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

Annotations already declare read-only, idempotent, non-destructive, closed-world behavior, so the safety profile is covered. The description adds value beyond that by disclosing the two invocation modes and the failure mode for invalid slugs (error rather than empty result). It does not describe result shape or any limits, but the output schema covers returns.

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?

Two sentences, no filler, and the discovery step is front-loaded before the error warning. The opening is a noun fragment ('A published ranking by its slug') rather than a clean verb-first statement, which slightly obscures the primary action for a tool named list_rankings.

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, zero-required tool with an output schema and full annotation coverage, the description supplies everything an agent needs: what a slug is, sample values, the enumerate-first workflow, and the error case. Return values need not be explained because an output schema exists.

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?

With one parameter at 100% schema description coverage, the baseline is 3, and the schema already explains omission behavior ('Omit to list all') and the slug pattern. The description adds real semantics: example slug values that convey the vocabulary space, and the error consequence of an unrecognized key, which is not captured in 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?

The description states a specific resource ('published ranking') and its identifier ('by its slug'), plus concrete examples (largest employers, top taxpayers) that make the data domain unmistakable. It also covers the enumeration mode via 'Call WITHOUT a key first to list the available slugs'. It does not explicitly contrast itself with any sibling, but the sibling set (company search/lookup tools) is a different domain, so differentiation is implicit rather than stated.

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 gives a clear operational workflow: call with no key first to discover slugs, then use one. It also warns that inventing a slug returns an error, which is genuine usage guidance. What is missing is naming an alternative tool for adjacent needs (e.g., when a sibling search is more appropriate), so it stops short of the 5 bar.

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