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

Annotations already cover the safety profile (readOnlyHint, idempotentHint, destructiveHint=false, openWorldHint=false), so the bar is lower. The description still adds genuine context: the pricing model that explains the numbers ($4/number/month, 2¢/segment), the fact that funds are held for in-flight texts, and the important workflow constraint that money cannot be added here and the returned link must be surfaced to the user.

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?

One front-loaded sentence with a colon-delimited list of returned values, followed by two short behavioral clauses. The pricing parenthetical is dense but earns its place by explaining what the balance figures mean; no sentence is pure 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-parameter read tool, the description covers purpose, safety, pricing context, and the follow-up action when the user wants to add funds. It also previews return contents even though an output schema exists — lightly redundant but harmless, and the agent needs nothing else to call this correctly.

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 no parameters, so the schema baseline is 4 and there is no parameter meaning for the description to supply. Nothing here misrepresents or omits input semantics.

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?

Names a specific resource ('the account's prepaid balance for business numbers') and enumerates exactly what the call returns: balance, held amount, today's spend against the daily limit, and recent entries. An agent can distinguish this from get_stats or get_number purely from the description.

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?

States the operational boundary clearly — 'Read only. You can't add money: give the user the link it returns' — which tells the agent both what not to attempt and what to do instead when a top-up is requested. It does not name any sibling tool as an alternative, but with zero parameters and an unambiguous purpose there is little else an agent could confuse it with.

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