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

Annotations declare a non-read-only, open-world, non-idempotent operation, and the description adds substantial context beyond them: no charge occurs at call time, the checkout link expires in 30 minutes, and the post-payment email/redemption flow is spelled out. The seasonal cutoff (24 December) is exactly the kind of constraint annotations cannot express.

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-loaded with the outcome and price before the flow narrative, and the Args block is scannable. The middle passage describing the parent's redemption steps is longer than strictly needed for an agent deciding whether to call the tool, though it does inform expectations about what happens after.

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 correctly names the return value (checkout_url) and what it is, plus the expiry window. Combined with full parameter coverage and the end-to-end flow, an agent has everything needed to invoke this correctly and explain the result to a user.

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 description coverage is 0%, so the description carries the full burden and mostly does: it explains that buyer_name is shown to the family, buyer_email drives receipt and gift link, recipient_email sends the link onward, and message is capped at 300 characters. Minor gap — promo_code gets only a restatement ("a promotion code, if the buyer has one") with no format or validity guidance.

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?

Opens with a specific verb and resource: "Start buying a gift: a phone call from Santa for a child in the UK, £6.99." It immediately distinguishes itself from siblings like check_gift_status by making clear this initiates a purchase rather than checking one, and names the artifact returned (checkout_url).

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?

Usage context is clear — this is the entry point of the purchase flow, usable until 24 December, and nothing is charged until the buyer pays on the Stripe page. It does not explicitly name alternatives or say "use check_gift_status after payment," so the agent must infer the sequencing from the sibling names.

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