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

The description repeats the read-only/idempotent annotations but adds new facts: the verified data source (deps.dev, Google) and a 1-hour cache. These details give the agent practical expectations about freshness and provenance beyond the annotation hints.

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 description is well-organized: purpose first, then parameters, then use case, then provenance/cache, then fallback instructions. It is somewhat verbose and repeats annotation facts, but every section has a purpose and the key information is front-loaded.

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 read-only lookup tool with three required parameters, an output schema, and rich annotations, the description covers use case, alternatives, source, cache, and fallback behavior. It does not describe error handling for missing packages/versions, but that is a minor omission given the other structured data available.

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?

The description gives examples for each parameter and emphasizes 'Exact version string' and requiredness, which the schema also documents at 100% coverage. However, the ecosystem list in the description (including RubyGems and capitalized names) differs from the list in the schema (lowercase, six ecosystems), creating ambiguity about acceptable values.

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 opens with a specific verb-plus-resource statement: 'Fetch the SPDX licence identifier for an open source package version.' It names the sibling alternative (security_fetch_package_vulnerabilities) and clarifies the licence-focused use case, clearly distinguishing it from the many security tools in the sibling list.

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 explicitly states when to use the tool ('to verify licence compatibility before including a dependency') and when to use the alternative ('Use security_fetch_package_vulnerabilities instead when checking for security issues not licences'). It also provides an escalation path to report_feedback if the result is unsuitable.

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