con-duct pp   .duct/logs/info.json   # human-readable dump of one run
con-duct plot .duct/logs/usage.jsonl # resource-usage-over-time chart

For a compact, dependency-free review summary suitable for pasting into a notebook entry or a PR review, use the helper in this skill:

python skills/duct/scripts/summarize_run.py .duct/logs/info.json
python skills/duct/scripts/summarize_run.py .duct/logs/          # newest run under a dir
python skills/duct/scripts/summarize_run.py .duct/logs/info.json --json

It reads info.json, prints exit code, wall-clock time, peak/average memory, and peak/average CPU in readable units, and flags whether the run produced any samples (very short runs may record none).

Guardrails

1. Respect any parent constitution, project policy, or task-level write constraint already in scope.
2. Treat the captured numbers as best-effort: duct polls at an interval, so very short runs may record zero samples and report null for peak memory or CPU. Lower --sample-interval or report that the run was too short to measure.
3. duct tracks a process session; a command that starts a brand-new session escapes tracking, and duct stops collecting once the primary process exits even if children remain. Note this when reviewing background jobs.
4. Write logs to a per-run prefix (keep {datetime} and {pid}) so parallel runs never overwrite each other.
5. Do not capture secrets: streams written to stdout/stderr files may contain credentials or tokens; use -c none or scrub before sharing when output is sensitive.
6. Report the wrapped command's real exit code; do not let the wrapper mask a failure as success.

With labnb

When running an experiment under the [labnb](../labnb/SKILL.md) skill, wrap the verify or benchmark command with duct and store the logs inside the experiment's artifacts/ directory:

duct -p "$EXPERIMENT_DIR/artifacts/verify-{datetime}-{pid}_" -- $VERIFY_COMMAND

This satisfies the labnb requirement to instrument runs well enough to inspect logs, progress, and resource consumption from outside the process, and it gives a later resume or review step real numbers to compare against the recorded metric.

The captured info.json can also be fed straight into labnb's loop breaker so a slice is cut short on runaway memory:

python skills/labnb/scripts/monitor_slice.py check \
  --experiment-dir "$EXPERIMENT_DIR" \
  --usage-file "$EXPERIMENT_DIR/artifacts/verify-_info.json" \
  --max-rss-bytes 8000000000 --max-pmem 90

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: sensein
• Source: sensein/agent_skills
• License: Apache-2.0

Install and usage instructions live in the source repository linked above.