fair-meter: the Fair Offer kit for x402 sellers

What a seller needs to keep the Fair Offer Standard 0.4, in one small Python package with no required dependencies:

Signing receipts needs eth-account, which the x402 SDK already installs. Licence: Apache-2.0 (the standard's text, schema and examples in the rest of this repository are CC BY 4.0).

Install

From this repository until it is published:

pip install "git+https://github.com/Agoranome/fair-offer.git#subdirectory=kit"

A metered route with the x402 SDK (Python, 2.25)

Register x402's upto scheme beside exact (the facilitator must list upto on your network), offer the metered route with both, and declare the block:

from x402.mechanisms.evm.exact import ExactEvmServerScheme
from x402.mechanisms.evm.upto import UptoEvmServerScheme
from fair_meter import declare

server.register(NETWORK, ExactEvmServerScheme())
server.register(NETWORK, UptoEvmServerScheme())

BLOCK = declare.extension(declare.block(
    signer=RECEIPT_ADDRESS,                       # the address that signs receipts
    sample="/v1/rows/sample",
    meter=declare.metered("row", "0.0002"),       # one public price a row (FO-9)
    ledger="/receipts/ledger", reports=None,      # the open book (FO-10)
))
# On the route's config: both options at the same price (for upto it is the ceiling), and extensions=BLOCK.

In the handler, guard the result, settle only what was delivered, sign the receipt and put it in the book once the payment settles:

from fair_meter import guard, meter, receipt

UNIT = meter.unit_atomic("0.0002")               # 200 units of USDC

async def rows(request):
    requirements = request.state.payment_requirements
    found = guard.guard(lookup(request))          # FO-6: refuse to send instructions; the failed call isn't settled
    charged = len(found["rows"]) * UNIT
    if requirements.scheme == "upto":
        charged = meter.charge(len(found["rows"]), UNIT, int(requirements.amount))
    paid = receipt.build(receipt_id=new_id(), resource=str(request.url), network=requirements.network,
                         asset=requirements.asset, pay_to=requirements.pay_to, amount_atomic=charged,
                         result=found, ceiling_atomic=int(requirements.amount), units=len(found["rows"]),
                         unit="row")
    response = JSONResponse(receipt.attach(found, paid, RECEIPT_KEY))
    if requirements.scheme == "upto":
        name, value = meter.overrides(charged)   # the middleware settles this amount, never the ceiling
        response.headers[name] = value
    return response

When the facilitator returns the settlement, record it: book.record(receipt_id, payer=..., amount_atomic=..., tx_hash=...), and serve book.view() at your ledger path and book.proof(receipt_id) beside it. A metered call that charged nothing has no transaction and stays out of the book.

Read the field names on requirements from your SDK version; the shape above is the 2.25 Python SDK's.

A public share (FO-7)

If you promise a share of what you earn to a cause, publish the pledge ledger beside the receipts ledger:

from fair_meter import pledge
page = pledge.ledger_doc(book, share_bps=1500, proof_url="https://you.example/receipts/{receipt_id}/proof",
                         recipient=CAUSE, transfers={"2026-11": [{"tx_hash": "0x...", "amount_atomic": 300000}]})

Each month shows its root, its basis, what it owes (the share, rounded up), what was sent and its standing: a month that owes under $1 carries over and shows as owed, and nothing is carried past the end of a quarter (carry-1). Send one USDC transfer on Base to the recipient each month; anyone can read it on the chain.

What a buyer checks

• receipt.verify(body) (or any EIP-191 library over the receipt's RFC 8785 bytes): the receipt is signed by its signer and names the hash of the result it came with.
• agoranome-inspector check-delivery answer.json recomputes the delivery hash with no trust in the seller.
• The ledger's month roots fold every receipt's proof; Agoranome's inspector can copy them into its open book (anchor-lab) so a past month can't be changed quietly (verify-receipt).
• https://api.agoranome.com/v1/fair-offer/check?url= reads your live 402 against your block, free.

Tests

cd kit && pip install -e ".[test]" && pytest

The ledger's roots are tested against the construction Agoranome's lab ledger uses, so the same tools check both.