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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds genuinely new context: the currency is EUR and topping up is impossible through this tool because only the account owner can do it on the website, which prevents a futile search for a top-up sibling. It does not discuss staleness of the balance or permission semantics, keeping it just short of a 5.

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?

Two sentences with no filler. The returned values come first and the top-up limitation, the actionable constraint, comes second, so an agent gets the essentials immediately.

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, rich annotations and an output schema, the remaining burden on the description is small, and it covers the currency and the top-up limitation. The only unaddressed item is what the permission list contains or how a limit exhaustion should be handled, which is minor for a zero-argument read.

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. There is nothing for the description to disambiguate in the input schema, and the brief note that the amount is denominated in EUR is the only meaningful semantic addition.

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 the exact resources returned: EUR account balance, remaining spending limit for this app, and permissions for this connection. That is specific and clearly distinct from the sibling tools (buy_number, send_sms, list_messages). It is phrased as a noun list rather than a verb+resource statement, so it is clear but slightly less crisp than 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?

Usage is only implied: an agent can infer this is the tool to call to check funds or permissions before attempting a purchase such as buy_number, but the description never says so. The note about top-ups being made by the owner on the website is useful spillover guidance (do not search for a top-up tool), yet it is not framed as when-to-use versus an alternative.

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