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

Annotations already declare readOnly, idempotent, and non-destructive, so the safety profile is covered. The description adds real value beyond that: text is only present 'when an extraction exists' (conditional return), and it explicitly flags the sender header and body as untrusted prompt-injection surface that must never be forwarded as a destination or approval id. It does not cover not-found behavior, which keeps it below 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?

Two sentences, front-loaded with what is returned, followed by the security caveat and the scope. Every clause carries information; 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?

An output schema exists, so return-shape explanation is not required, yet the description still characterizes the payload well and covers the critical untrusted-input risk. The remaining gap is the undocumented inbox_id and unspecified behavior when the fax or extraction is absent.

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 0% and the sole parameter inbox_id is undocumented in both schema and description. The 'Scope: inbox:read' line hints at inbox scoping, but the description never explains what inbox_id refers to or where to obtain it, so it only partially compensates for the gap.

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 names a specific resource and scope: a single received fax plus its metadata, document reference, and extracted text. 'One received fax' implicitly distinguishes it from list_faxes and preview_fax, though it never names a sibling explicitly.

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 is implied rather than stated: 'One received fax' signals single-item retrieval versus listing, and 'Scope: inbox:read' tells the agent what authorization it needs. There is no explicit when-to-use/when-not guidance or named alternative for the listing case.

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