iana-registries-mcp-server

Look up a Private Enterprise Number

iana_lookup_pen

Read-onlyIdempotent

Look up a Private Enterprise Number (PEN). Pass exactly one of pen (a number such as 32473, or an OID under 1.3.6.1.4.1 such as 1.3.6.1.4.1.32473.1.2) or organization (words matched against organization names). Returns the organization and its OID prefix. Arcs below the enterprise number are assigned by the enterprise, not IANA. Contact details are not returned.

Input Schema

Name	Required	Description
`pen`	No	Enterprise number, e.g. "32473", or an OID under 1.3.6.1.4.1 naming it, e.g. "1.3.6.1.4.1.32473.1.2" (a leading dot and the iso.org.dod.internet.private.enterprise prefix are accepted). Pass this or organization, not both.
`limit`	No	Requested maximum results, 1–1,000,000; values above 100 are clamped to 100 and disclosed as requested_limit and effective_limit. Default 25.
`offset`	No	Number of organization matches to skip; pass the next_offset of the previous response to get the next page. Default 0.
`organization`	No	Words matched as whole tokens against the names of assigned entries, e.g. "cisco systems"; look up a number with pen to see a reserved or unassigned one. Pass this or pen, not both.

Output Schema

Name	Required	Description
`cap`	No	The limit applied.
`mode`	No	Which lookup ran.
`error`	No	Present when the call failed. Absent on success.
`found`	No	True when a returned number is assigned to an organization.
`shown`	No	Results returned.
`notice`	No	Guidance on a miss, a cut list, or another condition.
`source`	No	Provenance and freshness of the answer.
`sub_arcs`	No	Arcs below the enterprise number, e.g. "1.2"; the enterprise assigns these.
`truncated`	No	True when more matches exist than were returned.
`totalCount`	No	Matches before the limit was applied.
`enterprises`	No	Matching entries: exact organization-name hits first, then number order.
`next_offset`	No	Pass as offset for the next page; absent on the last.
`requested_oid`	No	The OID as requested, when it named arcs below the enterprise number.
`effective_limit`	No	The response limit after clamping, present when clamped.
`requested_limit`	No	The requested limit, present when clamped.

Schema Changelog

Changes observed during successful MCP inspections.

Changed4 schema fields changedOct 17, 2026

changedInput schema / properties / limit / description

Previous value: -"Maximum number of results to return, 1–100. Default 25."New value: +"Requested maximum results, 1–1,000,000; values above 100 are clamped to 100 and disclosed as requested_limit and effective_limit. Default 25."

changedInput schema / properties / limit / maximum
```
Previous value: -100New value: +1000000
```

addedOutput schema / properties / effective_limit

Added value: +{
+  "description": "The response limit after clamping, present when clamped.",
+  "type": "number"
+}

addedOutput schema / properties / requested_limit

Added value: +{
+  "description": "The requested limit, present when clamped.",
+  "type": "number"
+}

First observedOct 2, 2026

TDQS

A4.2/5.0

Behavior4/5

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

Annotations already establish readOnlyHint, idempotentHint, and openWorldHint, so the safety profile is covered. The description still adds real behavior beyond them: what is returned (organization and OID prefix), what is explicitly not returned (contact details), and the scope boundary that sub-arcs are enterprise-assigned rather than IANA-assigned.

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

Conciseness4/5

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

Front-loaded with the action and resource, then the input constraint, then return semantics and caveats in a compact block. The input-selection sentence partially duplicates the schema's own "Pass this or organization, not both" wording, which is the only real slack.

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, the description does not need to enumerate return fields, and annotations carry the safety profile. What remains — how to supply the identifier, mutual exclusivity, the scope caveat about enterprise-assigned arcs, and the absence of contact data — is exactly the residual an agent needs to call this correctly.

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 the schema already documents the pen pattern, the OID example, the mutual exclusivity, and the limit clamping behavior in more detail than the description. The description's restatement of the either/or constraint and the OID example adds no meaning beyond the schema, so baseline 3 is appropriate.

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?

States a specific verb (look up) and a precisely named resource (Private Enterprise Number), with the canonical form and both accepted input shapes. It is immediately distinguishable from siblings such as iana_lookup_port, iana_lookup_media_type, or iana_lookup_uri_scheme, which target entirely different registries.

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?

"Pass exactly one of pen ... or organization" gives explicit selection guidance between the two input modes, and the note that arcs below the enterprise number are assigned by the enterprise rather than IANA implicitly warns against using this tool to resolve sub-arcs. It stops short of naming an alternative tool for those cases, so it is clear context without full when-not routing.

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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