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

Annotations only indicate readOnlyHint=false, openWorldHint=false, and destructiveHint=false, leaving the safety profile ambiguous. The description adds critical behavioral detail: it spends 1 of 50 daily joins only on success, explains the meaning of paired:false and its sub-reasons, and includes a security note about key handling. This significantly enriches the agent's understanding beyond the 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?

The description is longer than average but each sentence serves a purpose: stating the action, the cost, response semantics, and a security caveat. It is front-loaded with the core action and then provides necessary caveats. No filler sentences exist; the structure is logical and efficient.

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?

The description explains the failure reason format and the cost, but it does not describe the success response shape, how to obtain the challenge, or any preconditions like authentication state. Given the tool has no output schema and the request requires a challenge and signature, the description leaves some gaps that an agent would need to infer or look up elsewhere. It is adequate but not fully complete.

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 every parameter has a description in the schema. The description adds some context about the signature covering canonicalJson(body) and the body structure, but these details are largely echoed in the parameter descriptions themselves. It does not substantially deepen parameter understanding beyond what the schema already provides, 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 opens with 'Join a lobby (game, variant, division)', which is a specific verb and resource. It clearly distinguishes the action from siblings like lobby_leave, and the parenthetical enumerates the three dimensions of a lobby. The purpose is unambiguous and immediately actionable.

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?

The description explains that joining spends a daily join and what the response means when paired:false, but it does not state when to use this tool versus alternatives (e.g., when you should join a lobby vs. create one, or conditions that would make joining inappropriate). The context is implied by the action, but there is no explicit when-to-use or when-not-to-use guidance.

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