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

With no annotations, the description carries the full burden and does meaningful work: it discloses the signup side effect and its consequence ('a new email is signed up with 200 free emails a day'), states that a session key is the return value, and documents how to use it ('Authorization: Bearer <session_key>'). It omits key lifetime, invalidation, and failure behavior for a bad OTP.

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?

Three tight sentences, front-loaded with the purpose, then the two auth flows, then the downstream header usage. No filler and no repetition of the name.

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?

No output schema exists, so the description correctly states the returned session key and the signup side effect, and all three params are optional so the alternative flows account for that. It stops short of error cases (invalid/expired OTP) and session key lifetime, which an auth tool arguably should mention.

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%, so per-parameter text is already covered, but the description adds cross-parameter semantics the schema cannot express: email alone triggers an OTP send, email+otp yields a session key, and api_key is a mutually exclusive alternative. That mapping is genuinely additive beyond the individual property descriptions.

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 states a specific verb and resource ('Sign up or sign in to email59') and spells out the two concrete paths: email → OTP → session key, or API key swap. It is instantly distinguishable from adjacent siblings like create_api_key (which issues keys rather than exchanging one) and the many send/check tools.

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 an explicit call sequence ('Pass email to have a 6-digit code emailed, then call again with email and otp') and names the alternative path (api_key). What is missing is explicit guidance about which path to choose when both are available, or when not to use this tool.

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