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Subdomain Finder MCP

by io.github.theluckystrike
OverviewDiscussionsAdmin

Resolve subdomains (DNS over HTTPS)

resolve_subdomains
Read-onlyIdempotent

Resolve up to 40 hostnames now through Cloudflare DNS over HTTPS (A, AAAA and the CNAME chain). Each host is marked live, nxdomain, no_records or dangling_cname (a CNAME whose target does not exist, a subdomain takeover lead), and CNAME targets on cloud platforms (S3, Azure, Heroku, GitHub Pages, Netlify, Vercel and others) are named. Also probes a random label per parent domain to detect wildcard DNS. DNS lookups only; no connection is made to the hosts.

Input Schema

NameRequiredDescriptionDefault
hostsYesHostnames, for example ["www.example.com", "api.example.com"]. Max 40.
check_wildcardNoProbe one random label under each parent domain (max 3) to detect wildcard DNS.

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observed

TDQS

A4.5/5.0
Behavior5/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Beyond the annotations, the description discloses important behavior: DNS-only lookups, no host connections, result statuses (live, nxdomain, no_records, dangling_cname), cloud CNAME naming, and wildcard probing. This adds substantial operational context.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is front-loaded with the core action and remains dense with useful detail. Every sentence contributes to understanding the tool's output or safety profile.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

With no output schema, the description compensates by explaining result statuses, CNAME chain handling, cloud platform naming, wildcard detection, and the no-connection guarantee. It provides enough context for correct invocation.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, so both parameters are already documented. The description reiterates the 40-host limit and wildcard probing, but does not add syntax or meaning beyond what the schema provides.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description uses a specific verb and resource: resolve hostnames through Cloudflare DNS over HTTPS. It clearly distinguishes resolution from discovery by saying 'now' and describing DNS-only lookups, which helps separate it from siblings like find_subdomains.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

It gives clear context that this is for resolving known hostnames immediately and that no connections are made to hosts. However, it does not explicitly name alternatives such as find_subdomains or state when not to use this tool.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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