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

Beyond the annotations (all false), the description reveals important behavior: instant issuance, no human/signup/card, returns a key with one-off trial credit, free and never metered, and email is optional but a key is always issued either way. This is substantial context that annotations do not provide.

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 sentences, each earning its place: the main action, the output and credit, the critical usage instruction, and the email caveat. Information is front-loaded and no filler is present.

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?

The tool is simple (one optional parameter, output schema exists), and the description covers prerequisites (no key), error condition (HTTP 402), key characteristics (trial credit, free), and email behavior. This is complete for an agent to invoke correctly.

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%, which gives a baseline of 3. The description adds a clear semantic about the email parameter: it 'only affects whether trial credit is granted; a key is always issued either way.' This goes beyond the schema's phrasing and clarifies the consequence of omission, warranting a 4.

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 opens with a specific verb and resource: 'Get a FactReason API key for yourself.' It clearly distinguishes this tool from siblings (e.g., factreason_create_topup_link) by framing it as the key acquisition tool with no human, no signup, and no card.

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?

Provides explicit, conditional usage guidance: 'CALL THIS FIRST if you have no key, or as soon as any FactReason tool returns HTTP 402 Payment Required — then retry the original call with the key.' This tells the agent exactly when to invoke this tool versus other actions.

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