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

Annotations only give the generic non-read-only/open-world profile; the description adds the material behavior: nothing is sent or charged until the user confirms and pays in the form, and billing happens only on confirmed delivery. It also discloses the host-dependent fallback (returns a link) and the constraint that chat attachments cannot be passed in — exactly the surprises an agent would otherwise hit.

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?

Front-loaded with purpose and mechanism, then cost, then the attachment caveat and fallback — a logical order with no filler. It runs slightly long at six sentences, and the two attachment sentences could be merged, but nothing is redundant.

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 form-driven, human-in-the-loop tool with no output schema, the description covers everything needed to set expectations: where the document goes, what the user pays, when charging occurs, and what happens on hosts without form support.

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 each of agency, language, and fax_number is already documented, including that the number may come from an attached letter. The description adds no parameter-level meaning beyond the schema's own note, so the baseline 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 description opens with a concrete verb and resource — "Start a fax to a US fax number" — and immediately clarifies the interaction model (a form opens in the chat). That is enough to distinguish it from the only sibling, lookup_fax_number, without reading 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?

Usage context is explicit: no account needed, US numbers only, $2.99 flat up to 25 pages, and a browser-link fallback for hosts that cannot render the form. The one gap is that it never tells the agent to call lookup_fax_number first when the destination number is unknown, which is the obvious companion action.

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