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

Adds substantial context beyond the annotations: the API-key requirement, the human-in-the-loop constraint that the agent cannot pay the Stripe link itself, the fact that stamps land on this key's account and never expire. These are exactly the behavioral traits an agent needs and cannot infer from openWorldHint/destructiveHint/idempotentHint.

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?

Front-loads purpose, then mechanism, then pricing in a single dense block with no filler sentences. The price list is long but each entry carries information absent from the schema, so it earns its place; structure is slightly run-on rather than perfectly scannable.

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 compensates by naming the return value (`url`) and its meaning, plus the post-purchase workflow. Nothing needed to invoke the tool 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?

Schema coverage is 100% and the enum already lists the bundle ids, so the baseline is 3. The description adds real decision value by pairing every bundle id with its stamp count and price, which the schema does not fully do (it only prices bundle_5).

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 ("Start a purchase of prepaid stamps for your key") plus the scope of the purchase, and is instantly distinguishable from siblings like get_balance or create_account. An agent knows exactly what this tool initiates.

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 clear next-step context: requires an API key, returns a Checkout URL that a human must open, and tells the agent to show the link to the user and then poll get_balance. It names a follow-up sibling (get_balance) but offers no explicit when-not-to-use condition, since no alternative purchase path exists.

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