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

With no annotations, the description carries the full behavioral burden and does well: it discloses the 250 requests/month limit, English-only scope, personal-use restriction, required user confirmation, and how to use the returned key (Authorization: Bearer <key>). It does not state whether the creation call itself requires auth, whether multiple keys are allowed, or how the key is returned, leaving minor gaps.

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?

Four tight sentences, front-loaded with the action, each sentence adding a distinct constraint, alternative, or usage instruction. No wasted words or redundant restatement.

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 2-parameter creation tool with no annotations and no output schema, the description covers purpose, constraints, alternative behavior, and key usage well. Minor omissions remain around call authentication and whether the key is returned in the response, but the definition is largely self-sufficient.

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 50%: personal_use already has a schema description, but email has none. The description compensates by clarifying that email identifies the key recipient and adds an actionable requirement for personal_use ('confirm that with the user before setting personal_use=true') beyond the static schema description. It still adds no format or validation details for email.

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 specific action: 'Create a free API key ... for an email address.' It also scopes the offer (English cards, 250 requests/month) and contrasts with the paid alternative, making it distinguishable from the read-only card siblings.

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 explicitly gives when to use the free key (personal, non-commercial use), when not to (commercial use, other languages, printings, images), and names the alternative endpoint for paid plans (POST /v1/billing). It also instructs the caller to confirm personal use with the user before setting personal_use=true.

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