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

Annotations already declare readOnlyHint, idempotentHint, non-destructive, and closed-world, so the safety profile is covered. The description adds real value on top: it declares pagination, enumerates the returned fields, and clarifies the multi-condition semantics of the features filter ('ALL listed must be supported').

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?

Single front-loaded paragraph with the core action first and filters grouped by theme (routing, capacity, status). It is long but every clause maps to a real parameter; only the dense parenthetical listing hurts readability slightly.

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 no output schema, the description compensates by enumerating returned fields (id, number, country_iso, channels, status flags, billing_cycles_count, description, expires_at) and pagination meta. Combined with 19 fully documented params, an agent has everything needed to call it correctly.

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 the schema already documents all 19 parameters, setting a baseline of 3. The description adds meaning beyond it by spelling out AND-semantics for features and the routing/capacity interpretation of flags, though it mostly parallels the schema text.

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?

Opens with a specific verb+resource ('List the authenticated customer's DIDs') and immediately scopes it with the filter dimensions. It is trivially distinguishable from get_did, delete_did, and update_did without opening any 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?

The description explains what each filter accomplishes (e.g. has_trunk/has_sms_trunk false surfaces numbers 'routed NOWHERE', is_configured means 'routed AND with channels'), which is effectively usage guidance. It stops short of naming alternatives like get_did for a singleton lookup or the assign_* tools for non-list operations.

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