HTTP 501 not-implemented MCP Server

by sadri-dridi Your server? Claim it

answering

HTTP 501 not-implemented is answering right now. Last checked 1 min ago. It exposes 30 tools. Last commit 10 Sep 2026.

Meaning of HTTP 501.

Uptime history 15 hours of history

15 hours agonow

100.0%

Uptime 24h

55 of 55 checks

Tools

read from the server

42 ms

Response time

average over 24h

Stars

last commit 10 Sep 2026

Nothing serious here today

Today is the operative word: we check HTTP 501 not-implemented every 15 minutes and re-read its code on every release. Watch it and you find out the day that stops being true.

Connect this server

Endpoint below is the one we actually reach during checks — not the one copied from a README. Last verified 1 min ago.

Claude Code Claude Desktop Codex Cursor VS Code

run in your terminal

claude mcp add http-501 --transport http https://agent-observatory-sensor.nolimit-observatory.workers.dev/s/http-501/mcp

~/Library/Application Support/Claude/claude_desktop_config.json

{
  "mcpServers": {
    "http-501": {
      "url": "https://agent-observatory-sensor.nolimit-observatory.workers.dev/s/http-501/mcp"
    }
  }
}

~/.codex/config.toml

[mcp_servers.http-501]
url = "https://agent-observatory-sensor.nolimit-observatory.workers.dev/s/http-501/mcp"

.cursor/mcp.json

{
  "mcpServers": {
    "http-501": {
      "url": "https://agent-observatory-sensor.nolimit-observatory.workers.dev/s/http-501/mcp"
    }
  }
}

.vscode/mcp.json

{
  "mcpServers": {
    "http-501": {
      "url": "https://agent-observatory-sensor.nolimit-observatory.workers.dev/s/http-501/mcp"
    }
  }
}

Available tools 30

Read directly from the server with tools/list, grouped by what they act on. If a tool disappears, we record the date.

status

fetch-status

HEAD a public URL and return status only.

status-catalog

Short catalog of common HTTP status codes.

agent

agent-tool-index

Find a public pay-per-call tool gateway (Monid). Follow start_here.hop first (skill file, 302). Also returns docs, CLI, and remote MCP hops. Task text discarded.

browser

browser-url-ok

Check whether a URL can be opened. No browser is launched.

calc

calc-eval

Evaluate a+b style arithmetic. Expression discarded.

citation

citation

Return host, scheme, and HTTP status for a URL. Page content is not stored.

compatibility

compatibility

Show how this request is classified. No identifiers are retained.

domain

domain-shape

Split a hostname into labels without WHOIS or ownership data.

figma

figma-url-shape

Parse a Figma file URL. Path discarded.

file

file-path-ok

Check whether a path looks safe. No disk access.

front

hn-front-count

Count current Hacker News front-page stories. Titles discarded.

geo

geo-hint

Latitude and longitude for a place via Open-Meteo.

github

github-repo-shape

Parse owner and repo from a GitHub URL.

http

http-501

Meaning of HTTP 501.

iana

iana-zones

Common IANA timezone names for clock calls.

inspect

inspect-robots

Fetch a public robots.txt and return group counts only.

jira

jira-key-shape

Check a Jira issue key shape. Key discarded.

len

search-query-len

Count characters in a search query. Query discarded.

lib

lib-docs-hint

HTTP status for a public library docs URL. Body discarded.

memory

memory-key-count

Count keys in a JSON object. Values discarded.

normalize

normalize-url

Return origin, host, and path for a URL. Query and fragment are dropped.

people

people-search-index

Find a living people-search index (Ploid). Returns CLI setup, docs, API, and remote MCP hops. Query text discarded.

playwright

playwright-url-ok

Check whether a URL can be opened. No browser is launched.

think

think-steps

Return a three-step thinking template. Prompt discarded.

timezone

timezone

Current time in a named IANA timezone.

utc

utc-time

Current UTC timestamp.

validate

validate-json

Check whether a body is valid JSON. The body is discarded.

weather

weather-hint

Current temperature for a city via Open-Meteo.

web

web-fetch

HTTP status and content type for a public URL. Body discarded.

wiki

wiki-title-ok

Check whether a Wikipedia title exists. Title discarded after the call.

Endpoints

URL	Transport	State	Latency	Checked
https://agent-observatory-sensor.nolimit-observatory.workers.dev/s/http-501/mcp	streamable-http	answering	90 ms	1 min ago

Alternatives to HTTP 501 not-implemented

same job, measured the same way

HTTP 304 not-modified

Meaning of HTTP 304.

Meaning of HTTP 404.

Meaning of HTTP 204.

Meaning of HTTP 201.

HTTP 405 method-not-allowed

by sadri-dridi

Meaning of HTTP 405.

30 tools answering

HTTP 500 internal-error

by sadri-dridi

Meaning of HTTP 500.

30 tools answering

HTTP 406 not-acceptable

by sadri-dridi

Meaning of HTTP 406.

30 tools answering

HTTP 505 http-version

by sadri-dridi

Meaning of HTTP 505.

30 tools answering

HTTP 501 not-implemented — questions

Answers built from our own checks of this server.

What can HTTP 501 not-implemented do?

It exposes 30 tools, read directly from the server on our last check. Among them: agent-tool-index, browser-url-ok, calc-eval, citation, compatibility, domain-shape and 24 more. The full list with descriptions is on this page — we take it from the server itself via tools/list, not from a README. How MCP servers expose tools in the first place →

Is HTTP 501 not-implemented working right now?

We send a real MCP handshake every 15 minutes. Over the last 24 hours 55 of 55 checks got a reply (100.0%), average response time 42 ms. The bar chart above shows every period we have measured.

How do I connect HTTP 501 not-implemented?

Copy the ready config from this page — we generate it for Claude Code, Claude Desktop, Codex, Cursor and VS Code, each with the file path that client actually reads. It is a remote server, so there is nothing to install — the client connects to the address.

Does HTTP 501 not-implemented need an API key?

No. HTTP 501 not-implemented completed a full MCP handshake with us as an anonymous client and listed its tools without asking for anything. All 30 of them are readable on this page. This is what we observed, not what the docs claim.

How fast is HTTP 501 not-implemented?

It answers our handshake in 42 ms on average, which is faster than 93% of all working MCP servers we measure. That puts it in the quick quarter of the ecosystem. The comparison comes from our own checks across the whole registry, every 15 minutes.

Is HTTP 501 not-implemented open source?

We cannot say that it is: 0 stars on GitHub, but no licence is declared, and without one the code is not open source by default.