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

With no annotations, the description carries the full behavioral burden and does well: it discloses that nothing is sent and nothing is charged, and that both pricing tiers are returned in two units (cents and USDC atomic units). It does not cover failure modes (e.g., an unpriceable or invalid number) or auth/payer-credit behavior, 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?

Three short sentences, front-loaded with the action and scope, followed by the return shape, then the non-side-effect guarantee. No 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 read-only pricing tool with no output schema, the description adequately specifies both the input scope and the shape of the response (two tiers, two unit representations). Minor gap: it does not say what happens for unsupported numbers or how the optional payer credit affects the quote.

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 the schema already documents all four parameters including the note that the reply always lists both tiers. The description only reinforces the 'to' geography and the dual-tier return, adding little meaning beyond the structured fields; baseline 3 applies.

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 ('Price one fax') with explicit geographic scope (US or Canadian number), and the sentence 'Does not send the fax' distinguishes it from the send_fax sibling without the agent needing to open either 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?

'before sending it' establishes the workflow position (a pre-flight estimate ahead of send_fax) and the negation rules out using it as a send action. It does not explicitly name send_fax or get_fax_status as alternatives, so the routing is implied rather than spelled out.

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