tollwarden (Python)

Official Python SDK for TollWarden — the payment security firewall for x402 micropayments. One call before your agent settles a payment; allow/flag/block comes back with machine-readable reasons.

Python 3.9+. Single dependency (cryptography, for Ed25519 attestation verification).

pip install tollwarden

30 seconds

from tollwarden import TollWardenClient, TollWardenBlockedError

tollwarden = TollWardenClient(agent_id="my-agent")  # mints a free API key on first use (100 free scans)

try:
    tollwarden.guard_outgoing(payment, expected_price_usd=0.01)
    # verdict was allow (or flag) — safe to hand to your wallet
except TollWardenBlockedError as e:
    print("Payment blocked:", e.scan["checks"])  # machine-readable reasons

The one-line diff: scan every payment by default

Wrap your x402 payment-capable transport and every payment is scanned before it settles:

from tollwarden import TollWardenClient, wrap_transport_with_tollwarden

tollwarden = TollWardenClient(agent_id="my-agent")
guarded = wrap_transport_with_tollwarden(my_x402_transport, tollwarden)
# use `guarded` anywhere a transport goes

Non-402 responses pass through untouched (zero overhead). On a 402, the payment is guarded as an outgoing payment (overpayment, address poisoning, velocity, injection provenance — anything you observe()d feeds the detector), the offer is scanned as an incoming request (URL risk, credential demands, asset verification, reputation), and only passing verdicts reach the paying transport. A block raises TollWardenBlockedError before any payment is signed; unparseable 402 offers fail closed. Options: strict, scan_offer, expected_price_usd, on_scan telemetry, base_transport, and enforcer — pass a TollWardenEnforcer and every passing verdict is registered as signing authority, so a guard_signer()-wrapped account inside the paying transport signs only what was scanned (see enforcement below).

The important part: provenance tagging

TollWarden's strongest detector catches payments triggered by prompt-injected content — but it needs to know where your agent's decision came from. Tell it:

# After EVERY tool result / fetched page your agent reads:
tollwarden.observe(tool_result_text, source_url="https://api.example.com/page")

# The next scan (within 5 min) is automatically tagged:
#   context.origin = "fetched_content" | "tool_result"
#   context.content = the observed text (truncated to 8 KB)
# If the pay-to address turns out to have COME FROM that content -> block.

# When the decision is the agent's own plan, or a human said so:
tollwarden.note_planning()
tollwarden.note_user_instruction()

Each observation is consumed by one scan; unrelated later scans aren't mislabeled. LangChain/CrewAI users: call observe() in your tool-output callback and guard_outgoing() in your payment tool — two lines total.

Verified verdicts (on by default)

Every scan response carries an Ed25519 attestation binding the verdict to the exact payment. The SDK pins the server's verdict key (fetched once, or pass verdict_key_hex to hard-pin), verifies the signature against the pinned key, recomputes the payment commitment sha256(network|pay_to|asset|amount|nonce) locally (rejecting attestations issued for a different payment — replay defense), and enforces expiry. Any failure raises AttestationError.

The verifier is cross-validated in CI against attestations signed by the production Node signer, so Python and TypeScript agree byte-for-byte.

Wallet authors: verify_attestation(scan, payment, trusted_key_hex) and compute_payment_commitment(payment) are importable standalone.

Enforcement: a wallet that refuses unscanned payments

Everything above is advisory — a compromised agent can skip the scan. The enforcement kit closes that gap at the signing layer:

from tollwarden import TollWardenClient, TollWardenEnforcer
from eth_account import Account

tollwarden  = TollWardenClient(agent_id="my-agent")
enforcer = TollWardenEnforcer(trusted_key_hex=tollwarden.verdict_key())
account  = enforcer.guard_signer(Account.from_key(PRIVATE_KEY))
# hand `account` to your x402 client exactly as before — it is a drop-in proxy

scan = tollwarden.guard_outgoing(payment)  # raises on block
enforcer.approve(scan, payment)         # registers the allow-verdict locally
# x402 pay-and-retry now succeeds. ANY other payment authorization the wallet
# is asked to sign — different recipient, amount, asset, chain, or nonce —
# raises TollWardenEnforcementError before the signature exists.

How the binding works: the wrapped signer intercepts EIP-712 payment authorizations (EIP-3009 TransferWithAuthorization/ReceiveWithAuthorization — the x402 "exact" scheme — plus ERC-2612 Permit; eth-account's positional, keyword, and full_message= call shapes are all recognized), reconstructs the payment from the typed data itself, and recomputes the commitment sha256(network|pay_to|asset|amount|nonce). Only a live approval for exactly that commitment lets the signature happen — so "scan payment A, sign payment B" fails structurally, not by convention.

Pre-sign approvals. In the default path you scan the 402 offer, and an offer has no nonce — the x402 client mints the EIP-3009 nonce when it signs. An approval registered from a nonce-less payment binds (network, pay_to, asset, amount) and admits exactly one authorization carrying those facts, whatever nonce it ends up with (single-use is what stops a second). An approval registered with a nonce still requires that exact nonce. The two compose in one line:

enforcer = TollWardenEnforcer(trusted_key_hex=tollwarden.verdict_key())
account  = enforcer.guard_signer(Account.from_key(PRIVATE_KEY))
guarded  = wrap_transport_with_tollwarden(paying_transport_for(account), tollwarden, enforcer=enforcer)
# every 402: scanned → verdict registered → the guarded account signs that authorization and nothing else

Guarantees and options: approvals are verified against the pinned verdict key at approve() time (tampered/replayed/expired attestations raise), are single-use by default (reusable=True to opt out), expire with the attestation (tighten with max_age_s), gate on allow-only verdicts (allow_flagged=True to accept flags; accept_overrides=True to accept human-approved override:allow verdicts from step-up approvals — opt-in because a self-webhooked agent could approve its own flags), and can be revoke()d. Unrecognized typed data passes through by default; strict_types=True makes the signer deny-by-default. Enforcement is fully local and fail-closed — if TollWarden is unreachable, nothing new can be approved. For flags that pause for a human (scan["approval"] present), client.wait_for_approval(scan, payment=payment) polls until the operator decides and returns the signed override.

Local policy: allowlist + spend caps. The verdict gate answers "was this exact payment scanned and allowed?" — local policy answers a different question: "is this payment inside the bounds I set, no matter what any scan said?" Configure it on the enforcer and it is checked against the typed data at signature time, entirely offline and independent of approvals:

enforcer = TollWardenEnforcer(
    trusted_key_hex=tollwarden.verdict_key(),
    allowed_recipients=["0xKnownMerchantA…", "0xKnownMerchantB…"],  # hard allowlist (case-insensitive; [] = deny all)
    max_amount_atomic=1_000_000,   # per payment: 1 USDC (6 decimals)
    max_total_atomic=10_000_000,   # cumulative across this enforcer's lifetime: 10 USDC
)

Even a payment carrying a valid allow-verdict is refused if it pays an unlisted recipient or exceeds a cap — so if everything upstream is confused or compromised, the wallet can still only move bounded amounts to known parties. Unparseable values under a cap are refused (fail-closed); enforcer.total_authorized_atomic reports the running total. Atomic units are only comparable within one asset (for x402 that's USDC); bound multi-asset flows with separate enforcers.

Growing the allowlist. The agent can never extend the list — that's the point (an injected agent's first move would be to add the attacker). New recipients are added out of band, by whoever owns the enforcer config. For a smoother path there's one opt-in escape hatch: override_admits_recipient=True (requires accept_overrides) lets a human-approved override:allow from step-up approvals satisfy the allowlist for exactly the payment it binds — the human admits one commitment-bound payment, the list itself never changes, spend caps still apply, and a plain allow-verdict never admits. It inherits the accept_overrides security note: only meaningful when the approval webhook receiver is out of the agent's reach.

Delivery outcomes (automatic). The payment-path wrapper also closes the loop after settlement: x402 delivery is synchronous, so it observes the paid response mechanically and reports the outcome — 2xx → delivered, 5xx or a second 402 → not_delivered, with status/bytes/latency evidence — bound to the scan it just performed (scan_id + payment_commitment, one outcome per scan, so delivery history can't be faked). Reported on a daemon thread: it never delays the response. Opt out with report_outcomes=False; settling another way? call client.report_outcome(scan, outcome, ...) yourself. Sellers with low measured delivery rates get flagged on everyone's future scans.

The gate is cross-validated in CI: an attestation signed by the production Node signer authorizes a signature through the Python enforcer end to end, so both SDKs enforce identical semantics.

Scope note: this guards the typed-data path x402 uses. If your signer also exposes raw sign_transaction, gate that at your policy layer too.

Paying for scans and plans (x402)

Your first 100 calls per key are free. Beyond that, pass a payment-capable transport — any callable (method, url, headers, body_bytes) -> (status, headers, body_bytes) that settles x402 challenges (e.g. wrapping an x402 Python client):

tollwarden = TollWardenClient(agent_id="my-agent", transport=my_x402_transport, auto_renew=True)

tollwarden.get_plans()        # free catalog: Starter / Pro ($4.99/30d, $0.005/scan) / Scale ($19.99/30d, $0.002/scan)
tollwarden.subscribe("pro")   # pays $4.99 over x402, upgrades this key for 30 days

Plans raise your own velocity/spend thresholds and cut per-scan price. Replay detection, merchant pinning, asset verification, and PII scanning are identical on every tier — no plan can relax them.

Reputation

tollwarden.report("0xbad...", "non_delivery", "paid, no data")  # always free
tollwarden.reputation("0xsomeone...")                           # report summary (paid / free-tier)

API surface

TollWardenClient — scan_outgoing, scan_incoming, guard_outgoing, guard_incoming, observe, note_planning, note_user_instruction, get_plans, subscribe, report, reputation, ensure_api_key, verdict_key, plus free_calls_remaining / plan state. Payment path — wrap_transport_with_tollwarden, payment_from_offer. Enforcement — TollWardenEnforcer (approve, guard_signer, assert_approved, assert_approved_for, revoke, clear), payment_from_typed_data. Standalone — verify_attestation, compute_payment_commitment. Errors — TollWardenError (.status, .body), TollWardenBlockedError (.scan), AttestationError, TollWardenEnforcementError (.commitment, .primary_type).

BUSL 1.1 (source-available; using this SDK against the hosted service is expressly permitted, including in commercial products). TollWarden is advisory and non-custodial: this SDK never touches your keys, wallet, or funds.