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

The description adds behavioral context far beyond the annotations: it discloses that this is a composite call fanning out to two services, that partial failures degrade gracefully, that bundlephobia's first measurement can take 5-30 seconds, and that sources_failed will list timeouts while the rest still returns. This is valuable information not present in readOnlyHint/idempotentHint/etc.

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?

Every sentence earns its place: first sentence defines purpose and sources, second gives usage triggers, third lists return shape, fourth scopes the ecosystem and points to alternative, fifth explains failure behavior. There is no redundancy or filler; the density is high but well-organized with dashes and semicolons.

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?

Despite having no output schema, the description enumerates the full return block (summary fields, per-advisory detail, links, alternative versions) and explains edge-case behavior (new-version timeout, sources_failed list). For a two-parameter tool with this complexity, the description covers all essential aspects of use and output.

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 provides 100% coverage with clear descriptions for both parameters ('package' and 'version', including the default behavior for version). The description adds no additional parameter-specific detail beyond what the schema contains, so the baseline of 3 is appropriate.

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 starts with a specific, compound purpose ('should I add this npm package to my project' check in ONE call) and details exactly what it fans out to (deps.dev and bundlephobia) and which metrics are returned. It clearly distinguishes from any sibling tool by scoping to NPM packages and naming the alternative for other ecosystems.

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 when to use the tool: 'Use whenever an agent asks "is X safe / popular / small" or "what does adding lodash cost me"'. It also gives an exclusion/alternative instruction: 'NPM ecosystem only in v1; PyPI / Maven / Cargo / Go fall under deps.dev:version directly.' This is clear guidance on when to use and when not.

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