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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so safety is covered. The description adds genuine behavioral nuance beyond annotations: the assignment-vs-activation caveat, the requirement that an owner phone be on file, and forwarding verification. It also discloses the API correspondence ('Mirrors GET /api/v1/numbers'). No contradiction with annotations.

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?

Three sentences, with the core purpose front-loaded and every subsequent clause earning its place — the caveats prevent misinterpretation and the sibling references guide follow-up. The second sentence is dense, but each clause is non-obvious and operationally relevant. Slightly more verbose than strictly necessary, but no waste.

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 low-complexity tool (1 optional filter param, full annotations, output schema present), this is nearly complete. The main risks — misreading assignment as activation and skipping owner-phone setup — are explicitly addressed. Minor omissions like pagination or ordering behavior are acceptable given the output schema and the tool's simplicity.

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 description coverage is 100% — the single optional parameter business_id is already documented in the schema with its filtering meaning. The description's 'accessible business(es)' phrasing loosely aligns with the param but adds no syntax, format, or semantic detail beyond the schema. Baseline 3 is appropriate when the schema carries the full parameter burden.

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+resource+scope: 'List phone numbers assigned to the accessible business(es), with capability and compliance flags.' This clearly distinguishes it from sibling tools like search_available_numbers (unassigned numbers), get_activation_status (setup state), and get_extra_number_status (status of a specific number). An agent can tell exactly what this tool returns.

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?

The description provides clear context on when this tool's data is insufficient: 'Assignment does not prove inbound routing, activation, or customer coverage' and explicitly routes the agent to get_activation_status / set_owner_phone for full setup verification. It doesn't formally name an exclusion for searching unassigned numbers, but the 'assigned to' scope plus the caveats make the boundary obvious.

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