Credit balance

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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), so the description's job is added context. It delivers that by disclosing the 402 error-recovery use case, the test-vs-live key distinction, and that the call itself consumes no credits. It does not discuss any failure modes beyond 402 or how stale the balance may be, which keeps it 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?

Three tightly packed lines: what it returns, when to call it, and a cost note. The return content is front-loaded and every sentence carries distinct information with no redundancy.

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-only tool with an output schema already defined, the description supplies exactly the missing pieces: the behavioral triggers and the test/live key semantics. Nothing an agent needs in order to decide to call it 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 there is no parameter meaning to add and the baseline of 4 applies. The description correctly adds nothing spurious about inputs and instead spends its words on the return content.

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 and resource ("Returns the account credit balance") and goes further by enumerating the returned breakdown (total, starter, subscription, purchased) plus the key's environment. An agent can distinguish it from siblings like get_store_requirements or get_job_status without opening any schema.

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?

Gives concrete triggers: call before create_screenshot_set or generate_screenshot when using a live key, or after a 402 INSUFFICIENT_CREDITS response. It names the sibling tools explicitly and notes test keys never spend credits, which implicitly tells the agent when the check is unnecessary.

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