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

Annotations already cover the read-only safety profile, and the description adds genuinely new behavior: a 404 result when no contact owns the number. That error semantics is the main operational fact an agent needs beyond the annotation.

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 short sentences with zero padding; the core action, the format hint, the miss behavior, and the endpoint are all front-loaded and each 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 single-parameter read tool this covers action, format, and failure mode. With no output schema, a note on what the returned contact contains would fully close the gap, but it is nearly 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?

Schema coverage is 100% and the phone_number field already documents E.164 preference with an example. The description repeats that same hint ('+31612345678 works best') without adding format rules, normalization behavior, or what happens with non-E.164 input, so it stays at baseline.

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: 'Find the contact that owns a phone number.' This clearly separates it from rinkel_get_contact (ID lookup) and rinkel_list_contacts, even though no sibling is named explicitly. The endpoint path reinforces the scope.

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 by the lookup framing and the 404 note tells the agent what a miss looks like, but it never says when to prefer this over rinkel_get_contact or rinkel_validate_phone_number. An agent must infer the routing itself.

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