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

Annotations only declare readOnlyHint=false, destructiveHint=false and openWorldHint=false; the description adds substantially more: that balance is debited (financial side effect), the credential/port topology returned, and a non-obvious ~5 second propagation delay that would otherwise cause wrong-country exits. This is exactly the kind of behavior that cannot be inferred from structured fields.

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 action and its cost, then returns, then the two actionable warnings. No filler; every clause carries operational information.

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?

No output schema exists, and the description compensates by naming the returned credentials and port semantics. Combined with the timing warning and pricing prerequisite, an agent has everything needed to call and use the result 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 baseline is 3, but the description adds real meaning beyond it: it ties rotation_interval_minutes to the returned port (824 = rotating, 10000 = sticky) and explains that the country filter propagates asynchronously. It stops short of restating enum values or syntax for type/traffic_gb, which the schema already covers.

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+resource ('Buys a proxy') plus the key side effect ('deducting the account balance') and what is returned. An agent can distinguish this from sibling extend_proxy or create_topup 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?

Gives a concrete prerequisite ('Check get_pricing × traffic_gb against the balance first') naming a sibling tool, plus a post-purchase sequencing rule. It does not, however, say when to use extend_proxy instead for topping up an existing proxy, so the alternative-selection guidance is incomplete.

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