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

Annotations already declare readOnlyHint=true and openWorldHint=true, so the safety/openness profile is covered. The description adds real value beyond them by disclosing the workflow dependency: the section slugs it returns are the inputs mirror_headlines accepts. It does not mention pagination or result size, which is the only remaining gap.

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 tight sentences, zero filler. The core action and the headline-routing purpose are front-loaded, and the sibling disambiguation trails it — exactly the right ordering.

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 zero-parameter read-only lister with an output schema already describing the returned section objects, the description supplies everything else needed: what it returns, how that output is consumed by mirror_headlines, and when to prefer siblings.

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 tool takes 0 parameters, so per the rubric the baseline is 4 with nothing to disambiguate. The description's mention of the 'section slug' describes an output the agent can feed into another tool rather than an input, which reinforces the chaining contract without needing parameter documentation.

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+resource ('List the public Mirror section inventory') with a clear scope qualifier ('accepted by mirror_headlines'), and explicitly differentiates itself from mirror_news and mirror_article. An agent can distinguish it from the sibling section/headlines tools without opening a schema.

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 explicit when-to-use routing: use mirror_news for the general feed, mirror_article for a single story, and this tool when you want the section inventory or a section-specific headline feed via a returned slug. Alternatives and the selection condition are named outright.

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