Read pixels

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

With annotations already declaring readOnlyHint=true and openWorldHint=false, the safety profile is covered. The description adds meaningful behavior beyond annotations by specifying the exact return structure ('list of distinct colours plus a row-major index grid') and a performance/cost hint ('look is cheaper'). This gives an agent useful expectations about result shape and relative cost, going beyond the safety annotations.

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 with zero filler: the first defines the operation and output, the second gives concrete use cases, the third names the cheaper alternative for visual inspection. Information is front-loaded and every sentence earns its place, making the description efficient and scannable.

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 read-only tool with five optional parameters, a nested region object, and an output schema, the description is fully sufficient. It covers the result format, when to use the tool, and when to prefer a sibling. The schema handles parameter details and an output schema exists, so nothing an agent needs to call this correctly 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 description coverage is 100%, so the input schema fully documents all five parameters, including defaults and fallback behavior (e.g., 'Omit to use the active frame'). The description does not add parameter-specific details, but its mention of 'row-major index grid' and 'rectangular region' clarifies the semantics of the region and output. Baseline 3 is appropriate because the schema carries the parameter-documentation burden.

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 opens with a specific verb and resource: 'Read a rectangular region as structured data: a list of distinct colours plus a row-major index grid.' This precisely states both the operation and the output format, leaving no ambiguity about what the tool does. It also differentiates from siblings by contrasting with 'look' ('For eyeballing, look is cheaper'), making the tool's niche clear.

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 description explicitly tells an agent when to use this tool: 'Use this when you need to compute over pixels (sample a palette from art, find a silhouette edge, copy a region) rather than just look at them.' It then names the alternative ('look') and the condition for choosing it ('For eyeballing'), giving direct routing guidance instead of leaving it to inference.

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