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

With no annotations, the description carries the burden and does well: "no eval" discloses that arbitrary code is not executed, and "FREE, no key" discloses the auth/cost profile, both meaningful traits. It does not mention error behavior on malformed expressions or numeric precision/return format, 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?

A single dense sentence, front-loaded with the free/no-key and safety facts. The trailing operator list is long but earns its place because the schema does not enumerate supported functions.

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 one-parameter, no-output-schema utility, the description covers cost, safety, and accepted input syntax. Only return formatting and error handling are unstated, which are minor for a deterministic arithmetic tool.

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% and the schema already gives an example expression, so the baseline is 3. The description goes beyond it by specifying the accepted syntax (operators, %, !, sqrt, trig, constants like pi/e), which is exactly the semantics an agent needs to construct a valid expression.

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 capability (a safe, non-eval calculator) and enumerates the exact operations it supports, which makes clear it evaluates arithmetic expressions rather than converting units or validating data. It does not explicitly name a sibling to differentiate from, but the operator list makes the scope unambiguous.

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?

"FREE, no key" tells the agent there is no auth barrier, but nothing states when to prefer this over siblings like unit_convert, position_size, or json_validate. Usage is only implied by the enumerated syntax.

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