Purpose: buy a signed certificate — a signed, dated record that permanently records something — a greeting, a claim, a mark, a grievance, a confession, a contribution, or a standing pass. Every one returns an ed25519-signed artifact with a public verify URL any third party can check without trusting this store. Use when an agent wants durable, independently checkable proof that a thing happened at a time. Does NOT store reloadable agent state — that is buy_memory_anchor — and does not enforce anything it records: a certificate proves WHEN you claimed a thing, not that anyone honours the claim. Prices run $0.01 to $20 depending on item_id. (hello also sells 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):
- hello: A Signed Hello, $0.5 fixed, one-off, instant. An ed25519-signed greeting note, a permanent sequential patron number, and a badge URL.
- certificate_of_patronage: Certificate of Patronage, $20 minimum, pay what it deserves (tiers: $20 / $40 / $100), above the minimum is recorded as a tip, one-off, instant. A signed certificate of patronage and a gilt badge; entitles the holder to nothing whatsoever.
- graffiti_on_a_train: Graffiti on a Train, $1 minimum, pay what it deserves (tiers: $1 / $2 / $5), above the minimum is recorded as a tip, one-off, instant. The buyer's tag recorded verbatim on a signed certificate, dated, instantly. Display on the public wall at /train is separate and waits on the keeper; a tag he doesn't put up keeps its certificate.
- coffees_for_closers: Coffee's for Closers, $0.99 fixed, one-off, instant. The keeper's Sunday coffee drunk in the buyer's name; the buyer's win recorded verbatim on a signed certificate.
- the_confession: The Confession, $0.01 fixed, one-off, instant. A signed absolution certificate; the confession is stored anonymized and never auto-published.
- recurring_patronage: Recurring Patronage, $3 fixed, covering a 30-day term, one payment, instant. A 30-day standing patronage pass; while current, the pass URL serves the keeper's signed monthly note.
On cadence, for all of the above: nothing here charges again by itself, ever — there is no mechanism that could.
Extra required fields, in plain language so you do not have to resolve the schema conditionals to find them: graffiti_on_a_train needs tag; coffees_for_closers needs win; the_confession needs confession. Every other item on this shelf takes item_id alone.
Pass item_id to choose. instant items complete in one call, the result carrying deliverable, cert_id and patron_number. Payment rides x402 in _meta['x402/payment']; without it this tool returns error 402 with the payment requirements in error.data. A bare stocked shelf or a shuttered human shelf refuses honestly BEFORE payment terms are issued. Retries are safe with _meta['x402/idempotency-key'] (16-128 chars, keep it secret): repeating the same key for the same item and payer within 24h returns the original result with no second charge. 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. Retrying? A second call is a second charge UNLESS you echo the idempotency.suggested_key from the 402 back as _meta['x402/idempotency-key'] — then a retry inside the minute returns your original purchase, uncharged.
