# Idempotency-Key header

[5 · Act](/category/act/)

| Idempotency-Key header |
| --- |
| Category | [Act](/category/act/) |
| Also known as | Idempotency-Key, idempotent tool call |
| Related | [Agentic Commerce Protocol](/protocols/agentic-commerce-protocol/), [Machine Payments Protocol](/protocols/mpp/) |

[Act](/category/act/) An Idempotency-Key is a unique client-generated token attached to an HTTP POST/PATCH request or agent payment invocation so that if an agent retries after a network timeout, the server recognizes the duplicate and returns the original result rather than executing the action or charging the user twice. It is standard across Agentic Commerce Protocol, x402 / MPP payment retries, and non-idempotent MCP tool annotations (idempotentHint: false). Used by: agentic-commerce-protocol, mpp.

## Instances

Not applicable.

## See also

- [Agentic Commerce Protocol](/protocols/agentic-commerce-protocol/)
- [Machine Payments Protocol](/protocols/mpp/)

## References

- [https://github.com/agentic-commerce-protocol/agentic-commerce-protocol](https://github.com/agentic-commerce-protocol/agentic-commerce-protocol) (2026-04-17) VERIFIED

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