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

Beyond the annotations (readOnly, idempotent), the description reveals important runtime behavior: fan-out across two external services, partial failures degrading gracefully, bundlephobia's first measurement can take 5-30s, and 'sources_failed will list it if it times out'. This is valuable non-obvious information.

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?

The description is front-loaded with the core purpose, then gives usage trigger, output structure, ecosystem scope, and failure behavior. Each sentence serves a distinct purpose and there is no filler. Despite its length, it is concise for the complexity it covers.

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 composite tool with two parameters and no output schema, the description thoroughly explains what is returned (summary block fields, per-advisory details, links, alternative versions) and the failure mode (sources_failed). This is sufficient context for an agent to invoke and interpret results.

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?

The input schema already describes both parameters with 100% coverage. The description adds no further parameter-specific semantics beyond what the schema provides; however, it confirms 'package' is an npm package name and 'version' is optional, which is already in the schema. 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 opens with a specific verb and resource: 'Composite "should I add this npm package to my project" check in ONE call' and then details the two data sources (deps.dev, bundlephobia). It clearly distinguishes itself from siblings by stating it is a composite check for npm packages, which is unique among the listed tools.

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?

Explicit usage guidance is given: 'Use whenever an agent asks "is X safe / popular / small" or "what does adding lodash cost me".' It also provides exclusionary context: 'NPM ecosystem only in v1; PyPI / Maven / Cargo / Go fall under deps.dev:version directly.' This tells when not to use and directs to alternatives.

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