Check iMessage

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

Annotations already declare readOnlyHint=true and openWorldHint=true, so the safety profile is covered. The description adds one genuinely useful domain rule — 'An email address can only get iMessage' — which tells the agent how to interpret the result for email inputs, but says nothing about latency, failure modes, or the shape of the answer.

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, front-loaded with the core question and followed by the one non-obvious rule. Nothing is padded or repeated.

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-only lookup with no output schema, the description conveys enough: the question answered, the accepted input types, and the email special case. The only gap is that the return value (presumably a boolean) is implied rather than stated.

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 single parameter is already documented as 'A phone number or an email address.' The description restates that accepted input types include both, reinforcing but not extending the schema; there is no format or normalization guidance beyond it.

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 the exact question the tool answers ('Whether a phone number or email address uses iMessage') and the resource it operates on. It is clearly distinguishable from the messaging siblings (send_message, get_message, etc.) because it is a capability lookup, not a message operation, though it never explicitly contrasts itself with them.

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 gives a concrete reason to call it: 'so you know if a text will arrive as iMessage or as RCS/SMS', which tells an agent when this check is worth making before sending. It stops short of explicit exclusions or naming an alternative tool for the case where the answer is SMS.

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