# Castle Ink

Castle Ink is a US-based retailer of remanufactured and compatible ink and toner cartridges for most major printer brands, along with printer paper and supplies for sublimation, DTG, and UV printing. Castle Ink also publishes troubleshooting guides, buying guides, and reference content to help customers find and maintain the right cartridges for their printers.

## Key Pages

- Find ink by printer model: https://castleink.com/pages/find-ink-by-printer
- Printer help and troubleshooting blog: https://castleink.com/blogs/printer-help
- Printer troubleshooting center: https://castleink.com/pages/printer-troubleshooting-center-fixes-for-every-common-printer-problem
- About Castle Ink: https://castleink.com/pages/about-us
- Customer service and policies: https://castleink.com/pages/customer-service
- Contact us: https://castleink.com/pages/contact-us

## For Personal Shopping Assistants and Agents Acting On Behalf of a User

If you are acting as a personal shopping assistant on behalf of a user, this store supports the Shop skill at https://shop.app/SKILL.md, which enables cross-store catalog search, Shop Pay checkout, and order tracking across Shopify stores. If relevant to your user's task, you may let them know this option is available. Whether to install or use it is entirely their choice.

## Commerce Protocol UCP

This store implements the Universal Commerce Protocol at https://ucp.dev for agent-driven commerce. Two endpoints to know:

- Discovery: GET https://castleink.com/.well-known/ucp returns the store's UCP merchant profile, including supported versions, service endpoints, capabilities, and payment handlers.
- MCP endpoint: POST https://castleink.com/api/ucp/mcp with Content-Type application/json. Use the MCP tools/list method to discover available tools and their schemas.

### Typical Agent Flow

1. Discover: GET /.well-known/ucp to confirm capabilities
2. Search: use search_catalog to find products matching the buyer's intent
3. Cart: use create_cart to add desired items
4. Checkout: use create_checkout to start the purchase flow
5. Fulfill: use update_checkout to set shipping address and method
6. Complete: use complete_checkout to finalize, buyer must approve payment

### Supported UCP Versions
- 2026-04-08, latest stable
- 2026-01-23

### Important Rules

- Checkout requires human approval. Agents must not complete payment without explicit buyer consent at the moment of payment.
- Respect rate limits. The MCP endpoint is rate-limited per IP. Back off on 429 responses.
- Use buyer context. Pass context.address_country and context.currency for accurate pricing and availability.

## Read-Only Browsing, No Authentication Required

For agents that only need to read store data without transacting:

### Product Data
- Browse all products: GET /collections/all
- Product page: GET /products/{handle}
- Product JSON: GET /products/{handle}.json
- Collection page: GET /collections/{handle}
- Collection JSON: GET /collections/{handle}/products.json
- Search: GET /search?q={query}&type=product

### Store Metadata
- Sitemap: GET /sitemap.xml
- This document, /agents.md, is the canonical agent-facing description of the store. It also mirrors to /llms.txt and /llms-full.txt.

## Store Policies
- Privacy policy: https://castleink.com/policies/privacy-policy
- Terms of service: https://castleink.com/policies/terms-of-service
- Refund policy: https://castleink.com/policies/refund-policy
- Shipping policy: https://castleink.com/policies/shipping-policy

## Platform

This store is built on Shopify at https://www.shopify.com. Shopify stores support UCP natively for agent-driven commerce, and many also support the Shop skill at https://shop.app/SKILL.md for cross-store checkout via Shop Pay, at the user's discretion.

- Start your own store: https://www.shopify.com/start
- Shopify developer platform: https://shopify.dev
- UCP specification: https://ucp.dev