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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, yet the description adds material context beyond them: an unknown id returns an isError result with the message shape, and the call is free with no auth required. That is exactly the extra behavioral disclosure the description should carry.

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 the core action and no filler sentences; the field enumeration and error/auth notes all earn their place. The middle sentence is a fairly long multi-clause run, but it stays readable and purposeful.

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 a rich output schema handling return values and annotations covering the safety profile, the description covers the remaining agent needs: id sourcing, alternatives, error semantics, and auth/cost. Nothing required to invoke it correctly is missing.

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 id parameter is already well documented there, so the baseline is 3. The description goes further by explaining the provenance of the id value and noting that the prefix is absent for changes_since entries, which adds meaning beyond 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?

Specific verb+resource ('Fetch one third-party library source card by id') plus an explicit enumeration of the returned fields. It also distinguishes itself from siblings by naming search, search_library, and get_article as different tools for different situations.

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?

States exactly where a valid id comes from (sources[].id from search, search_library, latest_sources, or changes_since library entries) and when to use alternatives instead ('Use search or search_library when you have no id, and get_article for Agent Paywire's own articles'). Routing is fully explicit.

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