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

Annotations already declare read-only, open-world, non-destructive, non-idempotent, but the description adds substantial behavior the annotations cannot express: per-call pricing, the x402 payment handshake, the exact unpaid-call outcome (isError with a PaymentRequired object and fresh invoiceId), and the retry path via _meta["x402/payment"] or payment_signature. It also discloses that unpaid calls cost nothing, which is the key risk-reassurance an agent needs before calling.

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?

It is a single dense paragraph with no wasted sentences, but the pricing sentence leads and largely duplicates the title, pushing the actual purpose to the second sentence. For an agent scanning for selection, purpose should be front-loaded ahead of payment logistics.

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 no output schema, the description still tells the agent what comes back (count plus source and observed_at), what a failure looks like, how to recover from payment failure, and the data-range limit. That covers everything needed to invoke this tool correctly on the first or second attempt.

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%, so baseline is 3; the description adds real meaning by stating the 18-month maximum range, which the schema's period field does not mention, and by explaining the payment_signature parameter's role relative to the _meta channel. Package and period syntax themselves are already fully documented in the schema, so the gain is incremental rather than comprehensive.

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 verb and resource: npm's published download count for one package over a stated period, drawn from npm's documented public API. The phrasing "solves one question: How many npm downloads did this package have in this period?" pins the scope tightly and separates it from adjacent tools like mcp_registry_counts, which covers a different registry.

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?

Gives clear when-to-use framing (one question, one package, one period) plus explicit exclusions: "Counts only; not unique users or popularity" and a hard cap of 18 months for date ranges. It does not name an alternative tool to use for popularity or unique-user questions, so it stops short of full routing guidance.

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