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

With no annotations, the description carries the full behavioral burden; it does disclose the return shape ('spelled in Persian words with its unit') and the per-call cost with payment rail, which is useful context. It says nothing about invalid input handling, currency/unit validation, or what happens if unit is omitted.

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 the core action and scope before the use case and the pricing note. Little waste, though the pricing sentence is operational rather than selection-relevant.

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 tool with no output schema, no annotations, and 0% schema coverage, the description covers purpose, return content, cost, and use case adequately but omits parameter-level detail and error behavior. An agent can invoke it, but the two non-required parameters remain opaque.

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 description coverage is 0% across three parameters, so the description must compensate. It implies the unit domain ('rial or toman') and that amount is the payable value, but the 'number' parameter is never explained nor is its relationship to 'amount' clarified, leaving a required-vs-optional ambiguity unresolved.

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+resource ('Write an amount of Iranian rial or toman out in Persian words') and names the domain artifact it serves (مبلغ به حروف on cheques, invoices, contracts). It does not, however, distinguish itself from the sibling fa_numberToWords, leaving the amount-vs-number boundary to inference.

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 a concrete usage context ('render a payable amount for a Persian-speaking payer') tied to real document types, which is more than implied guidance. It stops short of any exclusion or explicit alternative (e.g. when to use fa_numberToWords or fa_rialToman instead).

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