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 randommonicle/claude-skills --skill outbound-side-effect-idempotencygit clone --depth 1 https://github.com/randommonicle/claude-skillsWrote 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/randommonicle/claude-skills/outbound-side-effect-idempotency)<a href="https://agentmods.dev/skills/randommonicle/claude-skills/outbound-side-effect-idempotency"><img src="https://agentmods.dev/badge/skills/randommonicle/claude-skills/outbound-side-effect-idempotency/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/randommonicle/claude-skills/outbound-side-effect-idempotency"><img src="https://agentmods.dev/badge/skills/randommonicle/claude-skills/outbound-side-effect-idempotency.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 | $0.00104 | $0.00603 |
| Opus 5 | $0.00052 | $0.00302 |
| Sonnet 5 | $0.00021 | $0.00121 |
| Haiku 4.5 | $0.00010 | $0.00060 |
Grade A, and why
outbound-side-effect-idempotency 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 11d 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 — 44 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Outbound side-effect idempotency
The genuine distributed-systems tradeoff hiding in every send-then-record path: "never lose it" and "never duplicate it" cannot both be absolute. A review-request sender kept fail-closed semantics (mark sent only after the provider accepted) and then had to confront the race that design leaves open: two concurrent passes both reading "not sent", both sending.
The rules
- Decide which failure harms the recipient less, and design for that. A lost password reset is worse than a duplicate; a duplicate payment is worse than a retried one. The choice is per-path, explicit, and recorded.
- Fail-closed for never-lose-it: mark sent only after the provider's 2xx. A pre-marked "sent" that then fails to send is a silent loss with a clean audit trail — the worst combination.
- A read-then-write pre-check is not a lock. "Is it already sent? No → send" protects
sequential retries only; concurrent callers both pass. True at-most-once needs an atomic
claim: a
sendingstate transition the row passes through, or a unique constraint the second sender violates. - When both properties matter and the claim state is deferred, make the residual case detectable: a conditional mark that records the race happened is the honest interim — you cannot yet prevent the double-send, but you will know about it instead of finding out from the recipient.
What this skill does not do
It does not govern internal retry loops with no external recipient (lock-at-the-chokepoint) and does not verify delivery (email-delivery-verification). It chooses and implements the failure semantics of the send itself.
Why
Every send-then-record path embodies this tradeoff whether or not its author noticed; the unexamined default is usually at-least-once with a silent-loss edge, which is the wrong choice for most recipient-facing sends. Evidence: ICC LESSONS_LEARNED L-021 and the reviewRequest fail-closed pattern; the same check-then-act race on an internal path in ASH LESSONS_LEARNED 2026-06-04 (shared evidence with lock-at-the-chokepoint).
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.
- 11d ago First seen · 44 lines · 104 tokens per session scan A e974ea7c2bce
outbound-side-effect-idempotency is a skill published in the GitHub repository randommonicle/claude-skills (23 stars, last pushed 4d ago), licensed Apache-2.0. It adds 104 tokens to every session and 603 once invoked, about $0.0005 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 2026-08-31.
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