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

Annotations already declare readOnly, idempotent, non-destructive, open-world. The description adds real context beyond that: the lifecycle state vocabulary and the fact that delivery status is proxied live from Phaxio (consistent with openWorldHint). It doesn't mention latency, rate limits, or what a not-yet-sent fax returns, so it stops short of a 5.

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?

A single front-loaded sentence with no filler; the enumeration of return fields is the useful payload. The em-dash aside makes it slightly dense but every clause earns its place.

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 two-parameter read tool with full schema coverage, full annotations, and an output schema, the agent has everything needed: what it inspects, what statuses mean, and where the delivery data originates. Nothing material is missing.

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% — fax_id and api_key are both documented in the schema, including the create_fax_draft linkage. The description adds no parameter meaning beyond 'a fax', so the 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?

States a specific verb+resource ('Check a fax') and enumerates exactly what comes back: lifecycle status with the DRAFT/APPROVED/SENT/FAILED scale, page count, quote, and live delivery status. This cleanly separates it from siblings like send_status or request_send, which act on sending rather than inspection.

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?

Implies usage context by noting the live delivery status is only available 'once sent', which hints at when the tool is useful. However, it never explicitly says when to call this versus send_status, or that it should be polled after send_fax; the routing is left to inference.

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