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 one genuinely useful behavioral note — that a miss means only that IANA has no entry, not that the header is invalid — but otherwise restates return content that the output schema already carries.

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

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

Three sentences, front-loaded with the purpose, then the argument constraint, then the caveat about misses. No filler and nothing that fails to earn its place.

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

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 need not enumerate return fields, and the annotations cover safety. The mutual-exclusion constraint and the false-negative caveat are the two things an agent most needs, and both appear. Pagination mechanics are left entirely to the schema, which is acceptable but a minor gap.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so name, keyword, status, limit, and offset are all documented in the schema, including the mutual-exclusion rule in each property. The description reinforces but does not extend that (case-insensitivity and keyword token matching are both already in the schema), so the baseline 3 applies.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

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 (header or trailer) name") and names the exact identifier spaces it searches. It is clearly separable from siblings like iana_lookup_media_type or iana_lookup_http_status based on the resource alone.

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

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

Gives the key operational rule — "Pass exactly one of name or keyword" — and explains when keyword mode applies ("words matched against field names and comments"). It also warns that many widely used headers are unregistered, which guides result interpretation. No explicit routing against siblings such as iana_search_registries, so it falls short of a 5.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.