iana-registries-mcp-server

Look up an HTTP field name

iana_lookup_http_field

Read-onlyIdempotent

Look up a registered HTTP field (header or trailer) name. Pass exactly one of name (case-insensitive, e.g. "Cache-Status") or keyword (words matched against field names and comments). Returns the registration status — permanent, provisional, deprecated, or obsoleted — the Structured Field type when registered, and the defining reference. Many widely used headers are unregistered; a miss means only that IANA has no entry.

Input Schema

Name	Required	Description
`name`	No	Exact field name, case-insensitive, e.g. "Content-Type" (a trailing ":" is ignored). Pass this or keyword, 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 keyword matches to skip; pass the next_offset of the previous response to get the next page. Default 0.
`status`	No	Keep only fields with this registration status. Applies to both modes.
`keyword`	No	Words matched as whole tokens against field names and registry comments, e.g. "cache". Pass this or name, 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 registered field matched.
`shown`	No	Results returned.
`fields`	No	Matching fields: exact name hits first, then registry order.
`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.
`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, 1–100. 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 declare readOnlyHint, idempotentHint and openWorldHint, so the safety profile is covered. The description adds real behavioral context beyond that: the expected return shape (registration status, Structured Field type, defining reference) and the important negative-result caveat that a miss only means IANA has no entry rather than that the header is invalid.

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?

Four sentences, front-loaded with the purpose before the parameter contract, and each sentence carries distinct load: what it does, how to call it, what comes back, and how to read a negative result. Nothing is padding.

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, the description needn't enumerate return fields, yet it still previews them usefully. It covers mode selection, status filtering, and the false-negative caveat; pagination and limit clamping are left entirely to the schema, which is acceptable but leaves the description one notch short of fully self-contained for a multi-mode lookup.

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 each of the five parameters is already documented in the schema, including the name/keyword mutual exclusion, case-insensitivity, keyword token matching, and the limit clamping behavior. The description largely restates these, adding the concrete example "Cache-Status" but no syntax or semantics not already present.

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 and resource ("Look up a registered HTTP field name") and scopes it precisely to header/trailer field names, which distinguishes it from siblings like iana_lookup_http_status, iana_lookup_media_type, and iana_lookup_uri_scheme. An agent can route correctly without opening any schema.

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?

Gives explicit invocation guidance ("Pass exactly one of `name` ... or `keyword`") and a clear condition for the keyword mode ("words matched against field names and comments"). It stops short of naming alternate sibling tools for when the caller actually wants status codes, language tags, or registry browsing, so it is clear context without explicit alternatives.

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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