firstmate is an agent distribution that lets one coding agent coordinate a crew of autonomous agents working in separate git worktrees. It is for running parallel fixes, investigations, plans, and audits while receiving completed pull requests, merges, or reports through one supervising agent. The catalogue entries provide skills, hooks, instructions, and settings for operating this workflow.
Borrowing it
Nothing to install: this file belongs to kunchenguid/firstmate. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.
curl -O https://raw.githubusercontent.com/kunchenguid/firstmate/main/.agents/skills/diagnostic-reasoning/SKILL.mdgit clone --depth 1 https://github.com/kunchenguid/firstmateWrote 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/kunchenguid/firstmate/diagnostic-reasoning)<a href="https://agentmods.dev/skills/kunchenguid/firstmate/diagnostic-reasoning"><img src="https://agentmods.dev/badge/skills/kunchenguid/firstmate/diagnostic-reasoning/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/kunchenguid/firstmate/diagnostic-reasoning"><img src="https://agentmods.dev/badge/skills/kunchenguid/firstmate/diagnostic-reasoning.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- NVIDIA SkillSpector pass
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 5.1 | $0.00021 | $0.00652 |
| Opus 5.5 | $0.00008 | $0.00261 |
| Sonnet 5.5 | $0.00004 | $0.00130 |
| Haiku 4.5 | $0.00002 | $0.00065 |
Grade A, and why
diagnostic-reasoning 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 4d 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 — 52 lines — stays where its author put it; the contents beside it link to each section on GitHub.
diagnostic-reasoning
Use this procedure before scoping a reported bug and before acting on a diagnostic report. This skill is the single owner of Firstmate's bug-diagnosis reasoning procedure. Firstmate applies it when briefing delegated investigation and evaluating the resulting evidence, without taking over project-specific investigation itself.
Establish the observed behavior
Start from the end user's experience rather than an internal error string or an implementation hypothesis. Require an end-to-end reproduction aligned with the real user path whenever it is feasible and safe. If a faithful reproduction is not feasible, record the exact limitation and use the closest representative path without presenting it as equivalent evidence. Capture the expected behavior, observed behavior, setup, inputs, and repeatability before assigning a cause.
Separate these three facts explicitly:
- The initiating trigger is the event, input, or transition that starts the faulty behavior.
- The masking condition is the independent state, environment, timing, cache, configuration, or path difference that hides or exposes the fault.
- The visible symptom is what the end user or operator can actually observe.
Do not collapse those facts into one label. A masking condition may explain why a fault appears only sometimes without being the initiating cause, and the visible symptom may be several layers downstream from both.
Test the causal explanation
Inspect the failing path and a proven path where the intended behavior is known to work. Compare their inputs, state transitions, dependencies, timing, and control flow to find the earliest meaningful divergence. Inspect relevant history, including blame, commits, migrations, and prior implementations, when it can explain why the paths diverged or which invariant was intended. Do not treat the most recent nearby change as causal without evidence.
Identify the smallest counterfactual that should change the outcome if the leading explanation is true. Change one condition at a time where practical, and record whether the symptom appears, disappears, or remains unchanged. Seek disconfirming evidence deliberately: name what observation would falsify the leading explanation, run that check when feasible, and retain contradictory results instead of explaining them away. Compare the final explanation against the proven path and show why the proposed causal boundary accounts for both the failure and the success.
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.
- 4d ago Changed · -2 lines · -36 tokens per session 74083d93a321
- 1mo ago First seen · 54 lines · 57 tokens per session scan A e3df3c616fd5
diagnostic-reasoning is a skill published in the GitHub repository kunchenguid/firstmate (8,295 stars, last pushed today), licensed MIT. It adds 21 tokens to every session and 652 once invoked, about $0.0001 per session on Opus 5.5. 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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