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

readOnlyHint=true already tells the agent this is a safe, non-mutating render operation, so the bar is lower. The description adds the return form ('Returns an image link'), which is genuinely useful. It says nothing about the 5000-point data cap, cost/latency, or what happens on invalid dates, leaving real behavioral gaps.

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 short sentences: what it draws, when to use it, what it returns. Front-loaded with the visual identity and zero filler. Nothing could be cut without losing 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?

For a render-only tool with no output schema, the definition covers identity, purpose and return type adequately. However, the most important input (the data array's shape and limits) is never described, and the theme/brandTheme distinction is left entirely to the schema, so it is only minimally complete.

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%, and the undescribed remainder includes the central 'data' array and the 'theme' enum. The description adds no parameter meaning at all: nothing about date format, value scaling, colour ramp, or when to use brandTheme. An agent must read the schema to learn anything about inputs.

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?

Names a specific verb and resource plus the visual form: 'Draw a calendar heatmap (GitHub-style contribution grid): one cell per day coloured by value.' That is enough to picture the output. It does not explicitly distinguish itself from the sibling generate_heatmap_chart, which is the most likely confusion point.

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?

'Use for daily activity over weeks or months' gives a clear positive condition based on data shape. There is no when-not guidance and no mention of when to prefer generate_heatmap_chart instead, so the agent must infer the boundary from the sibling name alone.

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