x402 Buyer: Pay Safely from an Agent Wallet

Diagnoses why an HTTP API sold over x402 v2 cannot be paid by standard buyer agents or never appears in the Coinbase CDP Bazaar catalog, using an ordered checklist and a decision tree over the pasted 402 response, headers and symptoms. Names the exact field that is wrong or missing and how to prove the fix. Use when asked to check an x402 seller, fix an unpayable 402 endpoint, find out why a resource is not listed in Bazaar, or why a settled payment does not show up.

Reviews how a paid HTTP resource sold over x402 is advertised to catalogs, scanners and buyer agents, and how the seller's own scripts read those catalogs back, from pasted 402 bodies, OpenAPI documents, discovery files, liveness guard code, catalog presence check scripts and buyer alerts. Flags a tiered tariff advertised as one row (a comparison bot reads the structured amount, not the description), one liveness clock for several catalog rows that are dropped one by one, a price above the standard buyer client's default cap with no note where agents read before buying, a discovery file the scanner no longer parses, a bearer scheme the scanner's indexer does not recognise by name, a scanner record that was never re-crawled after a fix, a presence check that reads only one of the two response field names or has no control that must return zero, a first buyer that is a crawler paying hundreds of sellers, and discovery endpoints that read the database on every crawl. Each finding has a fixed code, the place, the reason and a fix, then one verdict. Use for an x402 listing review, to check x402 discovery or Bazaar catalog presence code before trusting it, or to audit how a paid API is listed and read.

Reviews ONE piece of retry code, or a written retry design for an agent or a service client, and lists only the five mistakes that turn a failure into an outage or a double write, one line each with a fixed code, the place, why it hurts and the fix - or exactly No findings. It catches waits that grow without a random part, a loop with no limit on tries or on total time, a POST repeated with no idempotency key, a 429 or 503 answer whose Retry-After header is never read, and retries on client errors that cannot change. It leaves out the safe look-alikes, such as a GET retried with a cap and jitter, a retry set of 408, 429 and server errors only, and a wait taken straight from Retry-After. Use to review the retry loop of an API client, to audit how an agent calls tools that fail, or before shipping a client to production.

Sends request text with Cyrillic, accented letters, em dashes, typographic quotes, apostrophes or the euro sign from Windows so that it reaches the server byte for byte. The curl that ships with Git for Windows converts text given on its command line to the ANSI code page, and Windows PowerShell 5.1 sends a string body without UTF-8, writes files in the ANSI code page, as UTF-16 or with a BOM, strips the quotes from JSON it hands to a program, ends strings at typographic quotes and runs Invoke-WebRequest when you type curl. The server still answers 200 and the page later shows � or ?. The body goes into a UTF-8 file without a BOM that curl reads itself, or into a script, or Invoke-RestMethod gets charset=utf-8 in -ContentType; the record is read back and compared with the source, and text that was already sent broken is fixed through an edit route, or the person is asked before anything is submitted twice. Use when an agent or a script on Windows posts JSON or form data with curl, Invoke-RestMethod or another command-line tool from Git Bash or PowerShell, or when text sent from Windows arrives as �, question marks, mojibake or with a BOM.

Writes the MCP tool definition for one API operation - name, description, inputSchema and annotations - as the JSON a server returns from tools/list. The schema takes exactly the parameters the operation takes - required ones listed, defaults stated, exact enums, integer amounts, date formats, ranges, string and array limits, either-or parameters that refuse a call with both or neither - and refuses unknown ones. Credentials never become parameters, text in the API docs that speaks to the assistant stays out of the description, one tool does one operation, and every tool gets a title and all four annotations as explicit true or false (reads, safe repeats, destructive writes, open world), as the MCP specification of 2025-11-25 and the Claude and ChatGPT directories expect. Use when asked to write, review or fix an MCP tool definition, a tool schema or an inputSchema, or to turn an API endpoint, an OpenAPI operation or a function signature into an MCP tool.