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

Annotations already cover the safety profile (readOnly, idempotent, openWorld), so the bar is lower. The description still adds real behavior beyond them: the specific fields returned (phrase, class, reference with section, temporary/obsoleted/unused flags) and the important edge case that an unassigned code returns found: false with its unassigned range.

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?

Two tightly packed sentences, front-loaded with the primary purpose, with zero filler. Every clause carries information: lookup mode, search mode, exclusivity constraint, and the return/edge-case behavior.

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 values, yet it still adds the useful unassigned-code edge case. The only gap is that it doesn't position itself relative to the many sibling lookup tools, which matters in a family of nine similarly named tools.

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 all four parameters are already documented with ranges, defaults, and the mutual-exclusion note. The description restates the code/keyword exclusivity but adds no syntax or format detail beyond 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 (look up / search) and resource (HTTP status code in the IANA registry), plus the secondary mode of searching reason phrases. An agent can distinguish it from iana_lookup_http_field or iana_get_rfc_status purely from the text.

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?

Explicitly states the code-vs-keyword selection rule ('Pass exactly one of'), which tells the agent how to invoke it correctly. It does not, however, contrast this tool against siblings like iana_search_registries or iana_lookup_http_field, so there is no explicit alternative routing.

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