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

Annotations only declare the generic non-readonly/non-idempotent/open-world profile; the description adds the substantive traits: a 300 req/min rate-limit payoff, the exact header format for the returned session key, and the significant side effect that unknown emails are auto-signed-up and issued an api_key. That side effect is precisely the kind of non-obvious behavior an agent must know before calling.

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 dense sentences, front-loaded with purpose then the two paths then the output header convention; essentially no filler. Slightly long as a single undifferentiated paragraph, and the parenthetical about new signups is easy to miss.

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, and the description compensates by naming the return value (session key) and how to transmit it. For a zero-required-parameter auth tool with three alternative inputs, everything an agent needs to complete either flow is present.

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 baseline is 3; the description adds relational semantics the schema cannot express — that otp is only meaningful paired with email, and that the two credentials are mutually exclusive alternatives rather than combinable inputs. Required parameters are zero, yet the description makes the effective requirements clear.

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 in to b59.link') and immediately qualifies it with the concrete benefits (ownership of created resources, 300 req/min). Siblings are all content-creation/lookup tools, so authentication is unambiguously distinguishable without opening the schema.

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 lays out the two invocation paths — pass api_key for an immediate session key, or pass email then email+otp — which is exactly the branching guidance an agent needs. It stops short of saying when to call this relative to the sibling tools (e.g. 'call first if you need higher rate limits or ownership'), so no explicit exclusion/precedence rule.

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