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

Beyond the annotations (readOnlyHint=false, idempotentHint=false), the description discloses the concrete price (TEST PRICE 500 credits), that the effect fires immediately and is non-reversible in practice ('fires IMMEDIATELY'), that it consumes the caller's own handle and emits a public announce banner with a class badge, and that auth plus shop scope are required. These are exactly the operational facts an agent cannot derive from annotations or schema.

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?

Information is front-loaded: trigger, eligible items, cost, effect, exclusion, requirements, example. The leading 'Arena Zing Buy.' merely restates the title and is dead weight, and the ALL-CAPS emphasis is dense but each remaining clause carries real signal.

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?

Output schema exists and cost/auth/side-effects are covered, so most of the calling picture is complete. However, all four parameters are optional and the sole example is 'arena_zing_buy {}', leaving unexplained which item is purchased when no item_id is supplied — a real ambiguity for a purchase that spends credits and fires under the agent's handle.

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 aliasing of id/item_id/catalogId and the key parameter is already documented upstream; the description only reinforces that item_id comes from arena_shop_browse. It adds no guidance on which alias to prefer or how to resolve the id, 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 gives a specific verb and resource (buy a standard-tier zing) and enumerates the eligible items (zing-nuke, zing-love, zing-virus, zing-alarm, zing-zombies), plus names the source catalog arena_shop_browse. The closest sibling, arena_shop_buy, is never mentioned, so the boundary between this specialized zing purchase and the generic shop purchase is left to inference.

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 'WHEN:' clause states the trigger condition explicitly (buying a standard-tier zing with Arena play credits), and 'GODMODE is never sold' functions as an exclusion. What is missing is the counter-case: when an agent should call arena_shop_buy rather than this tool, which is the most likely routing decision it faces.

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