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

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false. The description adds 'Side effects: none (read-only)' which reinforces but doesn't expand beyond annotations. It doesn't add context about response format or pagination, but output schema exists. No contradiction.

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 with zero waste. The core purpose is stated first, and the read-only note is separate, contributing to clarity.

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 tool's low complexity (2 params, simple read), the description is mostly sufficient. The output schema exists, so return values are documented elsewhere. Minor gap: no mention that 'user_id' can be used to access another user's numbers, but the schema covers it.

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%, so parameters are well documented in the schema. The description adds no extra semantics beyond what the schema provides. Baseline of 3 is appropriate.

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 states 'Get information about a specific fax number', which is clear as a verb+resource. The name 'get-number' is accurately reflected. It doesn't explicitly distinguish from sibling tools like 'list-numbers' or 'get-fax', but the specificity of 'fax number' helps.

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 implies usage for querying information about a fax number, but does not explicitly state when to use this instead of 'list-numbers' (for multiple numbers) or 'get-fax' (for fax transmissions). The 'side effects' line is about behavior, not usage guidance.

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