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

Annotations already declare readOnly, idempotent and non-destructive, so the safety profile is covered. The description still adds genuine value beyond them: the billing consequence of leaving a machine running until its removal time, plus "Free; changes nothing," which tells the agent there is no cost or side effect in polling it often.

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: the first front-loads what comes back, the second front-loads the reason to call it. No filler, and the fragments "Free; changes nothing" carry real information rather than padding.

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 parameters and no output schema, the description carries the burden of describing returns, and it does — state, price per hour, SSH command, removal time, host notice. It also closes the loop on the cost model, which is the one thing an agent could get wrong about a free read on a billing resource.

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 there is nothing for the description to disambiguate and the baseline applies. The description correctly spends no words on arguments and instead describes the result shape.

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 names the resource and its scope precisely — "Every machine on the account" — and enumerates what each entry carries (state, price/hour, SSH command, removal time, host notice), which cleanly implies the list-all behavior in contrast to the singular get_instance. The only shortfall is that the verb is implicit and no sibling is named outright, so differentiation relies on the agent inferring singular vs. plural from the name.

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?

"Check it at the start of a session" gives an explicit trigger condition, and the billing rationale explains why. It stops short of naming alternatives (e.g., use get_instance for one machine, list_images before renting), so the when-not guidance is missing.

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