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

Adds substantial context beyond annotations: the purchase only creates a payment link settled via HTTP 402 / Stripe shared payment token, withdrawal rights end on install, and uninstalled eSIMs are refundable within 30 days (less fee, full within 14 days in EU/UK). This is exactly the commercial and legal context an agent cannot get from readOnly/openWorld/idempotent hints.

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 purpose and payment mechanism, then prerequisites, then refund terms. Every sentence carries information, though the refund/withdrawal details are dense and could be trimmed without losing call-critical guidance.

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?

For a paid, open-world mutation with an output schema and full annotation coverage, the description supplies the payment flow, consent requirement, and post-purchase refund rules. Nothing needed to call it correctly and explain the outcome to a customer 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%, so the baseline is 3, but the description meaningfully elaborates customer_agreed_to_terms by quoting the terms and the consequences of agreeing, and reinforces confirming plan and email before the call. The added meaning goes beyond the schema's own field text.

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 ('Starts the purchase of one eSIM plan') and the immediate result ('returns a payment link'), which separates it cleanly from the sibling find_esim_plans. The two-step flow (find a plan, then buy it) is unambiguous.

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?

Explicit preconditions: confirm plan, price and email with the customer and obtain agreement to the terms before calling. It does not explicitly name find_esim_plans as the source of plan_id in the prose, though the schema covers that, so usage routing is clear but not fully self-contained.

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