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

With readOnlyHint=true, the description adds useful context beyond the annotation: the benchmark source ('Cleanor Labs’ controlled benchmark') and the additional HEIC conversion tax output. It does not contradict annotations and provides a clear behavioral picture without needing to describe side effects.

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 sentences, front-loaded with the core value proposition, then the extra HEIC detail, and finally the use case. Every sentence earns its place; no filler or repetition of schema 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?

The tool has only 2 parameters with full schema descriptions, an output schema, and a clear description covering purpose, use case, and additional output. The agent has all necessary context to select and invoke the tool correctly.

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 adds meaning by explaining the 'matched perceptual quality' concept, which directly relates to the quality parameter. It also ties the format parameter to the comparison against JPEG, going beyond the schema's enum list.

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 clearly specifies the tool's function: it quantifies storage savings of WebP, AVIF, and JPEG XL over JPEG at matched quality. It also mentions the HEIC conversion tax and explicitly states the intended use ('justify a format choice'), distinguishing it from sibling tools like optimize_image or placeholder_image.

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 gives an explicit use case: 'Use to justify a format choice when building a site or app.' It clearly indicates when to use the tool. It doesn't name alternatives or exclusion criteria, but the context is sufficient for an agent to decide between this and other image-related tools.

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