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

The annotations already declare destructiveHint=true and idempotentHint=true, but the description adds meaningful semantics beyond them: cancellation is reliable only while queued, degrades to best effort while sending, and cannot recall already-sent pages. The scope 'fax:cancel' also gives useful authorization context.

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 description is front-loaded and compact, with three short clauses that each carry operational meaning: queued behavior, sending fallback, and non-recall limitation. There is no redundant restatement of the tool name or annotations.

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 simple one-parameter mutation tool with rich annotations and an output schema, the description covers the key behavioral caveats and permission scope. The remaining gap is the undocumented fax_id parameter, but the core call semantics are otherwise complete.

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?

The schema has one required parameter, fax_id, with 0% schema description coverage, and the description never mentions it or its expected format. The phrase 'Cancel a fax' implies some identifier is needed, but the description does not compensate for the missing parameter documentation.

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 gives a specific verb and resource ('Cancel a fax') and immediately clarifies the operational state: queued versus sending. It also states a key limitation ('Never a recall of pages already sent'), so an agent can distinguish this from related fax tools such as get_fax_status or send_approved_fax.

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 clearly indicates when cancellation is reliable ('while it is queued') and when it is only best effort ('once it is sending'). It also gives a strong exclusion ('Never a recall of pages already sent'), though it does not explicitly name an alternative sibling tool for checking status first.

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