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

Annotations already declare readOnly/idempotent/non-destructive, but the description adds pagination mechanics ('Returns 25 at a time, newest first', pass next_page back as page) and an auth behavior ('Needs the person to sign in to Faxette; calling it while they aren't asks the app to sign them in'). That auth trigger is behavior an agent cannot get from the annotations or schema.

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 trigger sentence, then pagination, then filtering, then alternative tool, then auth — a sensible order with no wasted clauses. The closing 'It changes nothing' is slightly redundant against readOnlyHint/destructiveHint, keeping it from a 5.

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 only two optional params and an output schema present, the description covers everything needed to call it correctly: trigger, page loop, filter semantics, auth side effect, and the sibling for detail lookups.

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 baseline is 3; the description adds value by explaining the pagination contract ('Pass next_page back as page for the next 25') and giving concrete filter examples (['failed'], ['delivered']). It doesn't enumerate the other status enum values, but the schema does that.

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 and resource (list faxes sent from a Faxette account) plus the scope cue 'what they sent last week or which faxes failed'. It also explicitly separates itself from the sibling get_fax, which returns a single fax's details.

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?

Opens with an explicit trigger ('when the person asks about faxes they've sent... like what they sent last week or which faxes failed') and names the alternative for the single-item case: 'Use get_fax for one fax's details.' The routing decision is fully specified.

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