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

Annotations declare readOnlyHint=false, destructiveHint=false, idempotentHint=false, openWorldHint=true, so the safety profile is covered. The description adds real context beyond that: the two-step OTP flow, the signup side effect and quota, and how to use the returned key via the Authorization header. Missing only operational details like code/session expiry or rate limits.

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?

Front-loaded with the core action, then the flows in call order, then the header usage. Dense but virtually every clause carries information; the parenthetical quota note is the only slightly tacked-on element.

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, and the description compensates by explaining that the flow yields a session key and how to send it. Both auth paths are covered. It stops short of error handling, session expiry, and OTP retry behavior, which would complete the picture for a mutation-capable auth tool.

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 the 3 baseline applies, but the description goes further by explaining the relationship between parameters (email triggers the OTP email, otp is the code from it, api_key is an alternative to email) and the e59_live_ prefix convention. That is genuine added meaning over the schema.

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?

States a specific verb and resource ('Sign up or sign in to email59') and immediately distinguishes itself from siblings like create_api_key or get_account by covering the session-acquisition role. An agent knows exactly what this tool is for.

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?

Explicitly routes between two paths: pass email to trigger a 6-digit code then call again with email+otp, or pass api_key to swap for a session key. It also discloses the side effect (new email = account with 200 free emails/day), giving the agent enough to choose deliberately.

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