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

With no annotations supplied, the description carries the full behavioral burden and does substantial work: it discloses the two payment paths, the 402 response with both prices, and queue ordering (priority faxes go ahead of other agent faxes, but website/app faxes still go first). Gaps remain on rate limits and, most importantly, what a successful call returns and whether sending is synchronous or queued.

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?

The core action is front-loaded in sentence one and the payment mechanics follow. Sentences are dense but each carries information; the payment sentence is packed tightly but not padded.

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 side-effecting send tool with no annotations and no output schema, the description covers payment and priority well but omits the response shape, so an agent cannot tell how to obtain the fax identifier needed to chain into get_fax_status. It also omits failure modes beyond the 402 case.

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 100%, so the schema already documents all four parameters. The description largely restates the schema (https pdf_url, priority meaning) rather than adding new semantics such as accepted number formats or how the priority price differs. 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?

The first sentence states a specific verb and resource with scope: "Send a PDF to a US or Canadian fax number." That is clearly distinguishable from get_fax_status and quote_fax by action alone, but the description never names or contrasts those siblings, so it stops short of a 5.

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 concrete usage conditions: pass an https pdf_url, pay via Bearer API key or the x402 PAYMENT-SIGNATURE header, and set priority true for priority pricing. The 402 fallback is described, which tells the agent what happens if it calls without payment. It does not, however, tell the agent when to call quote_fax first instead.

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