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

Beyond the annotations (readOnlyHint=false, destructiveHint=false, idempotentHint=true), the description discloses the key non-obvious traits: the link is handed to the user who pays externally, the assistant cannot pay, and calling the tool charges nothing. It also states the auth requirement ('Needs the user's account'), which the annotations do not cover.

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?

Three tight sentences, front-loaded with the trigger condition, then the return behavior, then the safety/auth caveat. Every sentence earns its place and nothing is redundant.

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 steps in and describes the return value (a link to the payment page), plus the auth requirement and the no-charge guarantee. An agent has everything needed to call it and explain the result to the 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 coverage is 100% and the letter_id description already explains the two modes (pay a draft vs. omit for credit), so the schema carries the parameter burden. The description's phrasing 'add credit, or to pay one specific draft letter' mirrors that without adding new syntax or constraints, which is the expected baseline 3.

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 states a specific verb and resource ('Returns a link to the payment page') and scopes it precisely to two cases: adding prepaid credit or paying one draft letter by card. No sibling (create_upload_link, get_balance, send_letter) shares this resource, so an agent can identify it without opening the 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 an explicit trigger: 'Use this when the user needs to add credit, or to pay one specific draft letter by card.' That is clear context for invocation, but no alternatives or when-not conditions are named (e.g. when to use get_balance or get_price_quote instead), so it stops short of full routing guidance.

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