Who is signed in

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

Annotations already declare readOnlyHint and openWorldHint, and the description adds value by disclosing the thrown-error behavior and the precise-code failure mode. It could have described the return shape or failure code format, but the conditional behavior is a useful addition beyond annotations.

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?

Two sentences, zero filler. The purpose is stated first, and the important caveat and alternative are packed into the second sentence without 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?

For a simple read-only whoami tool with one optional parameter and clear failure behavior, the description is complete. It explains what it returns, when it fails, and which sibling tool to use instead.

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%, and the single optional parameter is fully described in the schema ('Leboncoin site to use (fr). Defaults to the server's configured site'). The tool description adds no extra parameter meaning, so the baseline of 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?

The description uses a specific verb ('Returns') and names the exact resource: the Leboncoin account the stored profile is signed in as. It clearly distinguishes itself from session_status by noting the failure behavior.

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 states when to use this tool versus an alternative: 'use session_status instead if you want a diagnosis rather than a failure.' It also signals that a failure is thrown when no usable session exists, so agents know this is for identity queries, not session diagnostics.

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