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

No annotations are provided, so the description carries the full burden; "look up" implies a read-only, non-destructive call but that is never stated. It also says nothing about match behavior (exact 3-letter match only?), empty-result handling, or that an API key may be required — the auth hint exists only in the schema.

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 tight sentences: the first names the operation and key, the second gives the usage trigger. Nothing is redundant and the operation is front-loaded.

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?

For a single-required-parameter lookup with a fully described schema and no output schema, the description covers what the call does and roughly what it returns (CICs or carrier names). Only the exact result shape and any no-match behavior are unaddressed, which is a minor gap at this complexity.

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 coverage is 100%, so both the acna pattern and the api_key fallback are already documented. The description reinforces that acna is a carrier abbreviation tied to CIC/carrier-name results but adds no format or matching 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) and resource (CIC rows) and clarifies the lookup direction via the 3-letter ACNA key. It implicitly distinguishes itself from the sibling lookup_cic by making ACNA the input, but never names that sibling, so the differentiation is left for the agent to infer.

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?

"Use when the user has an ACNA and wants associated carrier identification codes or carrier names" gives a clear triggering condition and expected payoff. There are no explicit exclusions or pointer to the inverse tool (lookup_cic) for when the agent starts from a CIC instead.

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