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

Adds substantial value beyond the readOnlyHint/openWorldHint annotations: unknown packages yield 404, and each successful call costs $0.002 USDC via x402 (Base, Solana, Arc) with failures uncharged. That cost and error-profile disclosure is exactly what an agent needs before calling. It stops short of mentioning rate limits or result size, so not a 5.

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 dense sentences, front-loaded with the source and field list, then error and pricing behavior. The field enumeration is partly redundant given an output schema exists, which is minor waste but not harmful.

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 an output schema, the description covers source, coverage, error behavior, and payment model. The redundant field list and lack of keyword-search caveats are small gaps rather than blockers.

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 schema already documents 'name' as an exact CRAN package name with an example (ggplot2). The description adds no format guidance 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?

States a specific resource (CRAN R package metadata via the crandb JSON mirror) and enumerates the returned fields, so an agent knows exactly what data it gets. It doesn't differentiate itself from adjacent metadata sources like search_crossref or search_pubmed_article_metadata, and the name says 'search' while the behavior is an exact-name lookup.

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 notes the source is public and keyless, which is useful context, but never says when to pick this tool over a sibling metadata source or what its limits are (exact match only, no keyword search). No explicit when/when-not guidance is given.

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