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

Annotations already convey that the tool mutates state and is not idempotent, and the description adds a clear 'Side effects' line stating that it creates a webhook subscription. It does not go deeper into duplicate creation, replacement, or confirmation behavior, but it does not contradict the annotations.

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 short, front-loaded with the core operation, and includes a separate side-effects line with no wasted words. The optional-number behavior is explained in one clear sentence with an example.

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?

Given the output schema exists and schema coverage is 100%, the description covers all decisions an agent needs to invoke the tool correctly. It could add duplicate-handling or idempotency details, but the annotations already signal idempotentHint=false, so the remaining gap is minor.

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, but the description adds meaningful behavioral nuance beyond the schema: it explains the filtering effect of passing numbers and the all-numbers behavior when omitted. This helps an agent choose parameter values correctly even beyond the schema fields.

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 clearly states the action, resource, and inputs: create a webhook subscription for a fax event and target URL. It also explains the optional number-filtering behavior, making the tool's scope unambiguous and distinguishing it from webhook deletion/inspection tools.

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?

The description gives explicit usage guidance for when to provide numbers vs. omit them: pass E.164 numbers to filter to specific fax numbers, or omit to receive all events. It does not name alternative sibling tools, but the create operation is self-selecting and this guidance resolves the main conditional decision.

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