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

Annotations already declare readOnly, openWorld, idempotent, and non-destructive behavior. The description adds value by naming the returned fields, noting 'Keyless' access, and advising a companion vulnerability search—context not present in the schema. No contradiction with annotations.

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 sentences front-load the core action and deliverables, then add a pairing tip and auth note. No filler or redundant restatement of the schema.

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-parameter read-only lookup with rich annotations and no output schema, the description is complete: it enumerates the return fields, specifies that no key is needed, and points to the relevant vulnerability companion. An agent has sufficient information to select and invoke the tool correctly.

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% and the single parameter already has a clear description with examples ('express' or '@scope/pkg'). The tool description does not add parameter-level meaning beyond that, 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?

The description uses a specific verb ('Look up') and a specific resource ('npm (Node.js) package'), then enumerates the exact data fields returned. This immediately distinguishes it from sibling package/registry tools like pypi_package and cargo_crate.

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?

It provides clear context for when to call this tool—any time an npm package's metadata or download stats are needed—and explicitly recommends pairing with cve_search_by_keyword for vulnerability checking. It does not spell out exclusions such as 'use pypi_package for Python packages,' but the npm scoping makes the boundary obvious.

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