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

With no annotations, the description carries the full burden. It discloses a key behavior (no auth needed) and provides critical post-registration steps (api_key returned and required as a header on all future MCP calls). It does not mention potential errors or idempotency, but for a simple registration tool, this is sufficient.

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 two sentences, front-loaded with the main purpose, followed by a vital usage note. Every sentence earns its place with no filler or redundancy.

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 tool with one parameter and no output schema, the description covers the critical aspects: what it does, auth requirements, and what to do with the result. It does not explicitly state what the response contains (though 'api_key' is implied) or whether the email address uses the 'name' parameter, but the context is sufficient for correct invocation.

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 provides 100% coverage for the single parameter 'name', including its format. The description adds nothing beyond the schema, so it provides only the baseline value. It does not explicitly map 'name' to the email address, but that is implied.

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 ('Register') and the resource ('a new agent email at phobomail.com'). It is specific and naturally distinguishes itself from sibling tools like get_email or send_email, which are email operations after registration.

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 provides clear usage context: it emphasizes that no auth is needed for this call, and explicitly instructs to save the api_key and use it as a header on future requests. This implies this is the initial setup tool, but it does not explicitly contrast with alternatives or state conditions like 'only use once'.

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