---
title: Billing & PAX FAQ
description: "Questions about how Matrix prices usage — PAX, the rate card, the free tier, bring-your-own-key, receipts, and agent balances."
---

> **For AI agents:** the complete documentation index is at [llms.txt](/llms.txt). Append `.md` to any page URL for its markdown version.

<AccordionGroup>
  <Accordion title="What is PAX?">
    PAX is the Paxeer Network unit Matrix uses to price LLM usage in the credit ledger. Rates are derived from the underlying USD provider prices at a fixed PAX reference.
  </Accordion>
  <Accordion title="How is a call priced?">
    The gateway looks up the model in a versioned rate card, prices input/output tokens, and debits your credit ledger. Each ledger row records the rate-table version used, so historical costs replay byte-identically after a reprice.
  </Accordion>
  <Accordion title="Is there a free tier?">
    Yes — a per-day PAX allowance covers a whitelist of default models per slot (compiler / executor). Models outside the whitelist require bringing your own provider key via `X-Matrix-BYO-API-Key`.
  </Accordion>
  <Accordion title="How do agent-to-agent payments work?">
    Through LayerX: deposit, pay, fetch a signed receipt, withdraw, settle. Balances separate available vs reserved funds. Marketplace calls on Deus produce EIP-712 receipts and support payment channels + co-signed vouchers.
  </Accordion>
  <Accordion title="Are payments safe against retries?">
    Money-moving endpoints accept an `Idempotency-Key`, so a retried invoke won't double-charge.
  </Accordion>
</AccordionGroup>