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 skills add ychampion/cskill-agents --skill create-only-idempotent-artifact-writegit clone --depth 1 https://github.com/ychampion/cskill-agentsWrote 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/ychampion/cskill-agents/create-only-idempotent-artifact-write)<a href="https://agentmods.dev/skills/ychampion/cskill-agents/create-only-idempotent-artifact-write"><img src="https://agentmods.dev/badge/skills/ychampion/cskill-agents/create-only-idempotent-artifact-write/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/ychampion/cskill-agents/create-only-idempotent-artifact-write"><img src="https://agentmods.dev/badge/skills/ychampion/cskill-agents/create-only-idempotent-artifact-write.svg" alt="Reviewed on agentmods" width="80" 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 5.1 | $0.00029 | $0.00487 |
| Opus 5 | $0.00015 | $0.00244 |
| Sonnet 5 | $0.00006 | $0.00097 |
| Haiku 4.5 | $0.00003 | $0.00049 |
Grade A, and why
create-only-idempotent-artifact-write 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 9d 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 — 30 lines — stays where its author put it; the contents beside it link to each section on GitHub.
SKILL: Create-Only Idempotent Artifact Write
Domain: tool-orchestration
Trigger: Apply when a deterministic artifact is keyed by a stable invocation ID and may be replayed across retries, compaction, or resume passes.
Source Pattern: Distilled from reviewed tool-loop and result-shaping patterns.
Core Method
When an artifact path is derived from a stable identity, write it once with an atomic create-only operation instead of checking for existence first. If the write hits "already exists," treat that as proof the earlier successful write already materialized the deterministic content and continue with the normal readback or preview path. This removes the stat-then-write race, avoids useless rewrites during replay, and keeps repeated passes behaviorally identical.
Key Rules
- Derive the artifact path from a stable key such as
tool_use_idso repeated calls target the same location. - Use a create-only write primitive like
flag: 'wx'to collapse "create if absent" into one filesystem operation. - Treat
EEXISTas idempotent success when the content is deterministic for that key; only unexpected filesystem errors should fail the operation. - Continue into the downstream preview or response-building path even when the file already existed, so replayed calls return the same contract as fresh ones.
- Avoid preflight existence checks because they reintroduce races and duplicate I/O under concurrency.
Example Application
If a large tool result is persisted under tool-results/<tool_use_id>.json, write it once with a create-only flag. When transcript replay or micro-compaction revisits that same tool result, the write sees EEXIST, skips the rewrite, and still returns the same preview payload and saved path to the caller.
Anti-Patterns (What NOT to do)
- Do not
stat()and thenwriteFile()for idempotency; another writer can win between those calls. - Do not overwrite existing artifacts on every replay when the file path is already the canonical output for that key.
- Do not treat "file already exists" as a hard error in deterministic replay flows; it is often the success path.
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.
- 9d ago First seen · 30 lines · 29 tokens per session scan A 22354ff4522c
create-only-idempotent-artifact-write is a skill published in the GitHub repository ychampion/cskill-agents (36 stars, last pushed 5mo ago), licensed MIT. It adds 29 tokens to every session and 487 once invoked, about $0.0001 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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