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

Annotations already declare readOnly/idempotent/non-destructive/openWorld=false, so safety is covered. The description adds genuinely new behavioral facts that annotations cannot express: the ~15 minute delay, the cap of 10 symbols per side, and that only percent change is returned with no prices.

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 sentences, each earning its place: definition, when-to-use, and return shape/limitation. The scope ('today's biggest movers') is front-loaded before caveats.

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 carries the return-value burden itself and does so: up to 10 tickers per side, percent change only, no prices, ~15 minute delay. Nothing essential for correct invocation or interpretation of results is missing.

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 both parameters (side enum and limit range) are already fully documented in the schema, including the meaning of fire/ice/both and the 1-10 bound. The description restates 'up to 10 ticker symbols per side with percent change' but adds no syntax or defaults beyond what the schema carries, so the baseline 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?

Uses a specific verb+resource framing ('Today's biggest US stock movers') and precisely defines the domain terms fire/frozen as largest gainers/losers. An agent can immediately tell what it returns, but the description never references the sibling get_market_heat, so the boundary between a 'movers' list and a 'market heat' view is left for the agent to infer.

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 trigger ('Use this when someone asks which stocks are on fire, frozen, or moving the most today') and an implicit exclusion via 'no prices and no quote for a specific ticker', which steers agents away from using it as a quote lookup. It stops short of naming the alternative tool to use for those cases, so it is clear context rather than full when/when-not routing.

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