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‹ CHK-A04
CHK-A05AI search & agents · agentMedium

Structured errors returned in machine-readable format for agent flows

When an agent-facing endpoint is declared, errors are returned as structured machine-readable responses (JSON with an error code/field) rather than HTML error pages.

Check my store for this rule →Cite this
Free · no card · runs all rules at once · judged on 39 stores · updated
100%of the audited stores we could judge fail this rule
0 pass0 partial39 failPublished audits · September 2026
Stores judged
39
of 40 published audits
Score weight
7.7pts
of the AI Shoppers score
Audience
AI
reported separately

What earns each verdict

Same outcomes on every UXFix rule · how verdicts work
✓ Pass0

Error response is JSON with a machine-parsable error code and a field/reason reference.

~ Partial0

JSON returned but without an error code or field reference (message string only).

✕ Fail39

No agent endpoint declared (CHK-A01 fail), OR the endpoint returns an HTML error page, a bare status code with no body, or unstructured text.

not assessable is recorded when the rule applied but the store could not be judged on it (bot protection, a region lock, a step we could not reach), and not applicable when the store has nothing for the rule to judge. Those stores are left out of every number on this page, never guessed. Why we do that

How we test it

Scripted walk, fresh signed-out browser · the full walk
  1. 1Fresh browserSigned out, cookies cleared, desktop and mobile viewports, the store's live theme.
  2. 2Open a productA real, in-stock product page, found the way a shopper finds one.
  3. 3Add to cartDefault variant, quantity one, then the cart page itself.
  4. 4Walk the checkoutFrom the cart to the payment step. Nothing is ever bought.
  5. 5Read it like an agentThe markup and structured data a shopping agent reads, not the layout.
  6. 6Record the verdictPass, partial or fail, saved with the screenshot it was judged on.
What the agent looks for
When an agent-facing endpoint is declared, errors are returned as structured machine-readable responses (JSON with an error code/field) rather than HTML error pages.
Confidence
The rulebook rates this check’s confidence unstated. When the agent is not sure, the rule is recorded as not assessable rather than guessed.
Not judged
1 of 40 published audits: the rule did not apply or could not be judged. Dropped, never guessed.

The numbers behind the rule

Published UXFix audits · more statistics
Published audits failing this rule
and 27 more, newest audit first.
No pictures here: this rule is judged inside the page code, which no shopper ever sees, so there is nothing to photograph.
Where this rule sits among 12 agent rules
Most-failed rank
#1
Score weight rank
#11
Fail rate
100%

Score weight: a fail costs 7.7 pts of of the AI Shoppers score; a partial costs half that. Most-failed rank is among the 79 rules with enough published audits to report a rate.

Every store that fails this rule

The 39 audited stores that fail Structured errors returned in machine-readable format for agent flows, with each store's published score and the day it was audited.
#StoreScoreAuditedPublished audit
1Leathermanleatherman.com65Open audit
2Ooniooni.com64Open audit
3Archeryworldarcheryworld.co.uk52Open audit
4Bxbx.eu46Open audit
5Barnesandnoblebarnesandnoble.com61Open audit
6Mujimuji.us61Open audit
7Arhausarhaus.com60Open audit
8Blackstoneproductsblackstoneproducts.com60Open audit
9Tupperwaretupperware.com60Open audit
10Caspercasper.com62Open audit
11Thebrickthebrick.com61Open audit
12Wpswingswpswings.com57Open audit
13Lavernelaverne.com60Open audit
14Benettonbenetton.com58Open audit
15Keenfootwearkeenfootwear.com56Open audit
16Verabradleyverabradley.com58Open audit
17Everlaneeverlane.com62Open audit
18Davidsbridaldavidsbridal.com58Open audit
19Tomstoms.com52Open audit
20Bodenboden.com64Open audit
21Eddiebauereddiebauer.com62Open audit
22Forever21forever21.com57Open audit
23Gymsharkgymshark.com57Open audit
24Reebokreebok.com58Open audit
25Reebokreebok.com57Open audit
26Kathmandukathmandu.com.au63Open audit
27Bentibrandbentibrand.com66Open audit
28Ladounladoun.com41Open audit
29Toysrustoysrus.com57Open audit
30Brewdogbrewdog.com56Open audit
31Zulilyzulily.com61Open audit
32Mejurimejuri.com65Open audit
33Fashionphilefashionphile.com63Open audit
34Timextimex.com54Open audit
35Citstorescitstores.com65Open audit
36Enviafloresparadifuntosenviafloresparadifuntos.com56Open audit
37Wpfactorywpfactory.com70Open audit
38Iliabeautyiliabeauty.com62Open audit
39Tentreetentree.com58Open audit

What our auditors say about this rule

An error an agent cannot parse is an order it will either retry blindly or abandon.
Abdulhameid Grandoka, Co-founder, UXFix

Written by the person who audits this rule, and reviewed every quarter.

What this rule rests on

The published research and standards this rule is drawn from. Open them and check the reasoning for yourself.

Keep going

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Agent rulePDP-A04
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Agent ruleCRT-A01
Cart state reachable via stable URL or API endpointAI search & agents · Medium ›
Agent ruleCRT-A02
No session trap that breaks agent-driven cart hydrationAI search & agents · Medium ›
Agent ruleCRT-A03
Cart line items include machine-readable product IDs/GTINsAI search & agents · Low ›
Agent ruleCHK-A01
Agentic checkout readiness signals (ACP/UCP endpoints declared)AI search & agents · High ›
Agent ruleCHK-A02
No CAPTCHA-only gate without accessible agent pathAI search & agents · Medium ›
Cite this

«39 of 39 online stores audited by UXFix fail the rule "Structured errors returned in machine-readable format for agent flows" (CHK-A05) as of September 2026. UXFix, uxfix.ai/audits/rules/machine-readable-checkout-errors»

Rule record · JSON
Dataset →
{
  "id": "CHK-A05",
  "name": "Structured errors returned in machine-readable format for agent flows",
  "area": "agent",
  "severity": "medium",
  "weight": 7.69,
  "published_audits": {
    "judged": 39,
    "pass": 0,
    "partial": 0,
    "fail": 39
  },
  "updated": "2026-09-23"
}
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