Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx agentmods add skills/alchemiststudiosdotai/harness-engineering/differential-session-runnernpx skills add alchemiststudiosDOTai/harness-engineering --skill differential-session-runnergit clone --depth 1 https://github.com/alchemiststudiosDOTai/harness-engineeringWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/alchemiststudiosdotai/harness-engineering/differential-session-runner)<a href="https://agentmods.dev/skills/alchemiststudiosdotai/harness-engineering/differential-session-runner"><img src="https://agentmods.dev/badge/skills/alchemiststudiosdotai/harness-engineering/differential-session-runner.svg" alt="Measured on agentmods" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5 | $0.00049 | $0.01447 |
| Opus 5 | $0.00024 | $0.00724 |
| Sonnet 5 | $0.00010 | $0.00289 |
| Haiku 4.5 | $0.00005 | $0.00145 |
Grade A, and why
differential-session-runner scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 5d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 248 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Differential Session Runner
Use this skill when debugging requires a durable evidence trail rather than ad hoc notes.
This skill is for workflows where you are comparing:
- original vs rewrite
- implementation A vs implementation B
- baseline trace vs candidate trace
- before vs after replay output
- captured artifact vs regenerated artifact
The goal is not only to investigate. The goal is to leave behind a session artifact another operator or agent can continue.
When to Use
Use this skill when the user asks to:
- continue a differential debugging session
- compare two implementations and record mismatches
- create or update a debugging session log
- track replay / trace / capture divergence over time
- document what changed between mismatched and cleared runs
Core Principle
Every differential investigation should produce a reusable evidence packet.
A good session artifact lets another operator answer:
- what exact artifact was investigated?
- how was it identified?
- what commands were run?
- where did the first mismatch appear?
- what was learned?
- what changed?
- what validation proves the current state?
- what should happen next?
Preferred Storage
If the repo already has a native evidence location, use it.
Examples:
docs/.../sessions/docs/chunks/analysis/.../reports/- existing session indexes
If the repo does not already have a native convention, write to:
memory-bank/evidence/YYYY-MM-DD_HH-MM-SS_<topic>-session.md- optional index:
memory-bank/evidence/index.md
Workflow
1. Load the repo's evidence convention first
Before creating anything, search for:
- playbooks / runbooks
- session indexes
- prior session files
- report directories
- chunk/evidence templates
Read the relevant guidance and continue the repo's existing pattern.
2. Identify the artifact under investigation
Capture the strongest available identity for the artifact:
- artifact path
- SHA256 / content hash
- commit SHA
- case name / run id / replay id
- timestamp if needed
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 5d ago First seen · 248 lines · 49 tokens per session scan A 5f25dd647ab9
differential-session-runner is a skill published in the GitHub repository alchemiststudiosDOTai/harness-engineering (104 stars, last pushed 5mo ago), licensed MIT. It adds 49 tokens to every session and 1,447 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
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