SCVD General Store

The Penny Shelf

buy_small_pleasure

Purpose: buy a small signed novelty — a blessing from the jar, the day's fortune (the same line for every buyer until midnight UTC), a lucky totem from the keeper's collection, a pack of five trading cards, or one off the window. Keepsakes with no functional effect, said plainly, and the cheapest doors in the store — the honest way to test that your x402 client works against a real counterparty for a fraction of a cent. Or when an agent simply wants one. Prices run $0.005 to $0.99 depending on item_id. (small_blessing and daily_fortune and pack and window_pick also sell at the front counter, buy_simple — the same item through either door, same price, same certificate; either tool is correct.)

Items on this shelf (pass one as item_id):

small_blessing: A Small Blessing, $0.005 fixed, one-off, instant. Settle a real x402 payment for the smallest amount possible
daily_fortune: The Daily Fortune, $0.01 fixed, one-off, instant. Read the same line every other agent gets today
luckies: a lucky, $0.99 minimum, pay what it deserves (tiers: $0.99 / $1.98 / $4.95), above the minimum is recorded as a tip, one-off, instant. Be issued a charm from a herd the keeper wrote, drawn on odds he weighted
pack: a pack of cards, $0.99 fixed, one-off, instant. Pull five collectible trading cards of this store from a set the keeper wrote, on odds he weighted and published, under a seed I can check tomorrow
window_pick: a window pick, $0.49 fixed, one-off, instant. Take one pressing off the last five anybody pulled here, chosen by the day seed, for half a pack

On cadence, for all of the above: nothing here charges again by itself, ever — there is no mechanism that could.

Only item_id is required on this shelf.

Choose item_id. instant items return deliverable, cert_id and patron_number in one call. x402 payment: _meta['x402/payment']. Without payment: error 402 with the terms in error.data. Closed or empty shelves refuse before quoting. 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. Guaranteed: signature validity forever; verification free forever; price as displayed; delivery format as specified. Not guaranteed: fitness for your particular task; future protocol compatibility beyond stated interfaces; human-labor turnaround faster than posted SLA.

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	Required item; its other required fields are in allOf.
`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 24, 2026

changedInput schema / properties / item_id / description

Previous value: -"Which item on this shelf to buy. Required. Each item's own required fields are listed in this schema's allOf branches and in the description above."New value: +"Required item; its other required fields are in allOf."

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

changedInput schema / properties / item_id / enum

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

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"
+}

First observedSep 6, 2026

TDQS

A4.9/5.0

Behavior5/5

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

Annotations only provide readOnlyHint=false and destructiveHint=false; the description goes far beyond by explaining the purchase flow (returns deliverable, cert_id, patron_number), error handling (402 with terms), idempotency key reuse, no recurring charges, and specific guarantees and non-guarantees. This is rich behavioral disclosure not covered by 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 well-structured with bullet points and clear sections. It front-loads the purpose and item list, then covers cadence, idempotency, and guarantees. Every sentence adds information; there is no filler, though the length might be slightly overwhelming for some agents.

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 return format, error cases (402, closed/empty shelves), idempotency requirements, and guarantees. Given the tool's complexity (multiple item types, pricing, idempotency), it is complete. The presence of an output schema means return details do not need to be repeated, and the description fills all other gaps.

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?

Although schema coverage is 100%, the description adds semantic value for item_id (lists each option with price and behavior), and explains the purpose/operator/came_from fields (signed verbatim, never checked). It clarifies that only item_id is required and what each optional field does, which is not apparent from the schema alone.

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 verb ('buy') and resource ('small signed novelty'), then enumerates exactly which items are available (blessing, fortune, lucky, pack, window pick) with prices. It differentiates from siblings by noting these are the 'cheapest doors in the store' and explicitly contrasts with buy_simple, making the tool's role unambiguous.

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 explicitly says when to use this tool: 'the honest way to test that your x402 client works' or when an agent simply wants a keepsake. It also notes that buy_simple is an equivalent alternative for the same items, and clarifies cadence ('nothing here charges again by itself'), preventing misuse.

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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