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

Annotations already declare the mutation/safety profile (readOnly=false, destructive=false, idempotent=false, openWorld=false), so the bar is lower. The description adds genuinely useful non-structured context: this is a legacy path, registration through it is rejected, and plugin connections must use browser OAuth. It omits what happens on success (session/token issuance) and any lockout or rate-limit behavior.

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 short sentences, front-loaded with the primary use case followed by the two exclusion rules. No filler or repetition, and each sentence carries distinct routing information.

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?

With no output schema and no return-value documentation, the description leaves an agent guessing what a successful call yields (session token, cookie, user object) and how credentials persist. It fully covers when-to-use and rejection behavior, but the auth result contract is a meaningful gap for a sign-in 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 description coverage is 100%, so the schema already documents all four parameters, including the deprecated 'register' flag and its rejection. The description reinforces that registration is unavailable but adds no syntax, format, or constraint detail beyond what the schema states; baseline 3 applies.

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+resource combination ('sign-in for an existing password account') and flags the tool as legacy, which is enough to separate it from a modern auth flow. It does not name the OTP siblings (auth_request_otp/auth_verify_otp) directly, so differentiation is by scope rather than by explicit sibling reference.

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 scopes use to existing password accounts and rules out account creation and ChatGPT/Codex plugin connections, pointing those to browser sign-in/OAuth. It stops short of routing OTP-based or passwordless flows to the sibling OTP tools, leaving one alternative unstated.

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