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

With no annotations, the description carries the full burden and does so: it discloses the return value (bearer token), its lifetime and default (1 year, or token_ttl_days), session-scoped caching behavior, the required precondition, and a typical failure mode (401 invalid credentials). This is substantially more than the structured fields provide.

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-loads the core action and return value before prerequisites, which is good ordering. However, the register_account/no-password caveat is stated twice (in the NOTE and again in the 'Typical error' sentence), which is mild redundancy.

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 or annotations exist, yet the description still conveys the return token, its expiry semantics, the prerequisite, and the failure case. Nothing essential for correct invocation is missing.

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 only 33% (token_ttl_days alone is documented). The description compensates by explaining token_ttl_days' default of 1 year and by clarifying that password must have been established via set_password, adding meaning beyond the bare schema. email and password remain self-evident from their types.

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 (log in) plus the credential resource, and clearly differentiates itself from register_account, set_password, and refresh_token in the sibling set. An agent can select it without ambiguity.

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 covers the when-not case (register_account accounts have no password, so keep using the register token) and names the alternative action (call set_password while authenticated to enable later logins). The 'Requires: a password set earlier' line states the precondition directly.

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