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

No annotations are provided, so the description carries the full burden, and it does disclose two genuinely important traits: entry is free with a $100,000 paper-money starting balance, and the api_key is returned only once and must be stored. That last point is critical operational context an agent would otherwise miss. It does not cover idempotency, duplicate-registration behavior, or rate limits, so it falls short of a 5.

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 tight clauses, front-loaded with the action and scope, then cost/balance, then the return-value warning. Every sentence earns its place with zero filler.

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 17 parameters, a nested strategy_summary object, no annotations, and no output schema, the description does cover the essential return contract (agent_id and api_key, shown once) and the entry economics. It leaves the optional profile fields (pledge, icon, maker metadata) and duplicate-registration semantics unexplained, which is a real but secondary gap for a registration call.

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 53% with 17 parameters, so the description should compensate somewhat, but it adds no per-parameter meaning beyond the schema. Its only parameter-adjacent content is the league count and the starting-balance context, both already implied by the schema enum and its own description field.

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 and resource ('Register your agent') plus scope ('in any of the 24 leagues'), which is immediately distinguishable from siblings like arena_update_maker (modify an existing maker) or arena_set_icon (cosmetic change). An agent can tell this is the entry-point creation tool without opening a schema.

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?

Usage is implied rather than stated — the description tells you registration is free and gives the starting balance, and it cross-references arena_leagues//leagues.json for league status. But there is no explicit when-to-use vs when-not guidance, no mention of whether an agent should register once versus per league, and no prerequisites listed.

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