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

Annotations already cover safety (readOnly, idempotent, non-destructive), and the description goes well beyond them: it explains that a report URL exists only after delivery, failure or cancellation, that document links require being the signed-in owner while files are retained, that links expire after five minutes, and that delivery does not mean read. 'It changes nothing' merely restates readOnlyHint, slightly wasting a sentence, but the added operational context is substantial.

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 usage trigger is front-loaded, and most sentences carry unique information (return fields, URL availability rules, expiry, ownership limits). The line 'It changes nothing' duplicates the readOnly annotation and is the one sentence that does not earn its place; the rest is dense and purposeful.

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?

With an output schema present, return shape need not be described, and the description instead covers what the schema cannot: conditional URL availability, five-minute link expiry, ownership/retention limits, and the read-vs-delivered distinction. The only real omission is differentiation from sibling tools such as get_fax_status and get_fax.

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 both parameters (account fax id with its sign-in and mutually-exclusive-with-status_url constraint, and the guest status_url token) are already fully documented. The description adds indirect context — the 'signed-in owner of an account fax' and guest token framing — but does not directly explain parameter behavior beyond what the schema states, so 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 description uses a concrete verb ('Use this when the person asks for...') plus a specific resource (delivery report / transmitted document) and enumerates what is returned: destination, page count, delivery result, timestamps, attempts. It's clear what the tool does, but it never distinguishes itself from close siblings like get_fax_status or get_fax, which an agent could easily confuse it with.

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 an explicit trigger condition: 'Use this when the person asks for a delivery report or the transmitted document.' That is a strong when-to-use cue. It stops short of naming when-not-to-use or pointing to a sibling tool (e.g., get_fax_status for in-progress status), so it does not reach a 5.

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