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

With readOnlyHint=true already declaring the safety profile, the description's main addition is the return contract ('Returns an image link'), which is valuable since there is no output schema. It says nothing about the 5,000-item data cap, behavior when x/y categories are too numerous to render legibly, or what the image link's lifetime is.

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?

Two tight sentences: the visual definition comes first, the return format last. Every clause carries information and there is no filler or restatement of the title.

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?

For a six-parameter chart generator with an annotations-declared read-only profile, the definition covers the essential what and the output shape adequately. It falls short on sibling disambiguation against generate_calendar_heatmap and on any mention of the data-volume limits or category-count caveats that would matter when calling it.

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 67%, with title, width, height and brandTheme already documented in the schema, so the baseline of 3 applies. The description adds conceptual meaning by framing x and y as categories rather than coordinates, but contributes nothing about theme, sizing, or when to use brandTheme versus the built-in colorblind option.

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 ('Draw a heatmap') and then defines the visual form ('a grid of cells coloured by value, with x and y categories') with concrete examples. It clearly separates this from most chart siblings, but it never distinguishes it from the close sibling generate_calendar_heatmap, so an agent can't rule that one out from this text alone.

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 examples (product × region) hint at categorical cross-tabulation, but there is no explicit when-to-use, when-not-to-use, or alternative routing. Notably, 'weekday × hour' is offered as an example even though a dedicated calendar heatmap sibling exists, without guidance on which to pick.

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