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

Annotations already mark this as read-only and idempotent; the description adds an important authentication requirement ('Authorization: Bearer zs_...') and states that the response reflects current balance plus available packs. It doesn't cover error behavior, but for a safe read operation with an output schema, this is useful context beyond the annotations.

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 short sentences with no filler: the first front-loads what the tool returns, the second states the auth requirement. Every sentence earns its place and there is no redundant restatement of the tool name.

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?

This is a simple zero-parameter tool with an output schema and read-only/idempotent annotations, so the description only needs to convey purpose and the auth prerequisite, both of which are present. It stops short of explaining when to choose it over billing siblings, but that gap is covered under usage guidelines.

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?

There are no input parameters to document, so the empty schema plus 100% coverage leaves no ambiguity. The only parameter-like constraint is the auth header, which is mentioned in the description rather than in the schema. Baseline for zero-parameter tools is 4.

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 a concrete resource ('credit balance and available packs') and pairs it with the imperative title 'Check Credit Balance', so an agent knows this is a read query for current balance/credits. It doesn't explicitly contrast with siblings like get_pricing or buy_credits, but the resource is distinct enough.

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?

The only contextual cue is a required Authorization header, which is an invocation prerequisite, not a selection rule. No guidance is given on when to call this instead of get_pricing or buy_credits, and there are several billing-related siblings in the list.

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