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

Annotations already declare readOnly, idempotent, and non-destructive, so the bar is lower. The description still adds meaningful context beyond them by defining the accounting semantics (Charged = gross debit, Refunded tracked separately, Net = Charged - Refunded) and reaffirming 'Does not spend sats'. It says nothing about time granularity beyond 'today' or about rate/pagination behavior, keeping it out of 5 territory.

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 the payload ('Today's spend...') before the field definitions, and every sentence adds a distinct fact. Slightly dense with three separate one-line field definitions, but none 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 zero-argument, low-complexity, read-only tool with an output schema present, the description covers the essentials. Because the output schema already carries return structure, the extra field definitions are a bonus rather than a requirement, so the description is comfortably complete.

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 of 4 applies; there is no parameter surface for the description to explain. Nothing in the text contradicts or misleads about inputs.

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 the specific resource set it returns: today's spend, remaining budgets, and last 10 buys. An agent can distinguish this from balance (account balance) or orders (transaction list) because it defines itself as a spend-focused summary. It lacks a strong verb but the noun scope is precise enough to be clearly different from siblings.

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?

No when-to-use guidance is given; there is no condition, prerequisite, or named alternative (e.g., use balance for standing balance vs spend_summary for spend). 'Read-only. Does not spend sats.' reassures safety but does not tell the agent when to pick this tool over balance or orders.

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