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

Annotations already declare readOnly, idempotent, non-destructive, and closed-world. The description adds genuinely new context beyond that: data granularity per printing, USD currency, the 7-1000 day window, and the February 2024 data-start caveat. The 'past prices only, no forecasts' line usefully prevents a wrong expectation.

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 sentences: usage trigger first, return shape second, scope limit last. Every clause carries information an agent needs; nothing is repeated from the schema except a harmless restatement of the day range.

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?

An output schema exists so return formatting is covered, and annotations carry the safety profile. Combined with the explicit data-start, currency, and no-forecast notes, an agent has everything needed to call this correctly and set expectations.

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 the baseline is 3. The description earns an extra point by adding the data-availability caveat ('history starts February 2024, later for newer cards'), which tells the agent that a large `days` value may return a shorter series — information the schema does not carry.

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 pair: returns the daily market price per printing over the last `days` days in USD. The trigger framing ('going up or down', 'worth on a past date') separates it from snapshot tools like card_price, but no sibling is named explicitly.

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?

Opens with an explicit when-to-use trigger list covering trend questions, period movement, and historical point-in-time lookups. It does not state when NOT to use it or name the alternative (e.g. card_price for the current price), so routing is inferred rather than spelled out.

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