# Idempotent requests

> Send an Idempotency-Key header on every POST to the Anytime Capital Payments API so a retried request never creates a second link, invoice or email.

Source: https://anytime-capital.com/docs/idempotency · Markdown: https://anytime-capital.com/docs/idempotency.md · All docs: https://anytime-capital.com/docs/llms-full.txt

Every `POST` accepts an `Idempotency-Key` header: any unique string of 1 to 255 printable characters, such as a UUID. If a request times out or the connection drops, send it again with the same key. You get the original response back, with the header `Idempotent-Replayed: true`, instead of creating a second payment link or emailing an invoice twice.

```bash
curl -X POST https://anytime-capital.com/api/v1/payment-links \
  -H "Authorization: Bearer $ANYTIME_API_KEY" \
  -H "Idempotency-Key: 5f1c3e0a-7b2d-4c8e-9a61-2d4b8f0e7c13" \
  -H "Content-Type: application/json" \
  -d '{"title": "Consulting — October", "amountCents": 12500}'
```

## How keys behave

| Situation | What you get |
| --- | --- |
| First request with a key | The request runs and, if it succeeds, its response is remembered with the key. |
| Same key, same request, within the window | The remembered response, with `Idempotent-Replayed: true`. Nothing runs again. |
| Same key, different body or endpoint | `422 IDEMPOTENCY_KEY_REUSED`. Use a new key for a new request. |
| Same key while the first request is still running | The second request waits for the first, then gets its result. |
| The first request failed | Errors are not remembered, so you can fix the problem and retry with the same key. |
| More than 24 hours later | The key is forgotten and the request runs as new. |
| Empty, too long or unprintable key | `400 INVALID_IDEMPOTENCY_KEY`. |
| No header | The request simply runs. It is still recorded in your activity log. |

## Choosing keys

- Generate one key per operation you mean to happen once, such as “create the payment link for order 1042”, and store it with that order before you call the API.
- Reuse the stored key on every retry of that operation, including after your own process restarts.
- Never reuse a key for a different operation, even for the same customer.
- Keys are scoped to your business: another business using the same string doesn't collide with you.

> **MCP tools take the same key:** The MCP tools that change something (create, update, deactivate and send) accept an `idempotencyKey` argument and send it as this header, with the same behaviour. See [Build with AI](https://anytime-capital.com/docs/build-with-ai.md).

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Anytime Capital is a fintech company, not a bank. Payment processing, banking and payout services are provided by Anytime Capital's licensed partners. Rates shown are for US card payments; international cards, currency conversion and disputes may carry additional fees shown in your dashboard before you accept a payment.