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 rate-limit change (2 → 300/min), the account-creation side effect for new emails, and the exact session-key transport ('Authorization: Bearer <session_key>'). It does not cover session-key expiry or error cases, which keeps it short of a 5.

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 sentences, front-loaded with the payoff and reason to authenticate, then the two credential paths, then the header usage. No filler; each sentence carries distinct information an agent needs.

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 no annotations, so the description must cover returns and it does (a session key, plus an api_key for newly signed-up emails). Missing only lifecycle details such as session-key expiry or failure behavior, which are secondary for successful invocation.

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 meaning is already documented; baseline would be 3. The description adds value beyond the schema by explaining the relational, multi-step semantics of the parameters (email → emailed otp → second call with email + otp), which the flat schema cannot express.

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') plus the concrete benefit (rate limit 2→300 req/min, ownership of created resources). This clearly distinguishes it from the sibling tools (shorten_url, make_qr_image, get_stats_url), which are all anonymous content operations.

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 the two flows: pass api_key for an immediate session key, or pass email to receive a 6-digit code and then call again with email + otp. It even discloses the side effect that new emails are signed up and receive an api_key, so an agent knows which path to take without guessing.

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