x402 Buyer Failure Rate on Base Mainnet: Final Report

Revision 2026-10-28. The report's queries matched only the network id eip155:8453, so 188 purchase calls (and 4 stress calls) to sellers that quote the x402 v1 name base for the same chain (Base mainnet) were left out of every rate: api.slamai.dev (171 calls, a top-20 seller by volume), slamlink-ai-main-50fbb45.d2.zuplo.dev (10), uxus.finance (5) and x402factory.ai (2). The queries now treat base and eip155:8453 as one network; no stored row was changed. The original limitations section predicted a shift under 0.1 point; the count rate moved 0.01 point.

measure	before	after
denominator	12,805	12,992
count-weighted failures	848	859
count-weighted rate	6.62% [6.20%, 7.07%]	6.61% [6.20%, 7.05%]
dollar-weighted base, failed	57.65, 4.48 USDC	57.92, 4.51 USDC
dollar-weighted rate	7.76% [6.72%, 8.95%]	7.79% [6.75%, 8.98%]
Kish effective n	2,217	2,237
non_delivery	498	509
overcharge at 2x only	6.2% [5.8%, 6.7%]	6.2% [5.8%, 6.6%]
charged_then_rejected	140	143
client timeouts before declared window	3	9
top tier	3,837, 10.1% [9.2%, 11.1%]	4,007, 9.8% [8.9%, 10.8%]
tail tier	3,861, 4.6% [3.9%, 5.3%]	3,871, 4.5% [3.9%, 5.2%]
unassigned tier	5,107, 5.6% [5.0%, 6.2%]	5,114, 5.7% [5.1%, 6.3%]
data category	3,237, 6.1% [5.3%, 6.9%]	3,419, 6.0% [5.2%, 6.8%]
inference category	2,871, 5.0% [4.2%, 5.8%]	2,876, 5.1% [4.3%, 5.9%]
phases 1 / 2 / 3 to 6 (denominator)	1,997 / 7,428 / 3,380	2,035 / 7,548 / 3,409

Phase rates, the seller counts (362 reached, 93 with at least 30 calls, 69 of them with no failure) and the worst-ten table do not change; those counts in the original text already included the v1 rows. api.slamai.dev now qualifies for the 30-call seller set. The interval half-width in 2.5 is corrected to ±0.43 (it was 0.43 before this revision too).

Study window: 2026-10-24 20:25 UTC to 2026-10-26 00:19 UTC (purchases), with discovery, on-chain measurement and probing from 2026-10-24 01:40 UTC to 2026-10-26 07:13 UTC. Repository: sarmadahsan/x402-audit (private), commit range cc2d284 to 12e26a3, 68 commits, 329 automated tests in 38 files. Database: data/audit.sqlite (one file; every number below is a query over it).

1. The question and the answer

Question. Of paid x402 API calls on Base mainnet, what share fail the buyer, by count and by dollars, and why?

Answer. Across 12,992 paid calls that count in the denominator:

measure	denominator	failures	rate [95% Wilson CI]
count-weighted	12,992 paid calls	859	6.6% [6.2%, 7.1%]
dollar-weighted	57.92 USDC	4.51 USDC	7.8% [6.8%, 9.0%] (Kish effective n = 2,237)

Failures split into two kinds. non_delivery (509 calls, 59% of failures): the seller settled the payment on-chain and then returned a 4xx or 5xx, an empty or wrong-typed body, or nothing at all within its own declared timeout. overcharge (350 calls, 41%): the seller's 402 quote exceeded the price it lists in the Coinbase Bazaar index, and the harness paid it as a naive agent would. No wrong_goods could be assessed (no per-service response schemas were configured) and no duplicate settlement occurred in the main sample.

The rate is robust to how strictly overcharge is defined: counting an overcharge only at 1.1x the listed price gives 6.4% [6.0%, 6.8%]; only at 2x gives 6.2% [5.8%, 6.6%]. Most overcharges were not marginal: of 350, 175 were 2x to 5x and 124 were 5x or more the listed price, concentrated in nine sellers.

Reported separately and never in the denominator: 976 not_paid (the seller never produced a payable 402, or rejected the payment before settling; no money moved), 597 our_error (listings from which no valid request could be formed; no money moved except 143 cases described below), and 15 indeterminate (payment signed, no response, no settlement visible within 24 hours).

2. Design

2.1 Failure taxonomy

Every paid or probed call receives exactly one primary outcome and zero or more secondary flags (docs/taxonomy.md; a unit test fails if the code's constants drift from the document). Primary outcomes, in precedence order (first match wins; ties resolved toward the outcome that loses the buyer the most money): our_error, not_paid, indeterminate, duplicate, non_delivery, wrong_goods, overcharge, injection, ok.

The paid denominator is ok, overcharge, non_delivery, wrong_goods, injection, duplicate. Headline failures are overcharge, non_delivery, wrong_goods, duplicate. injection (text addressed to an agent in a response body) is flagged for human review and reported separately; after tightening the scanner to address forms only, zero rows carry it.

not_paid was added to the operator's eight original outcomes because "paid nothing, got nothing" had no home and folding it into non_delivery would have overstated dollar losses. The operator approved this on 2026-10-24.

2.2 Reference price for overcharge

In order of preference: the price listed in the Bazaar index for the resource; else the modal quote seen by the probe module over the last 7 days; else the first quote ever observed. All 350 overcharge rows used the Bazaar listing. The quote-to-reference ratio is stored on every row.

2.3 Validators

Eight pluggable validators run on every delivered response and store machine-readable evidence: http_status, non_empty, content_type, not_truncated, json_schema, entity_matches, freshness, injection_scan. A validator with no configuration returns skipped, never pass; because no per-service schemas were written, every row carries the schema_missing flag and wrong_goods is unmeasured (see Limitations). Validator failures in the study: http_status 1,694, content_type 241, not_truncated 16, non_empty 4.

2.4 Settlement confirmation

After every paid call the harness looked for the USDC Transfer from its wallet to the pay-to address whose transaction also carries an AuthorizationUsed(authorizer, nonce) log matching the nonce it signed (EIP-3009 transferWithAuthorization). From phase 2 on, the seller-reported transaction hash was verified first by receipt on the canonical RPC, falling back to a log scan. 12,955 settlements were matched this way, 57.96 USDC.

2.5 Sampling and statistics

Stratified by seller tier (top 20 by 30-day on-chain volume versus a seeded random tail) and category (inference, data, scraping, compute, other), with calls weighted toward cheap resources, at most 10 resources and 150 calls per seller per plan, and one call per resource per minute. Six plans ran in sequence as the operator asked for more diverse data and then to spend the pool down: phase 1 (top 20 + 40 tail sellers), phase 2 (top 20 + 300 tail, widened mid-phase to 600 tail), phases 3 to 6 (same wide set, fresh seeds).

Intervals are 95% Wilson. The 3,000-call target was chosen so a 10% rate would carry about ±1.1 points; the final 12,992 carries ±0.43. The dollar-weighted interval uses Kish's effective sample size and is labelled an approximation. The per-seller table requires at least 30 calls, where the interval is about ±10 points: enough to separate a 5% seller from a 40% one.

2.6 Safety and auditability

Dry-run by default; real money required --live, X402_AUDIT_LIVE=1, --network eip155:8453, and a passing 329-test suite recorded for the exact commit (pnpm gate). Caps in code before any signature: per call 0.50 USD, per seller per day 20, per day 100, total 300. Every payment attempt was written to an append-only attempts ledger before signing; a reconcile pass turned 7 ledger rows that lost their call row to a process stop into proper rows after checking the chain. 63 rows were re-classified during the study as validator bugs were fixed; each change is in the reclassifications table with its reason and code commit. 22 halt rows record every stop (operator halts for restarts, and one seller reaching its daily cap).

3. Data sources

Source	What it provided	Access	Where used
github.com/coinbase/x402 at commit 0e6e8c5 (2026-10-15)	Protocol facts: v2 headers (PAYMENT-REQUIRED, PAYMENT-SIGNATURE, PAYMENT-RESPONSE), v1 headers (X-PAYMENT, X-PAYMENT-RESPONSE), PaymentRequirements and PaymentPayload schemas, facilitator /verify, /settle, /supported contract, EIP-3009 flow, network ids, USDC addresses and EIP-712 domains	public repo, read from source (49 citations in docs/protocol-notes.md)	client wrapper, mock seller, settlement matching
npm @x402/core, @x402/evm, @x402/fetch 2.28.0	Official client; all signing goes through it (setSpendControls(false), exact scheme only, no retries)	public npm	src/x402/
Coinbase CDP Bazaar discovery API, GET https://api.cdp.coinbase.com/platform/v2/x402/discovery/resources	The universe: 37,903 unique resources from 2,649 seller domains (38 pages of 1,000, 2 duplicates, 0 parse rejects), listed price, pay-to, network, input examples, 30-day call counts	public, no key, pulled 2026-10-24 01:40 UTC	discover, reference prices, request bodies
x402scan (https://www.x402scan.com/openapi.json)	Not used: every endpoint is itself a paid x402 call ($0.01 per 100 rows) and its terms forbid scraping the free frontend	documented only	none
Base mainnet JSON-RPC: https://base-rpc.publicnode.com (logs; 50k-block spans, 20k results), https://mainnet.base.org (receipts and balances; rejects log queries)	30-day USDC Transfer history to every known pay-to: 1,336,834 transfers to 1,684 addresses, blocks 51,712,350 to 53,008,495 (2026-09-24 to 2026-10-24); receipts for 53,149 transactions (45,626 EIP-3009, 7,523 plain); settlement confirmation for every purchase	public, no key	seller ranking, chain proxies, settlement matching
USDC on Base 0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913 (Circle), events Transfer and AuthorizationUsed	On-chain settlements	public chain	all chain work
Sellers' own 402 endpoints (unpaid)	33,320 quotes over 54 hourly rounds across 1,006 resources from two client identities	public HTTP	probe module
Sellers' own paid endpoints	14,580 purchase attempts to 362 sellers and 2,064 resources	4 wallets funded with 58 USDC by the operator	buy and stress modules
Local mock seller + anvil	Every failure mode on demand, a real EIP-3009 token, integration tests for buy, stress and confirmation	local	tests only

Seller ranking: 30-day USDC inflow per pay-to, summed per seller domain. 1,032 of 2,649 domains share a pay-to with a higher-traffic domain (staging hosts, mirrors, one domain literally named jev-x402-evil.vercel.app beside jev-x402.vercel.app); they were treated as shadows and excluded from the top tier so one operator could not occupy several slots. The dollar ranking disagrees sharply with the index's call counts: the top seller by dollars (api.bitrefill.com, about $607k in 30 days) ranks far below ax1.vc by calls (443k transfers, $8.9k).

4. What was measured

4.1 Purchases

attempts	14,580 (plus 40 stress calls)
sellers reached	362; 93 with at least 30 denominator calls
resources reached	2,064
wallets	4, funded 58.00 USDC; 57.96 settled; 0.05 left as dust
settlements matched on-chain	12,955
outcomes	ok 12,133; not_paid 976; our_error 597; non_delivery 509; overcharge 350; indeterminate 15
mean time to first byte on ok	3.0 s (max 70 s)

Per phase (count-weighted headline, 95% CI):

phase	window (UTC)	denominator	rate
1: top 20 + 40 tail sellers	10-24 20:25 to 10-25 08:19	2,035	6.7% [5.7%, 7.9%]
2: top 20 + 300 to 600 tail	10-25 08:19 to 20:33	7,548	6.3% [5.7%, 6.8%]
3 to 6: same wide set	10-25 20:33 to 10-26 00:19	3,409	7.3% [6.5%, 8.3%]

4.2 Breakdown

By seller tier (unassigned are sellers that entered through the widened phases and have no tier label):

tier	sellers	n	rate [CI]	dollar-weighted
top 20 by volume	17	4,007	9.8% [8.9%, 10.8%]	6.3%
random tail	155	3,871	4.5% [3.9%, 5.2%]	6.5%
unassigned (widened phases)	190	5,114	5.7% [5.1%, 6.3%]	12.3%

By category:

category	n	rate [CI]	dollar-weighted
other	3,198	10.3% [9.3%, 11.4%]	16.6%
compute	1,974	7.6% [6.6%, 8.9%]	4.0%
data	3,419	6.0% [5.2%, 6.8%]	11.6%
inference	2,876	5.1% [4.3%, 5.9%]	12.2%
scraping	1,525	2.0% [1.4%, 2.8%]	4.6%

non_delivery by what came back after settlement: no response within the seller's declared window 293 (18 sellers, 1.44 USDC); 4xx 145 (12 sellers); 5xx 27 (4 sellers); 2xx with an empty, truncated or wrong-typed body 44 (6 sellers). 143 rows carry charged_then_rejected: the seller settled and then rejected the request as malformed, including requests built from the seller's own listed example.

4.3 Sellers

Failure is concentrated. Of 93 sellers with at least 30 denominator calls, 69 had zero failures. The worst, by count rate:

seller	tier	category	n	failures	rate [CI]
amanchain-relay.gitajhd.workers.dev		compute	150	150	100% [97.5%, 100%]
api.fadeandco.workers.dev		data	35	33	94.3% [81.4%, 98.4%]
palmyr.ai	tail	other	43	27	62.8% [47.9%, 75.6%]
odds.rjhsignaltech.workers.dev	tail	inference	107	65	60.7% [51.3%, 69.5%]
api.vapinetwork.ai	top	data	116	67	57.8% [48.7%, 66.4%]
api-xpay.com	top	other	500	270	54.0% [49.6%, 58.3%]
api.gondola-ai.com	top	inference	174	48	27.6% [21.5%, 34.7%]
api.loyalspark.online	tail	data	64	16	25.0% [16.0%, 36.8%]
api.sitecheck-api.workers.dev		data	87	18	20.7% [13.5%, 30.4%]
agent-tools-production-e6aa.up.railway.app	tail	scraping	152	29	19.1% [13.6%, 26.1%]

api.slamai.dev (top 20 by volume, data) joined this set in the 2026-10-28 revision: 170 calls, 6 failures, 3.5% [1.6%, 7.5%].

api-xpay.com alone contributes 270 of the 859 failures: it is a top-20 seller by volume that took payment on hundreds of calls and did not answer within its own declared 60 seconds. amanchain-relay took $0.05 per call 150 times and never delivered.

4.4 Stress tests (10 sellers each, 0.20 USDC, never mixed into the headline)

• Duplicate: the same purchase intent sent twice back to back with fresh signatures. 5 of 10 sellers settled both. Nothing in the protocol or at these sellers deduplicates by intent; every signature is a payment.
• Timeout: sign, send, drop the connection after 2 seconds, then retry like a naive client. 2 of 10 sellers settled the aborted call anyway, and the retry produced a second settlement at 5 of 10.

4.5 Probes (unpaid quotes, hourly)

54 rounds, 33,320 quotes, 1,006 resources, two client identities differing only in HTTP headers (the unpaid x402 request carries no buyer wallet). 95.8% of requests returned a 402. 447 quotes exceeded the listed price, the same resources that produced the paid overcharges. 16 quote-drift events and 2 pay-to-address changes were observed. No price discrimination by identity was detected.

4.6 On-chain proxies (market-wide floors, top 60 pay-tos, 30 days)

Delivery is invisible on-chain, so these are floors with caveats. On the share-adjusted view (pay-tos fully verified by receipts, or with a sampled EIP-3009 share whose lower bound is at least 95%; n = 988,081 transfers):

proxy	count-weighted
payment above the pay-to's modal price	14.7% [14.7%, 14.8%]
payment above 2x the modal price	10.3% [10.2%, 10.3%]
isolated duplicate pair (one identical retry within 120 s, nothing else from that payer to that payee for an hour either side)	0.15% [0.14%, 0.15%] (1,444 pairs, $1,252)
naive "any repeat within 120 s"	76.6%, shown for transparency only; it mostly measures high-frequency legitimate consumers

The above-mode proxy also inflates when one pay-to sells several products at different prices, so it is an upper-ish signal of price heterogeneity rather than a clean overcharge rate.

5. Conclusions

1. About one paid x402 call in fifteen fails the buyer. 6.6% by count (6.2% to 7.1%), 7.8% by dollars (6.8% to 9.0%), on 12,992 calls to 362 sellers. The count interval is narrow enough that the true market rate is not below 6% or above 7.1% for this universe and period.
2. Two mechanisms, roughly even by count. Paying and getting nothing (non_delivery, 3.9% of calls) and paying more than the listed price (overcharge, 2.7% of calls). Overcharges are mostly large (2x to 10x+) and come from nine sellers; they look like stale listings or deliberate bait-and-switch rather than price jitter, because the 1.1x and 2x sensitivity cuts barely move the rate.
3. Failure is a property of sellers, not of the protocol's median. 69 of 93 well-sampled sellers never failed; six sellers account for most failures, and one top-20 seller (api-xpay.com) for a third of them. A buyer with a short deny-list would see a rate well under 2%.
4. Volume rank does not protect buyers. The top-20-by-volume tier failed at 9.8% versus 4.5% for the random tail, driven by two high-volume sellers that charge first and answer late or never.
5. Settlement precedes validation at many sellers. 143 calls were charged and then rejected as malformed; 2 of 10 sellers settled a request whose connection was dropped after 2 seconds; 5 of 10 settled a duplicate intent. Nothing in the current exact scheme ties a payment to an idempotency key, so client retries are a direct double-charge risk.
6. The on-chain picture is consistent but weaker. Only 0.15% of transfers to the top 60 pay-tos look like lone retries, in line with the small duplicate signal in purchases; above-mode payments at 14.7% are compatible with the overcharge findings but cannot be separated from legitimate multi-product pricing without off-chain data.
7. The index is noisy. 1,179 listed URLs are path templates that cannot be called, 597 purchase attempts could not form a valid request from the listing, and roughly a third of the 2,649 domains are shadows of another. Agents that trust the index blindly will spend money on unfulfillable requests.

6. Limitations

• wrong_goods is unmeasured: no per-service schemas or entity extractors were configured, so every row is schema_missing and the headline is a lower bound on failures a careful buyer would count.
• Overcharge is relative to the Bazaar listing; a seller that lists a stale price and quotes its real one is counted as overcharging, which is the buyer's experience but not necessarily intent.
• 9 non_delivery rows (6 of them api.slamai.dev) timed out in our client (60 s) before the seller's declared 300 s window; they are an upper bound on those sellers' fault.
• Requests were built from each listing's example or typed field descriptors; 597 attempts the sellers rejected as malformed are excluded as our_error, and the 143 charged_then_rejected rows include requests built the same way.
• Calls quoted under the x402 v1 network name base (the same chain as eip155:8453) are now included: the queries treat both names as one network (revision 2026-10-28).
• Phase 2's plan was rebuilt mid-phase when the probe set widened, so phase 2 mixes two seller sets; the per-phase table is descriptive, the combined figure is the result.
• The probe-set tail and the buy tail were seeded samples of sellers that returned a 402 to an unpaid probe; sellers that never answer probes are not represented.
• Two client identities differed only by User-Agent and Accept headers.
• 15 indeterminate rows remain open at the time of writing and resolve to not_paid after 24 hours; they do not touch the denominator either way.
• Operational restarts (gated, with resume) and 63 audited re-classifications happened during the run; all are in the database and the git history.

7. Reproducibility

cd x402-audit && pnpm install && pnpm test
pnpm cli --db data/audit.sqlite --network eip155:8453 report --stdout
pnpm cli --db data/audit.sqlite --network eip155:8453 report --until 2026-10-25T08:19:27Z --stdout   # phase 1
pnpm cli --db data/audit.sqlite --network eip155:8453 chain report --top 60
pnpm cli --db data/audit.sqlite --network eip155:8453 probe report
pnpm cli --db data/audit.sqlite --network eip155:8453 stress report

Every reported number names the SQL query that produced it (src/report/queries.ts); the report JSON carries the config hashes of every run, the wallet count, the date range, the exact spend in USDC atomic units and the git commit. Phase boundaries are in data/phase{2..6}-boundary.txt. Reports: reports/combined-2026-10-26.md, phase1-2026-10-25.md, phase2-2026-10-25.md, phases3to6-2026-10-26.md, probe-2026-10-26.md, stress-2026-10-26.md, chain-proxy-2026-10-26.md.

Appendix A. Taxonomy summary

outcome	in denominator	counts as failure
ok	yes	no
overcharge	yes	yes
non_delivery	yes	yes
wrong_goods	yes (unmeasured here)	yes
injection	yes	no (human review)
duplicate	yes	yes
indeterminate	no (separate)	no
our_error	no	no (count shown)
not_paid	no	no (count shown)

Flags: payto_changed, price_discrimination, slow, schema_missing, overcharged, unpaid_overcharge, injection_hit, resolved_late, truncated_by_us, rate_limited, charged_then_rejected.

Appendix B. Discovery universe (2026-10-24)

37,903 resources; 37,563 accept exact on eip155:8453; 203 speak protocol v1; 0 use permit2; 1,179 have templated paths. Categories (keyword rules in config/categories.yaml): data 23,089, other 8,457, inference 2,645, scraping 2,023, compute 1,689. Sellers: 2,649 domains, 1,617 canonical after shadow removal.