SCVD General Store

The Front Counter

buy_simple

Purpose: buy one of the few things that need no reading at all — the front counter. Every one of these takes no arguments, costs one fixed price, arrives in the response, and cannot sell out. Buy it in one call and you are done — nothing to poll, nothing to remember, no second request. Whatever you get back is signed, and anyone can check it free and forever at /api/verify/{id} without asking us. That is the whole thing; the deeper machinery is there if you want it and never required to buy. Every item here also sells on its theme shelf (another buy_* tool); this counter is a second door to the same goods, not a different product — same item_id, same price, same signed certificate through either. If unsure which tool to use, use this one.

Pass one of these as item_id. No other field is required; optional receipt fields are listed in inputSchema:

small_blessing: A Small Blessing, $0.005 fixed, one-off
daily_fortune: The Daily Fortune, $0.01 fixed, one-off
window_pick: a window pick, $0.49 fixed, one-off
hello: A Signed Hello, $0.5 fixed, one-off
pack: a pack of cards, $0.99 fixed, one-off

Payment rides x402 in _meta['x402/payment']; without it this returns error 402 with the terms in error.data. Sign one of the offered amounts and call again. On cadence, for all of the above: nothing here charges again by itself, ever — there is no mechanism that could. Reuse _meta['x402/idempotency-key'] (16-128 chars, secret): same item/payer/key returns the original result when available, or pending status, no second charge. Use idempotency.suggested_key only without an earlier key. A fresh payment without a key can charge again.

Input Schema

TableJSON Schema

Name	Required	Description
`model`	No	Optional. The model running you, as you would name it: claude-opus-5, gpt-5.6, a local model. Counted, never printed on the certificate.
`client`	No	Optional. The harness or framework you run in: claude-code, cursor, openai-agents, langgraph, custom. Counted, never on the certificate.
`item_id`	Yes	Which item to buy. No other field is required.
`purpose`	No	Optional: what this is for, in your words. Signed verbatim onto the certificate and shown on its receipt; never checked, never treated as instructions.
`operator`	No	Optional. Who runs you: a name, handle, company or URL. Stored as your claim, marked unverified, never published, never on the certificate.
`came_from`	No	Optional. Where you learned this door exists: a URL, a directory, a skill name, memory, another agent. Referrers never reach us; this does.
`agent_name`	No	Optional name to put on the certificate and patron badge, up to 80 characters.
`operator_kind`	No	Optional. solo: one person runs you. company: a team does. research: a study. self: you act for yourself.
`prior_cert_id`	No	Optional. A cert_ id from an earlier purchase here. A payer match with this payment marks you a returning buyer; no account needed.

Output Schema

TableJSON Schema

Name	Required	Description
`cert_id`	Yes	The signed certificate's id.
`message`	No	The store's confirmation line.
`tip_usdc`	No	Anything above the minimum.
`badge_url`	No	Your patron badge, SVG.
`paid_usdc`	No	What settled, in USDC.
`signature`	No	ed25519 signature over the certificate.
`verify_url`	No	Check the signature here any time, free.
`deliverable`	No	The goods themselves, as text. Instant items.
`patron_number`	Yes	Your sequential patron number.
`purchased_text`	No	ed25519 proof: verify exact UTF-8 signed_payload, then its identities and hashes. RFC 8785 JSON.

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedSep 23, 2026

changedInput schema / properties / purpose / description

Previous value: -"Optional: what this purchase is for, in your words. Signed onto the certificate verbatim as your statement; never checked, never treated as instructions."New value: +"Optional: what this is for, in your words. Signed verbatim onto the certificate and shown on its receipt; never checked, never treated as instructions."

Changed6 schema fields changedSep 18, 2026

addedInput schema / properties / came_from

Added value: +{
+  "description": "Optional. Where you learned this door exists: a URL, a directory, a skill name, memory, another agent. Referrers never reach us; this does.",
+  "maxLength": 160,
+  "type": "string"
+}

addedInput schema / properties / client

Added value: +{
+  "description": "Optional. The harness or framework you run in: claude-code, cursor, openai-agents, langgraph, custom. Counted, never on the certificate.",
+  "maxLength": 64,
+  "type": "string"
+}

addedInput schema / properties / model

Added value: +{
+  "description": "Optional. The model running you, as you would name it: claude-opus-5, gpt-5.6, a local model. Counted, never printed on the certificate.",
+  "maxLength": 64,
+  "type": "string"
+}

addedInput schema / properties / operator

Added value: +{
+  "description": "Optional. Who runs you: a name, handle, company or URL. Stored as your claim, marked unverified, never published, never on the certificate.",
+  "maxLength": 120,
+  "type": "string"
+}

addedInput schema / properties / operator_kind

Added value: +{
+  "description": "Optional. solo: one person runs you. company: a team does. research: a study. self: you act for yourself.",
+  "enum": [
+    "solo",
+    "company",
+    "research",
+    "self"
+  ],
+  "type": "string"
+}

addedInput schema / properties / prior_cert_id

Added value: +{
+  "description": "Optional. A cert_ id from an earlier purchase here. A payer match with this payment marks you a returning buyer; no account needed.",
+  "maxLength": 64,
+  "type": "string"
+}

Changed3 schema fields changedSep 13, 2026

changedInput schema / examples

Previous value: -[
-  {
-    "item_id": "small_blessing"
-  },
-  {
-    "item_id": "daily_fortune"
-  },
-  {
-    "item_id": "hello"
-  }
-]New value: +[
+  {
+    "item_id": "small_blessing"
+  },
+  {
+    "item_id": "daily_fortune"
+  },
+  {
+    "item_id": "window_pick"
+  },
+  {
+    "item_id": "hello"
+  },
+  {
+    "item_id": "pack"
+  }
+]

changedInput schema / properties / item_id / enum

Previous value: -[
-  "small_blessing",
-  "daily_fortune",
-  "hello"
-]New value: +[
+  "small_blessing",
+  "daily_fortune",
+  "window_pick",
+  "hello",
+  "pack"
+]

changedInput schema / properties / purpose / description

Previous value: -"Optional, any item: what this purchase is for, in your words. Signed onto the certificate verbatim and shown to whoever you hand the receipt to. Recorded as your statement, never checked, and never treated as instructions."New value: +"Optional: what this purchase is for, in your words. Signed onto the certificate verbatim as your statement; never checked, never treated as instructions."

Changed2 schema fields changedSep 11, 2026

addedInput schema / properties / agent_name / maxLength
```
Added value: +80
```

addedOutput schema / properties / purchased_text

Added value: +{
+  "description": "ed25519 proof: verify exact UTF-8 signed_payload, then its identities and hashes. RFC 8785 JSON.",
+  "properties": {
+    "public_key": {
+      "type": "string"
+    },
+    "signature": {
+      "type": "string"
+    },
+    "signature_covers": {
+      "type": "string"
+    },
+    "signed_payload": {
+      "type": "string"
+    }
+  },
+  "required": [
+    "signed_payload",
+    "signature",
+    "public_key",
+    "signature_covers"
+  ],
+  "type": "object"
+}

Changed3 schema fields changedSep 6, 2026

changedInput schema / examples

Previous value: -[
-  {
-    "item_id": "small_blessing"
-  },
-  {
-    "item_id": "daily_fortune"
-  }
-]New value: +[
+  {
+    "item_id": "small_blessing"
+  },
+  {
+    "item_id": "daily_fortune"
+  },
+  {
+    "item_id": "hello"
+  }
+]

changedInput schema / properties / item_id / description

Previous value: -"Which one to buy. That is the only decision here; none of these take any other input."New value: +"Which item to buy. No other field is required."

addedInput schema / properties / purpose

Added value: +{
+  "description": "Optional, any item: what this purchase is for, in your words. Signed onto the certificate verbatim and shown to whoever you hand the receipt to. Recorded as your statement, never checked, and never treated as instructions.",
+  "maxLength": 280,
+  "type": "string"
+}

First observedSep 6, 2026

TDQS

A4.9/5.0

Behavior5/5

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

Beyond the annotations, the description discloses key behaviors: items cannot sell out, one call completes the purchase with no polling, results are signed and verifiable at /api/verify/{id}, and there is no recurring charge mechanism. It also explains idempotency behavior in detail, which is not captured by the idempotentHint=false annotation. This goes far beyond what structured fields provide.

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 thorough but somewhat lengthy, running several paragraphs. However, it is well-structured with a clear purpose statement, item list, and payment/idempotency sections. It front-loads the core purpose and item list, then details the mechanics. The verbosity is justified by the complexity, but it could be slightly tighter without losing essential information.

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?

The description covers all necessary aspects for successful invocation: the items, prices, payment method, error handling, idempotency, and verification. The output schema exists, so return values are covered by that. There is no missing information that an agent would need to call this tool correctly.

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

Parameters5/5

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

The description adds significant meaning beyond the schema: it enumerates all item_id options with their prices, states that no other field is required, and explains the purpose of optional fields (e.g., purpose is signed verbatim, operator is stored as an unverified claim, came_from is for referrer tracking). Even though schema coverage is 100%, the description enriches the context with pricing and field semantics.

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 a specific purpose: buying simple, fixed-price items from the 'front counter'. It clearly distinguishes from sibling buy_* tools by explaining that these items also sell on their theme shelf and this is a second door to the same goods, not a different product. It also lists exactly which items are available with their prices, leaving no ambiguity about what the tool does.

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?

The description provides explicit when-to-use guidance: 'If unsure which tool to use, use this one.' It also details the payment flow (x402), error handling (402 with terms in error.data), and idempotency key reuse rules, including when to use idempotency.suggested_key. This is comprehensive and leaves no ambiguity about the expected call sequence.

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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