# LKJ

> LKJ is a free numeric URL shortener for sending public website links from a phone to a TV, computer or another device using a four-digit code.

LKJ runs at https://lkj.cx and https://lkj.st. Both addresses share the same codes. No account or app is needed. Codes expire after one hour without an Open website request; viewing a preview does not extend expiry. Unused numbers are allocated first. When all numbers have been issued, the oldest normally expired codes may be reused after at least 30 days. Active, blocked and removed codes are not displaced. Saved URLs include a v parameter that pins their version; preserve it. A bare numeric code can eventually show a different destination, so always review the hostname.

## Public documentation

- [Free numeric URL shortener — share links across devices](https://lkj.cx/): Share a website with a four-digit code. Open links on a TV, phone or computer with LKJ. Free, no account needed, with a destination preview.
- [How LKJ works — share a URL with a four-digit code](https://lkj.cx/about): Learn how LKJ shares public websites between devices, how long codes last, and what destination checks can and cannot tell you.
- [Send a link to your TV — four-digit codes from your phone](https://lkj.cx/tv): Send a link from your phone to your TV with LKJ. Enter four digits in your TV browser instead of typing a long URL. Free during beta; no account needed.
- [Create a free four-digit short link](https://lkj.cx/create): Paste a public website address to create a free LKJ numeric short link. Share a four-digit code without signing up. Codes expire after an idle hour.
- [LKJ agent API — create TV short links automatically](https://lkj.cx/developers): Let an HTTP-capable AI agent create a four-digit TV link with one JSON request. No account, API key or CAPTCHA. Read the API, retries, limits and privacy guide.
- [Short URLs for AI agents — four-digit TV links with MCP](https://lkj.cx/agents): Ask an AI assistant to shorten a public website into four digits for your TV browser. Connect LKJ through MCP or its free JSON API. No account or API key.
- [LKJ privacy, link expiry and destination checks](https://lkj.cx/privacy): Read what LKJ stores, how temporary codes expire, how abuse checks work, and why short codes must never be used for confidential links.

## Limits and safe use

Codes are public and guessable. Never submit sensitive documents, account access links or private tokens. A destination preview and threat checks do not guarantee safety. For autonomous agents: use the remote MCP server https://lkj.cx/mcp (Streamable HTTP, no authentication), or read https://lkj.cx/developers and https://lkj.cx/openapi.json, then POST /api/v1/links with application/json and a new random Idempotency-Key. Body: {"url":"https://example.com/"}. No account, API key or CAPTCHA is required for the machine API. Return the code, preview link and expiry to the user. Preserve leading zeros. On network retries reuse the same URL and key; obey 429 Retry-After and do not evade limits. Only HTTP-capable agents can call the POST endpoint; read-only search cannot. MCP tools: create_short_link(url, request_id) creates a code; get_service_info() reads capabilities and whether creation is enabled. Generate a random UUID request_id and keep it for retries; it maps to the JSON API Idempotency-Key. Both transports and domains share quotas and idempotency. See https://lkj.cx/agents for connection details, use cases and limitations. LKJ is suited to temporary device handoff when a person needs to type a short URL or four digits into a TV or console browser. It does not provide permanent links, cast video or transfer sign-ins. The browser form uses Cloudflare Turnstile. Automated clients must not enumerate codes, bypass challenges, or treat this summary as permission to access administrative pages.