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

Annotations already declare readOnly, idempotent, and non-destructive. Beyond that, the description discloses that the account is derived from the cookie the MCP client passes, requiring no arguments, and that the account owns the library only when a session exists – useful behavioral context the annotations cannot convey.

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?

A single dense sentence that front-loads the resource. The trailing clause about ownership 'when there is a session' repeats the session-conditional idea already implied earlier, costing a little tightness but not much.

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?

For a zero-parameter read tool with no output schema, the description usefully enumerates the returned fields (e-mail, library size, user.id). Nothing essential is missing, though it could note behavior when no session exists.

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?

The tool takes zero parameters, so the baseline is 4. The description's reference to `user.id` and the session cookie is framed as identity/return context rather than an argument, which is appropriate and adds clarity without implying a parameter.

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?

The description names a specific resource (the session account) and the fields it exposes (e-mail, account library size), which disambiguates the otherwise cryptic tool name 'me'. It does not explicitly contrast itself with the adjacent get_library or contact siblings, so it stops short of a 5.

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?

There is no explicit when-to-use guidance, no mention of alternatives like get_library, and the only contextual hint ('when there is a session') is a data condition rather than an invocation guideline. Usage is left to inference.

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