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

The description discloses important behavioral traits: the API key is returned only once and never again, and the service is free with no card or trial clock. These details go beyond the basic schema and help the agent understand the consequences of calling this tool. However, it doesn't mention what happens if the email already exists or other error conditions, which would be useful.

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 concise and front-loaded: it states the core purpose first, then adds key behavioral details. Every sentence earns its place, and the structure is easy to parse. It's not overly verbose and covers the essential points.

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 simplicity (3 params, no output schema, no annotations), the description covers the essential context: what it does, when to use it, and key behavioral caveats. It could mention error handling or idempotency, but for a simple account creation tool, it's largely complete. The lack of an output schema is mitigated by the description stating that an API key is returned.

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 the schema already documents all parameters. The description adds context about the password ('Needed only to sign in to the web console') and the email ('A contact address'), but these are already in the schema. The description doesn't add significant new meaning beyond the schema, so a baseline 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 clearly states the tool's purpose: creating an account on the email service and receiving an API key. It uses a specific verb ('create') and resource ('account'), and it distinguishes itself from siblings by emphasizing that it requires no existing credentials, making it the entry point for agents with nothing.

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 explicitly states when to use this tool: 'Needs no existing credentials, so an agent with nothing can call this first.' This provides clear context and implies that other tools require credentials, effectively guiding the agent to use this tool as the first step. It doesn't explicitly name alternatives, but the context is strong enough.

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