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

Annotations already declare readOnlyHint, idempotentHint and destructiveHint=false, so the safety profile is covered. The description adds genuinely new behavioral facts: a required billing scope, that the call is free, that reserved funds are locked against other operations, and that credits cannot cross organizations. It stops short of describing freshness or caching, so it lands just under the top.

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 what is read and what fields come back, then the scope requirement, then the invariants. No filler and nothing repeated from the schema or annotations.

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 parameters and no output schema, the description carries the full burden and does so: it enumerates the returned monetary fields and their unit (USD), states the auth scope and cost, and explains the reserved/transfer constraints. An agent has everything needed to call and interpret this tool.

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?

The tool takes zero parameters, so the baseline is 4. The field names in the description (balance_cents, reserved_cents, available_cents, USD) describe the response rather than inputs, which is output information rather than parameter guidance, but nothing about parameters is needed here.

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 gives a specific verb and resource ('Read organization-bound prepaid balance') and names the exact fields returned (balance_cents, reserved_cents, available_cents), so an agent knows precisely what it gets. It does not, however, differentiate itself from the adjacent get_usage or purchase_credits siblings, which is the only thing keeping it from a 5.

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 states the prerequisite ('Requires billing scope') and the cost profile ('free'), which is useful invocation context. But it never says when to call this instead of get_usage or purchase_credits, so usage is implied rather than guided.

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