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Find subdomains

find_subdomains

Read-onlyIdempotent

List the hostnames found under a domain in TLS certificates: one row per hostname with the number of certificates and first/last seen dates. Covers certificates valid now or expired within the last 90 days; set history to true for every hostname certified since 2013 (needs an API key on the Pro Lite or Pro plan). Use this to enumerate subdomains; use search_certificates for the certificates themselves.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "domain": "<domain>"
    },
    "name": "find_subdomains"
  }
}

Input Schema

Name	Required	Description
`cursor`	No	The next_cursor of the previous call, to get the next page; omit for the first page
`domain`	Yes	A bare DNS name such as example.com, without a scheme, path or port
`history`	No	true lists every hostname certified since 2013 instead of the last 90 days; needs an API key on the Pro Lite or Pro plan

Output Schema

Name	Required	Description	Default
`count`	Yes
`hosts`	Yes
`scope`	Yes
`domain`	Yes
`has_more`	Yes
`next_cursor`	No

Schema Changelog

Changes observed during successful MCP inspections.

First observedNov 12, 2026

TDQS

A4.7/5.0

Behavior5/5

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

Annotations already declare a safe, idempotent read, and the description adds value beyond them: the data source is TLS certificates, the default coverage is now-or-expired-within-90-days, history widens it to 2013, and that mode requires an API key on Pro Lite or Pro. The plan gating is exactly the kind of auth prerequisite that annotations cannot express.

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?

Two sentences, both dense with information: the first front-loads what is returned, the second covers scope, the history escalation, and the sibling alternative. No filler or redundancy.

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 an output schema present and full schema coverage, the description only needs to carry scope, escalation behavior, prerequisites, and sibling routing — and it does all four. Nothing an agent needs to invoke this correctly is missing.

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 description coverage is 100%, so all three parameters are already documented, including the domain format and the history/plan caveat. The description restates the history semantics but adds no syntax or edge-case detail beyond the schema, so the baseline of 3 applies.

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 gives a specific verb and resource ("List the hostnames found under a domain in TLS certificates") and even states the row shape (one row per hostname with certificate count and first/last seen dates). It explicitly distinguishes itself from search_certificates, so an agent can choose between them without opening either schema.

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

Usage Guidelines5/5

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

Usage is stated directly ("Use this to enumerate subdomains") alongside the explicit alternative and its purpose ("use search_certificates for the certificates themselves"). It also documents the conditions for the history mode, including the plan/API-key prerequisite.

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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