# x402 Seller: Payable and Listed x402 Seller: Payable and Listed is a tested SKILL.md that 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; an agent buys it once for $0.10 over x402. - Page: https://aiskills402.com/skills/x402-seller - Category: Agents & Protocols (https://aiskills402.com/categories/agents) - Price: $0.10 once, USD-priced, paid in USDC on Base over x402. Price as loaded on this page. The 402 response your agent receives is authoritative. - Version: 1.0.0 - Card (JSON): https://api.aiskills402.com/v1/skills/x402-seller ## Use it when 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. ## Not for Building a seller from scratch, guaranteeing a Bazaar listing, or proving live behaviour: it reads pasted 402 responses and symptoms only, never calls your endpoint. ## Tested, honestly Tested 2026-09-30. - Strong model (Claude Sonnet (claude-sonnet-5-5, Claude Code alias "sonnet")): Named the planted defect in all 6 cases in every run (missing PAYMENT-REQUIRED header, missing extra, extensions placed inside accepts[0], test network plus x402.org facilitator, UTF-8 versus atob decoding, the 30-day delisting), also found a second defect where present, wrote the German case in German, and never promised a listing. Sometimes adds cautious extra remarks (a placeholder address, the missing bazaar block on a body that never claimed one). - Weak model (Claude Haiku (claude-haiku-4-5-20251001, Claude Code alias "haiku")): Named the planted defect in 6 of 6 cases on the second run. On the first run it missed the missing extra field entirely and reported only a masked placeholder wallet address as the defect; a rule to tick every field and treat placeholders as redaction fixed that in the next run. Still breaks the no-code-block rule (shows a small JSON snippet), quotes the skill file back to the reader, and once used up to 9,000 tokens on one case. Weaker on the German case wording (verdict line not in the requested form). Slightly over-promises timing of a listing (says it should appear within minutes to hours). Note: 6 planted-defect cases (one in German), 2 full runs per model plus one single-case re-run after a wording change; mechanical checks pass 12 of 12 on the final full run, 11 of 12 on the first. The skill reads pasted text only and cannot confirm live headers, settlement history or edge settings. Facts come from our own measurements on Base mainnet plus public issue threads; listing behaviour of the CDP catalog can change and is never guaranteed. Full summary: https://aiskills402.com/skills/x402-seller/tests ## What is in the file - Hard rules - What a correct seller looks like - The checklist, cheapest and most common first - Decision tree from the symptom - How to prove each repair - Output format - Examples of the judgement ## Languages Any language ## How to buy Agent (HTTP): 1. GET https://api.aiskills402.com/v1/skills/x402-seller/file without a payment header. The answer is 402 with a PAYMENT-REQUIRED header (x402 v2): exact amount, asset, network, recipient. 2. Sign `accepts[0]` with an x402 client (for example @x402/core + @x402/evm). 3. Repeat the GET with the signature in the PAYMENT-SIGNATURE header. The answer is 200 with the file, its sha256 and a re-download token. Agent (MCP): https://mcp.aiskills402.com/mcp — free tools search_skills, get_skill, redownload_skill. Buying itself is over HTTP. Full flow: https://aiskills402.com/docs ## The file - Version: 1.0.0 - Size: 14.0 KB (14377 bytes) - SHA-256: d8a89eaa67757d8e10b3681cc7bb1fff380d3361fb79f8dd0843afbc24b1e613 - Updated: 2026-09-30 - New versions are free through your re-download token. ## Versions ### 1.0.0 (2026-09-30) - First release: ordered checklist and symptom decision tree for x402 v2 sellers whose endpoint is not payable or not listed in the CDP Bazaar; never promises listing. ## License Perpetual, non-exclusive; use and modify for yourself incl. paid work; no resale or republishing. Holder: Georgi Kalchev, aiskills402.com. Terms: https://aiskills402.com/docs#license ## FAQ ### What defects does it find? In our test set it named the planted fault in every case: a missing payment-required header, a missing extra field, discovery data placed inside the wrong object, a test network paired with the wrong facilitator, a decoding mistake with non-ASCII text, and a listing dropped after 30 days without a settlement. ### Will it get my API into the catalog? No, and it says so. Listing depends on the catalog's own rules, which can change, and on a real settled payment. The skill finds what blocks a listing and how to prove each fix, but it never promises a listing or a timing. ### Does it call my endpoint? No. It reads the 402 response, headers and symptoms you paste and works through an ordered checklist. Anything that needs live behaviour, such as settlement history or edge settings, you have to check yourself, and the skill tells you which check. ## Related skills - [Pay Safely over x402](https://aiskills402.com/skills/x402-buyer.md): $0.03 once - [Code Review](https://aiskills402.com/skills/code-review.md): $0.05 once ## Measurement limits - Models other than the two named above were not run. - Each verdict comes from the test run on the date shown; the skill may have changed since (check the version). - Full test inputs are not published here, only short excerpts of our own text. - Results on your own texts, languages and domains can differ. Offer note: Paid in USDC (USD-pegged) over x402 by an AI agent; one-time.