iana-registries-mcp-server

Search IANA registries

iana_search_registries

Read-onlyIdempotent

Find IANA protocol registries and sub-registries by keyword over their titles and protocol categories, e.g. "tls cipher", "dns resource record", "protocol numbers". Returns the registry and sub-registry ids that iana_get_registry_records reads, with registration procedure and defining documents. Covers every registry linked from the IANA protocol registries index.

Input Schema

Name	Required	Description
`limit`	No	Requested maximum results, 1–1,000,000; values above 50 are clamped to 50 and disclosed as requested_limit and effective_limit. Default 15.
`query`	Yes	Words matched as whole tokens, singular or plural, against registry titles, categories, and ids, e.g. "tls cipher". An exact registry or sub-registry id ranks first, then the closest titles.
`offset`	No	Number of matches to skip; pass the next_offset of the previous response to get the next page. Default 0.

Output Schema

Name	Required	Description
`cap`	No	The limit applied.
`error`	No	Present when the call failed. Absent on success.
`shown`	No	Results returned.
`notice`	No	Guidance on a miss, a cut list, or another condition.
`source`	No	Provenance and freshness of the answer.
`truncated`	No	True when more matches exist than were returned.
`registries`	No	Matching index entries: exact id hits first, then the closest titles.
`totalCount`	No	Matches before the limit was applied.
`next_offset`	No	Pass as offset for the next page; absent on the last.
`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, 1–50. Default 15."New value: +"Requested maximum results, 1–1,000,000; values above 50 are clamped to 50 and disclosed as requested_limit and effective_limit. Default 15."

changedInput schema / properties / limit / maximum
```
Previous value: -50New 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

A4.1/5.0

Behavior4/5

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

Annotations already cover readOnly, idempotent, and openWorld, so the safety profile is handled. The description adds genuinely useful behavioral context beyond the annotations: it states the output includes registry/sub-registry ids plus registration procedure and defining documents, and asserts full coverage of the IANA protocol registries index.

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?

Three sentences that are front-loaded with the core action and resource, then scope, then output/workflow. Every sentence contributes, though the keyword examples and coverage claim make it slightly longer than strictly necessary.

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

Completeness4/5

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

With an output schema present and 100% parameter coverage, the description need not explain return values. It supplies the scope and the relationship to iana_get_registry_records, which is the remaining value-add; only explicit sibling differentiation 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 coverage is 100%, so all three parameters (query, limit, offset) are already documented, including token matching and pagination behavior. The description's example queries reinforce the schema but add no syntax or format detail beyond it, so the baseline 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 states a specific verb ('Find') and resource ('IANA protocol registries and sub-registries'), and explicitly scopes the search to titles and protocol categories with concrete examples. It also distinguishes itself from the sibling iana_get_registry_records by explaining that this tool produces the ids that tool consumes.

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?

The description clearly names the downstream consumer iana_get_registry_records, implying a search-then-fetch workflow so an agent knows why to call it. However, it does not explicitly contrast itself with the many lookup siblings (e.g. iana_lookup_port), leaving some routing inference to the agent.

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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