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

Annotations already declare readOnlyHint/openWorldHint/destructiveHint, so the safety profile is covered. The description goes well beyond that, explaining the data model: each assigned phone is its own subscription on its own monthly cycle, `lines` carries a per-phone renewal date, and `pending_orders` charge the first month when a phone is assigned. That is meaningful domain behavior an agent could not infer from 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, front-loaded with the core action, then return-field detail, then the 'start here' call to action. Dense but every clause earns its place; no filler or restated name/title.

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 carries the burden of explaining what comes back, and it does so concretely (per-phone renewal dates in `lines`, first-month charges in `pending_orders`). Combined with annotations covering the safety profile and zero parameters to document, nothing an agent needs to call this 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?

The tool takes zero parameters, so the baseline is 4. There are no inputs to document, and the description spends its space on return semantics instead, which is the right allocation here.

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 ('Read the authenticated user's phone rentals') and enumerates the returned fields (count, cost, renewal dates, pending orders). An agent can distinguish it from a generic list tool, though it never explicitly contrasts itself with close siblings like get-billing-request-tool or list-phones-tool.

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?

Ends with explicit directional guidance: 'Start here before proposing any billing change.' This tells the agent the context in which the tool should be called. It stops short of naming the alternative tools to use afterward or stating when-not-to-use it.

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