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

Annotations only declare readOnlyHint and openWorldHint; the description adds genuinely non-obvious context — a $0.002 USDC charge per successful call via x402 on Base/Solana/Arc, and that failed calls are not billed. That payment and failure-billing behavior is critical for an agent deciding whether to call. It stops short of covering rate limits, caching, or what happens for an unknown package.

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?

Three sentences, front-loaded with the capability, then input format, then pricing. Every sentence carries information, though the enumeration of returned fields overlaps with the output schema and could be trimmed.

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 only one required parameter and an output schema present, the definition covers the essentials: what it returns, the input format, and the payment model. The remaining gap is error behavior for nonexistent or private packages, which matters for a paid call.

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 is already documented in the schema. The description restates the optional @version convention with an example (react@18.3.1), adding only marginal clarity over the schema's own example, so the baseline of 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?

Opens with a specific verb+resource ('npm package bundle size') and names the data source (Bundlephobia's public size API), then enumerates exactly what is returned. No sibling tool in the list covers package bundle sizing, so the agent can select it unambiguously.

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 tells the agent what input to supply ('Give a package name, optionally with @version') but never states when to prefer this tool or what the boundaries are versus adjacent tools like agent_intel_bundle_sniper_detector. Usage is implied by the resource rather than explicitly guided.

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