iana-registries-mcp-server

Look up a URI scheme

iana_lookup_uri_scheme

Read-onlyIdempotent

Look up a registered URI scheme. Pass exactly one of scheme (e.g. "mailto"; a trailing ":" or "://" is ignored) or keyword (words matched against scheme names and descriptions). Returns the status — permanent, provisional, or historical — description, references, and well-known URI support.

Input Schema

Name	Required	Description
`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.
`scheme`	No	Exact scheme name, case-insensitive, e.g. "https" (a trailing ":" or "://" is ignored). Pass this or keyword, not both.
`status`	No	Keep only schemes with this registration status. Applies to both modes.
`keyword`	No	Words matched as whole tokens against scheme names and descriptions, e.g. "websocket". Pass this or scheme, 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 scheme matched.
`shown`	No	Results returned.
`notice`	No	Guidance on a miss, a cut list, or another condition.
`source`	No	Provenance and freshness of the answer.
`schemes`	No	Matching schemes: exact scheme hits first, then registry order.
`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

A4.3/5.0

Behavior4/5

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

Annotations already establish readOnly/idempotent/openWorld, so the bar is lower, and the description usefully adds the input-normalization behavior (a trailing ":" or "://" is ignored) and previews the returned fields. It doesn't discuss pagination limits or result-count behavior, which would have pushed it higher.

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?

Three tight sentences, front-loaded with the purpose, then the invocation rule, then the return context. No filler and nothing important buried.

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?

An output schema exists, so return values need not be explained in depth, and the annotations cover the safety profile. Combined with a 100%-documented parameter schema, the description supplies everything else an agent needs to call it 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 limit, offset, status, scheme and keyword are all already documented with ranges, defaults and mutual-exclusion notes; baseline 3 applies. The description restates the scheme/keyword exclusivity and the normalization rule but adds nothing the schema does not already carry.

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 URI scheme") that immediately separates it from the other iana_lookup_* siblings by resource type. It also names both lookup modes (exact scheme vs keyword), so an agent knows what the tool covers without opening the 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 a concrete selection rule — "Pass exactly one of `scheme` ... or `keyword`" — which tells the agent how to choose the mode. It does not, however, say when to prefer this over iana_search_registries for broader queries, so the alternative-routing is left implicit.

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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