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

Annotations already declare readOnly, idempotent, non-destructive, and closed-world, so safety is covered. The description adds genuinely new behavior: it returns the account e-mail as well as the balance, defines the unit ('1 credit = one 2K image'), and states the call is free — none of which is in the annotations. Return shape details beyond that are absent, but no output schema exists.

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 short sentences, front-loaded with what is returned, followed by the unit conversion and the cost note. Nothing is repeated from the title and no sentence is 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 parameterless read with no output schema, the description covers what comes back and what a credit is worth, which is the information an agent needs to interpret the result. It stops short of describing the numeric format or whether balances can be negative/zero, a minor gap.

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?

With zero parameters there is nothing to disambiguate, so the baseline is 4. The description correctly adds no per-parameter noise and instead spends its words on semantics of the returned value.

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 concrete verb and resource ('Returns the account e-mail and credit balance') and reveals an unexpected side output (the account e-mail) that an agent could not infer from the name. It does not explicitly contrast itself with siblings like get_credit_history or buy_credits, but the name-plus-description pairing makes the distinction clear 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?

No when-to-use or when-not-to-use guidance is given, and no sibling is named as an alternative (e.g., get_credit_history for past transactions, buy_credits for adding funds). Only the trailing word 'Free.' implies it is safe to call without spending credits, which is a hint rather than guidance.

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