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

Annotations already declare readOnly/idempotent/non-destructive/openWorld, so the bar is lower, yet the description adds real behavioral context beyond them: pricing ($0.001/call, 10 free/day, x402 payment-required result), error semantics (isError with message, not charged for invalid input/upstream failure), and data-quality flags (deprecated, no release in 2+ years). It does not describe response shape, but the output schema covers that.

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?

Front-loaded with capability, then what it returns, then usage routing, then cost and error behavior — a sensible priority order. It is somewhat dense and the ecosystem list is repeated from the schema, but nearly every clause carries actionable information.

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?

Given a simple 2-param read tool with an output schema and full annotation coverage, the description supplies everything else an agent needs: scope across three registries, staleness/deprecation flagging, pricing and payment flow, failure semantics, and an alternative tool for vulnerability checks.

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 description coverage is 100% and both parameters (name, ecosystem enum with default) are fully documented in the schema, so the baseline is 3. The description restates the ecosystems and confirms registry sourcing (plus npm/pypistats download counts), which adds only marginal meaning over the schema.

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 ('Any npm, PyPI or crates.io package') and enumerates exactly what is returned (version, release date, license, downloads, maintainers, repo, dependency count, deprecation). It also names a sibling (vuln_check) it must not be confused with, so an agent can disambiguate without opening 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?

Explicit 'When to use' line states the use case (package metadata for picking/vetting dependencies) and routes a different use case to a named alternative, vuln_check, with the server location. Both the when and the when-not are stated.

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