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

Beyond the annotations (readOnlyHint=false, idempotent=false, openWorldHint=true), the description discloses that this only starts a purchase, returns a checkout_url for a human to pay, that credits land only after payment clears, that credits never expire, and that a 5% (min $0.50) fee applies. That is meaningful behavioral context an agent cannot get from the structured fields alone.

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?

The description is front-loaded with the action and pricing, and every clause carries real information (price tiers, credit math, fees, checkout flow, policy gate). It is dense with parentheticals in the first sentence, which slightly impedes scanning, but nothing is filler.

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 three-parameter write tool with no output schema, the description covers preconditions, side effects, timing of credit delivery, cost, and the return value (checkout_url). An agent has everything needed to invoke it safely and 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 67%, and the description compensates by spelling out pack_id values (pack_5/pack_10/pack_50) and the credit-per-dollar ratio, which the schema leaves undefined. The agree and policy_version semantics largely duplicate the schema descriptions, but the added linkage to GET /v1/policy clarifies their source.

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?

The description names a concrete action (start buying a pack), enumerates valid pack values with prices, and explains what the resulting credits are. It is clearly distinguishable from siblings like get_pricing (lookup) and order_status (post-purchase), so an agent can route correctly without opening a 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?

It gives explicit preconditions and sequencing: show the refund policy and terms from GET /v1/policy first, then pass agree=true and the matching policy_version only after the person agrees, and use GET /v1/pricing for totals. It does not state when this tool should be avoided (e.g. already-pending purchase, auth requirements), so it stops short of full when/when-not guidance.

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