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

readOnlyHint=true already covers the safety profile, so the description's job is to add behavior it doesn't. It does: it discloses that contacts are returned nested by default (an expensive detail an agent would otherwise not anticipate) and offers the limit 0 escape hatch to avoid that cost. It omits pagination/truncation behavior beyond the schema's page-size note.

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 core operation plus the parenthetical definition come first with the special-case tip second. Every clause carries information an agent needs.

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 list tool with full schema coverage and no output schema, the description covers what is returned ("each with its contacts") and the main cost-control lever. It could say slightly more about result shape or default ordering, but nothing essential to calling 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%, so baseline is 3, but the description adds real meaning the schema lacks: the schema says limit has a minimum of 0 and a default of 100 without explaining what 0 does, and the description fills that gap. The interaction between search and excludeEmpty (search narrows contacts, excludeEmpty drops phonebooks left empty) is left to infer.

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 and resource ("List shared phonebooks") and even defines the domain concept ("CRM contact containers used for caller ID"), so an agent knows exactly what it gets back. It does not explicitly distinguish itself from the get_* siblings (get_phonebook, get_phonebook_contact), relying on the list/get convention instead of naming the boundary.

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?

"Use limit 0 to list phonebooks without contacts" is a genuine usage instruction, but only for one narrow calling pattern. There is no guidance on when to choose this over spokephone_get_phonebook, or on how search/excludeEmpty interact when browsing versus filtering.

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