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

Annotations already declare destructiveHint=true and idempotentHint=true, but the description adds substantial context they cannot: renewal fires one day before expiry (six hours for a one-day rental), a short balance triggers an email and no charge, and enabling it authorizes future wallet debits. What is missing is whether the number must be actively rented and what an error looks like. This is well beyond what the annotations carry.

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 compact sentences with no filler, and the behavioral consequence is front-loaded ahead of the confirmation requirement. The parenthetical timing clause makes the first sentence dense and slightly run-on, but 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 two-parameter mutation with no output schema, the description covers the important ground: timing, the insufficient-balance failure mode, and the consent requirement. It falls short on preconditions (must the rental be active?) and on distinguishing itself from renew_rental, which matters given how many rental siblings exist.

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 both parameters are already documented and the baseline is 3. The description still adds meaning by spelling out the consequence of autoRenew=true (a later wallet charge rather than an immediate one) and the exact timing of that charge, which the schema's 'true to turn auto-renew on' does not convey.

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 makes clear that this toggles the auto-renew flag on a rented number and explains what auto-renew means (renewal from the wallet before expiry). The verb is implicit ('Turning it on...') rather than stated as 'set', but the effect is unambiguous. It does not differentiate from siblings like renew_rental or cancel_rental, which a caller might reasonably confuse with this.

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?

It gives one concrete usage rule: confirm with the user before turning auto-renew on, since the wallet will be charged later. That is genuine guidance, not filler. However, there is no guidance on when to prefer renew_rental (a manual one-off renewal) or cancel_rental over this, so the agent must infer the boundary from sibling names alone.

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