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

Goes well beyond the annotations, which only say it is a non-idempotent write. The description discloses that it writes to the account, that the token side fills at the real quote so putInUsd can be under askedUsd, that fees are 0.25% to open plus 5% of earned fees, that it consumes 4 units and a heavy slot, and it enumerates four refusal conditions. That is exactly the behavioral context annotations cannot carry.

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?

Long but densely front-loaded: purpose and routing come first, then returns, then behavior, then cost, then errors, with no filler sentences. The run-on clause structure costs a little readability, but every clause carries information.

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 no output schema, the description compensates by enumerating the return fields (opened, cashUsd, open, say) and by covering prerequisites, cost model, resource consumption, and failure modes. An agent has everything needed to call it and interpret the result.

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%, so the baseline is 3, but the description adds genuine meaning the schema lacks: amountUsd is 'play dollars' drawn from account cash and may not be fully spent because the token leg fills at the pool's real quote. It also ties the band/rung error to lowPct/highPct and rungs, clarifying how those parameters interact.

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 ('Opens a paper position with play money in a real pool') and immediately scopes the pricing model ('laid out at the pool's price on the chain right now'). It explicitly distinguishes itself from plan_build, the sibling that opens a real position from a wallet, so an agent can route correctly without opening schemas.

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?

Gives an explicit precondition ('Use after arena_join'), a source for the required pool argument ('a pool from arena_pools or the token's default'), and names the alternative for the other case ('for a real position from a wallet use plan_build'). When-to-use and when-to-use-something-else are both covered.

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