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

Annotations already establish readOnly, idempotent and non-destructive behavior, so the description does not need to re-litigate safety. It adds genuinely new context the annotations lack: the call has no cost, and the response includes both the current balance and recent movements. It stops short of describing auth requirements or any rate limits for the movements data.

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, no waste, and the resource being fetched is front-loaded ahead of the cost note. Every clause earns its place.

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?

An output schema exists, so return-value detail is unnecessary in the description, and the absence of parameters means no input semantics are missing. For a simple read-only, parameterless tool, nothing an agent needs to call it correctly is absent.

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 behavior for the description to explain or compensate for.

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 a specific verb (Retrieve) and resource (credit balance for your API key) and adds scope detail (latest credit movements). The sibling tools (get_record, list_sources, screen_names) cover entirely different resources, so an agent can tell this apart from all of them immediately.

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?

"This tool is free to use" gives a cost-based cue about when it is cheap to call, which is mild usage guidance. However, it never states when to use this versus alternatives, nor any prerequisites or exclusions. Usage is only implied rather than articulated.

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