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

Annotations already declare readOnlyHint=true and openWorldHint=true, so the safety profile is covered. The description adds non-obvious behavior beyond that: documents are NOT downloaded, the implicit 30-day default window, and the pagination contract via next_skip.

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?

Four tight sentences: scope and returned fields first, then the important 'not downloaded' caveat, then defaults and pagination. No filler, and the most decision-relevant facts are front-loaded.

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?

There is no output schema, but the description enumerates the returned fields (sender, number, time, pages, duration, read flag), covers the default date window, and explains paging — everything an agent needs to call it correctly.

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 baseline is 3. The description adds meaning the schema doesn't: the from_date default window of 30 days and the pagination flow (skip = next_skip), which clarifies how skip is meant to be used across calls.

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 (List) and a precisely scoped resource (faxes received on the account's fax numbers), then enumerates the returned fields. The word 'received' cleanly separates it from the sibling notifyre_list_sent_faxes, so an agent can pick the right tool without opening a 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?

Gives actionable usage context: the default 30-day window when from_date is omitted, and that paging uses skip = next_skip. It doesn't explicitly say when to prefer this over sibling list tools, but the 'received' scope makes the alternative self-evident.

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