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

The description adds real behavioral context the annotations don't: cost ('Free to call'), an auth requirement (user's account), and the resulting mutation (credits added, new balance returned). However, 'Safe to repeat — an order is captured at most once' asserts at-most-once effect on repeat calls, which conflicts with the declared idempotentHint=false, so the behavioral disclosure is partially self-contradictory.

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 short sentences, front-loaded with the action and the trigger condition, with no filler. The trailing fragment 'needs the user's account' is terse to the point of vagueness, which costs it the top mark.

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 one-parameter mutation with no output schema, the description covers trigger, effect (credits added), return value (new balance), cost, and auth. What it leaves undefined is the second-call outcome (error vs. identical response), which matters given the idempotency claim.

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?

Only one parameter and schema description coverage is 100%, and the schema already documents order_id as 'returned by buy_credits'. The description reinforces the buy_credits provenance but adds no syntax, format, or validation detail beyond the schema, so the baseline 3 applies.

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: capturing an approved PayPal order and adding pack credits to the account. It also makes the distinction from the sibling buy_credits explicit by tying itself to the post-approval step, so an agent can tell the two apart without opening either 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 a clear trigger ('After the user approves the buy_credits link on PayPal') and a prerequisite ('needs the user's account'), which routes the agent correctly relative to buy_credits. It stops short of an explicit when-not rule, but the sequencing is unambiguous.

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