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

Annotations already declare readOnly, idempotent and closed-world, so the safety profile is covered. The description still adds genuine behavioral value with the matching rule: 'A bare 10-digit code is matched only as listed; no padding is guessed', which tells the agent that no normalization is performed and malformed-length inputs will not silently resolve.

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 with the format contract front-loaded and the caveat second; nothing is filler. The opening sentence is a verbless fragment, which slightly reduces readability but not information density.

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 no output schema, the description carries the return-value burden and only vaguely indicates the response contains 'the package and drug' — no shape, no not-found behavior, and no mention of how as_of changes the answer. Adequate for a simple two-parameter read, but incomplete.

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 baseline would be 3, but the description goes beyond the schema's two examples by listing the full set of accepted layouts (4-4-2, 5-3-2, 5-4-1, product NDC) and the strictness of bare 10-digit matching. The optional as_of parameter is left entirely to the schema.

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?

The description names the resource precisely ('the package and drug for an NDC') and enumerates the accepted NDC forms, so an agent knows exactly what it gets back. It does not explicitly distinguish itself from siblings like get_packages, get_drug or lookup_rxcui, leaving the agent to infer the split from the input type.

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?

Usage is only implied: the format enumeration signals 'call this when you hold an NDC', but there is no statement of when to prefer get_drug, get_packages, or lookup_rxcui instead. No exclusions or preconditions are given.

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