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

Annotations already declare readOnly, idempotent, non-destructive and openWorldHint=false, so the safety profile is covered. The description adds genuinely new behavioral context: no authentication is needed, the supported-country scope, and that premium-rate numbers will fail, which an agent cannot infer from the structured fields.

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, the auth fact, then the coverage/exclusion caveats. Reasonably tight, though the '(same list as sendfaxfast.com)' reference is an external pointer that adds little for an agent.

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 read-only pricing lookup with a fully documented two-parameter schema and no output schema, the description supplies the constraints an agent needs: supported geographies, premium-number exclusion, and no API key. Only the relationship to send_fax in a cost-estimation flow is left implicit.

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%, with fax_number's E.164 format and examples fully documented in the schema, so the baseline of 3 applies. The description implies the destination restricts valid numbers (country list, no premium rates) but adds no syntax or format detail beyond the schema.

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 specific verb and resource ('Calculate the pay-per-fax price for sending a fax to a specific number'), which cleanly separates it from send_fax and get_fax_status. It does not explicitly reference any sibling tool, so it stops short of full sibling differentiation.

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?

It gives clear conditions for when the result is valid: supported destination countries, premium-rate numbers excluded, and no API key required. It never states the workflow relationship to alternatives (e.g. call this before send_fax to estimate cost), so no explicit when-not/alternative routing is provided.

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