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

With no annotations, the description carries the full burden and does substantial work: it discloses a payment requirement and mechanism ('$1 USDC per call via x402 (Base)'), the create-vs-extend branch, and that repeat calls stack duration. It omits failure/refund behavior and what happens if payment fails, but the economic and state-transition behavior is unusually well surfaced.

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?

One dense but front-loaded sentence that leads with the core action and branches, then lists entitlements, then price. Every clause carries information, though the colon-separated feature list makes it slightly heavy for a single sentence.

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?

Given no annotations and no output schema, the description covers the essential call semantics: what is granted, key-or-create behavior, repeat-call extension, and cost per invocation. The main remaining gap is whether a key is returned and how payment/errors are surfaced.

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% and the single param is already described as optional in the schema, which sets a baseline of 3. The description adds genuine meaning beyond it by specifying the consequence of omission: a brand-new Pro key is minted instead of extending an existing one.

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 ('Adds 6 days of Pro to the given key') plus the exact entitlement quanta granted (30 watches, 1,000 alerts/day, safe tx alerts, proxy-upgrade reports). The purchase/extend purpose is clearly distinct from the watch-management siblings (add_watch, list_watches, etc.).

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?

'Call again to extend' tells the agent how repeat invocations behave, and the parenthetical explains the no-key case. It gives clear operational context but offers no explicit when-not-to-use or alternative-tool routing (arguably none exists among the siblings).

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