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

Beyond the readOnlyHint annotation, the description discloses significant x402 payment behavior: unpaid calls return PaymentRequired, clients can retry with _meta['x402/payment'], and execution fails closed if payment rails are unavailable. This is valuable behavioral context that annotations do not provide.

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 first sentence front-loads the core purpose and the sibling-routing guidance is concise. The payment-related sentence is long but earns its place given the tool's unusual payment requirement. Minor issues: 'MISSING' and 'canonical Kappa financial engine' are awkward and slightly obscure readability.

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 one-parameter, read-only metadata lookup with no output schema, the description covers purpose, usage routing, and payment failure modes. It does not describe the return payload, but that is reasonably inferable from the tool's purpose and is not critical for invoking it 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% for the single required parameter package_name, and the schema already includes an example. The description does not add any deeper semantics about format, validation, or endpoint behavior, so the baseline score 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 clearly states the tool's job: 'look up current npm package metadata.' This is a specific verb + resource, and the 'npm' scope distinguishes it from sibling tools like pypi_package_metadata and nuget_package_versions without needing to open their schemas.

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?

The description gives actionable guidance: prefer this tool when the task matches and do not first call list_verified_capabilities or search_verified_capabilities. It could have also explicitly distinguished npm lookups from pypi/nuget lookups, but the tool name and description make that implicit.

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