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

Beyond the annotations (which already declare read-only, idempotent, non-destructive, open-world), the description discloses that no code is executed, a free tier of 10 calls/day, then $0.001/call via x402 or X-API-Key credits, plus the hosting host. Auth/payment and rate-limit disclosure is exactly the kind of context annotations cannot carry. Left slightly short of 5 because failure/error behavior for malformed expressions is unstated.

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?

Front-loaded with the core capability, then safety, then cost — a sensible order. Sentences are dense but each carries information; the only disposable element is the bracketed host attribution at the end, plus the payment mechanics which are verbose relative to the arithmetic content.

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 values need no explanation, and pricing/auth are covered. Still, for an evaluator with a free quota and paid fallback, the description omits what happens when the quota is exhausted or the expression is malformed, and gives no hint about numeric precision or rounding of results.

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?

Schema coverage is 100%, so baseline is 3, but the description adds real meaning: it enumerates the operators and functions valid inside 'expr' and explicitly mentions named variables, clarifying that the vars parameter must be referenced by name in the expression. That is syntax guidance the schema's bare 'expression' / 'JSON object of variables' descriptions do not provide.

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?

Names a specific verb and resource (evaluate arithmetic) and enumerates the exact grammar it supports — operators, functions, constants, and named variables. 'No code execution' further pins down what kind of evaluator this is, so an agent can distinguish it from any code-running sibling without opening the 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?

'Evaluate arithmetic exactly instead of guessing' implicitly tells the agent to prefer this over mental arithmetic, which is useful framing. However, there is no explicit when-not guidance, no statement about invalid expressions, and no named alternative tool for related numeric tasks (e.g. apr_apy or currency_convert among the siblings).

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