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

Annotations already declare readOnlyHint=true and openWorldHint=false, so the safety profile is covered. The description adds genuinely useful context beyond them: the credentials the account sits behind (Authorization: Bearer ast_sk_…) and the fact that amounts are denominated in CHF as the book currency, which matters for interpreting results.

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?

A single dense sentence with no filler, front-loaded on balance and spend. The trailing 'of the account behind Authorization: Bearer ast_sk_….' is slightly awkwardly appended, but every clause carries information.

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, no output schema, and read-only annotations, the description's enumeration of returned fields plus the currency and auth context covers what an agent needs. Only the absence of guidance relative to estimate_cost keeps it from being fully 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, which sets the baseline at 4. Nothing is required to invoke it, and the description's only argument-like content (the auth header) is not a schema parameter, so no schema-level semantics are missing.

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 enumerates exactly what the tool returns (balance, credit limit, monthly spend, billing mode, payment currency), so an agent knows the resource precisely and can distinguish it from siblings like estimate_cost or get_model. It lacks an explicit verb, but the resource and returned fields are unambiguous.

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?

There is no when-to-use or when-not-to-use guidance. The closest sibling, estimate_cost, also concerns cost, yet the description never states that get_balance reports actual spend while estimate_cost projects future cost, leaving the routing decision to inference.

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