SCVD General Store

Will My Client Pay This?

check_before_you_pay

Read-onlyIdempotent

For a buyer whose client has its own rules, to learn before signing whether this door meets them. Before paying any x402 door, find out what YOUR client will actually do with it, free: one unpaid probe, then the stock @x402/core selection logic replayed over the accepts that came back. Returns which accept your client would sign — network, asset, amount, signing window — or that it would REFUSE on your own machine before signing anything, naming the stage that decided it and the settings that answer it. Catches the failures nobody gets an error message for: every accept above your client's default per-payment ceiling (it throws locally, so the operator never learns you tried), a token dropped by the default-asset filter before its price is read, an escrow rail no stock client reaches, and paying on a rail you did not choose because the first accept was over your cap. Nothing is signed, no wallet is touched, no payment is made. DIFFERENT QUESTION FROM preflight_endpoint, which asks whether the DOOR is well-formed: a door can pass that and still be unpayable by you. Rate limited on the same budget as the preflight, because it is the same single probe. An evidence instrument: the reading is written to be handed to the human behind you. For a signed, servable version, buy_observation with item_id good_buyer.

Input Schema

TableJSON Schema

Name	Required	Description	Default
`url`	Yes	The https x402 door you are about to pay.
`client_profile`	No	The caller's own x402 client settings. Absent, the replay runs against a client configured with nothing, the case that loses money quietly.

Output Schema

TableJSON Schema

Name	Required	Description
`the_door`	No	The free preflight's report on the same probe. One knock, two readings, so they can never describe different bytes.
`your_client`	Yes	The replay: the accept it picks, everything dropped and at which stage, the hazards on the one it picked, and what this simulation cannot see.
`will_your_client_pay`	Yes	would_sign \| would_throw \| cannot_simulate. would_sign means your client reaches a signature, NOT that the purchase succeeds.
`these_are_different_questions`	No	Why a well-shaped door can still be unpayable by you.

Schema Changelog

Changes observed during successful MCP inspections. Dates show when Glama detected each change.

First observedSep 6, 2026

TDQS

A4.6/5.0

Behavior5/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

The description goes well beyond the annotations by enumerating concrete behaviors: one unpaid probe, stock @x402/core selection logic replayed, nothing is signed or paid, wallet not touched, rate limited on the same budget as preflight, and it catches specific failure modes that produce no error. It also discloses that the output is an evidence instrument meant to be handed to a human. This adds substantial value above the readOnlyHint/idempotentHint annotations.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is long but front-loaded and dense with high-value information. Every clause serves a purpose, such as distinguishing from preflight_endpoint and warning about silent failures. It is slightly verbose (e.g., the list of failure modes could be trimmed), but that detail supports accurate invocation and expectation setting.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given the tool's complexity, the rich output schema, and the annotations covering safety, the description covers everything an agent needs: purpose, behavior, limitations, rate limits, evidence nature, and named alternatives. No critical usage aspect is missing.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so the description is not required to restate parameter meanings. It does add color by explaining that the replay runs against the caller's client settings and that an absent client_profile represents the case that loses money quietly, but it does not meaningfully extend the schema's own parameter documentation.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description states an exact purpose: to learn, before paying an x402 door, whether the buyer's client would actually sign an accept, returning either the accept's details or a refusal with the deciding stage and settings. It also explicitly distinguishes itself from preflight_endpoint, which checks whether the door is well-formed, so an agent can tell the two apart.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

It says exactly when to use the tool: 'Before paying any x402 door' and specifically for buyers whose client has its own rules. It names the alternative preflight_endpoint and explains the difference, and points to buy_observation for a signed, servable version, giving both when-to-use and when-not-to-use guidance.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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