What is idempotency-handling?

A way to make an operation safe to repeat without causing its effect more than once. It uses an idempotency key, a unique request identifier, to recognize retries.

Use it for payment processing, order creation, webhooks, message queues, bank transfers, and other APIs or distributed transactions that may be retried.

Network failures and retries can otherwise create duplicate payments, orders, emails, or other resources. It keeps repeated API requests and messages from applying the same action twice.

idempotency-handling — Skill for Claude Code and Codex by aj-geddes

Install

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.

Any agent

npx skills add aj-geddes/useful-ai-prompts --skill idempotency-handling

Clone the repo

git clone --depth 1 https://github.com/aj-geddes/useful-ai-prompts

Made for: Claude Code, Codex.

Wrote 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.

README.md

[![agentmods](https://agentmods.dev/badge/skills/aj-geddes/useful-ai-prompts/idempotency-handling/github.svg)](https://agentmods.dev/skills/aj-geddes/useful-ai-prompts/idempotency-handling)

Your own site

<a href="https://agentmods.dev/skills/aj-geddes/useful-ai-prompts/idempotency-handling"><img src="https://agentmods.dev/badge/skills/aj-geddes/useful-ai-prompts/idempotency-handling/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.

Your own site · 80×15

<a href="https://agentmods.dev/skills/aj-geddes/useful-ai-prompts/idempotency-handling"><img src="https://agentmods.dev/badge/skills/aj-geddes/useful-ai-prompts/idempotency-handling.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 39 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 605 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe. Third-party audits

Socket pass 14 Sept 2026
Snyk pass 14 Sept 2026

How audits are shown

Origin original No closer match found in the catalogue.

Token cost

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	$0.00039	$0.00605
Opus 5.5	$0.00016	$0.00242
Sonnet 5.5	$0.00008	$0.00121
Haiku 4.5	$0.00004	$0.00060

Measured 2mo ago against content hash 5ed3ee0194ee, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

idempotency-handling 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 2mo ago.

The scan reads SKILL.md. This mod also ships 1 executable file (scripts/validate-api.sh), listed below but not scanned — reading those needs a real analyzer, not pattern matching.

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.

skills/idempotency-handling/SKILL.md · 100 lines

How it starts

The opening of the file, as written. The whole thing — 100 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Overview

Implement idempotency to ensure operations produce the same result regardless of how many times they're executed.

When to Use

Payment processing
API endpoints with retries
Webhooks and callbacks
Message queue consumers
Distributed transactions
Bank transfers
Order creation
Email sending
Resource creation

Quick Start

Minimal working example:

import express from "express";
import Redis from "ioredis";
import crypto from "crypto";

interface IdempotentRequest {
  key: string;
  status: "processing" | "completed" | "failed";
  response?: any;
  error?: string;
  createdAt: number;
  completedAt?: number;
}

class IdempotencyService {
  private redis: Redis;
  private ttl = 86400; // 24 hours

  constructor(redisUrl: string) {
    this.redis = new Redis(redisUrl);
  }

  async getRequest(key: string): Promise<IdempotentRequest | null> {
    const data = await this.redis.get(`idempotency:${key}`);
    return data ? JSON.parse(data) : null;
  }
// ... (see reference guides for full implementation)

Reference Guides

Detailed implementations in the references/ directory:

Guide	Contents
Express Idempotency Middleware	Express Idempotency Middleware
Database-Based Idempotency	Database-Based Idempotency
Stripe-Style Idempotency	Stripe-Style Idempotency
Message Queue Idempotency	Message Queue Idempotency

Best Practices

✅ DO

Require idempotency keys for mutations
Store request and response together
Set appropriate TTL for idempotency records
Validate request body matches stored request
Handle concurrent requests gracefully
Return same response for duplicate requests
Clean up old idempotency records
Use database constraints for atomicity

Read the full file on GitHub · 100 lines

Files

What ships with it

6 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

Changes

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.

2mo ago First seen · 100 lines · 39 tokens per session scan A 5ed3ee0194ee

Subscribe to this mod's changes

idempotency-handling is a skill published in the GitHub repository aj-geddes/useful-ai-prompts (376 stars, last pushed 8mo ago), licensed MIT. It adds 39 tokens to every session and 605 once invoked, about $0.0002 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 2026-09-03.

Backend and APIs Finance and fintech

idempotency-handling next to 6 skills from other repositories
Mod	Type	Stars	Last push	Licence	Scan	Tokens	Works with
idempotency-handlingaj-geddes/useful-ai-prompts	Skill	376	8mo ago	MIT	A	39	Claude Code, Codex
x402browser-use/browser-use	Skill	120k	yesterday	MIT	A	175	Claude Code
tushareHKUDS/Vibe-Trading	Skill	38k	yesterday	MIT	A	79	Claude Code, Codex
stripe-best-practicesstripe/ai	Skill	2.0k	yesterday	MIT	A	161	Claude Code, Codex
wechatpay-payment-integrationwechatpay-apiv3/wechatpay-skills	Skill	372	5d ago	MIT	A	330	Claude Code, Codex
hyperliquid-websocketgalleonlabs/hypergrok-trading-desk	Skill	350	2d ago	MIT	A	89	Claude Code
crypto-market-data-service-guidenirholas/three.ws	Skill	269	yesterday	Apache-2.0	A	56	Claude Code, Codex

idempotency-handling