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

Annotations already declare readOnly/idempotent/non-destructive, but the description adds material context: nothing is decoded, a base64 prefix suffices, and errors come back as {"error": ...} rather than protocol errors, making retry-after-correction safe. That is behavioral information annotations cannot express.

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?

Front-loads the core capability and the 'FREE' cost cue, then groups usage and error behavior cleanly. The inline input/output JSON examples are verbose but earn their place by making the prefix-only contract concrete.

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 values need no explanation, yet the description still shows the shape. Size limits, error contract, and read-only idempotency are all covered, leaving nothing an agent needs to call this 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 goes further by stating that only a base64 prefix is needed and by showing a concrete input example. The remaining details (data-URL stripping) live in the schema rather than the description, so it does not fully extend the single parameter.

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: 'Read an image's format and pixel size from its header alone,' plus the supported formats (PNG, JPEG, GIF, BMP, WebP). It is clearly distinguishable from siblings like resize_plan and colour_check, which the description implicitly excludes ('before planning a resize', 'nothing here reads pixels').

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?

Gives explicit when-to-use ('Use to find out what you are dealing with before planning a resize') and explicit when-not ('Not for pixel content ... and not for EXIF'). This is exactly the routing information an agent needs against sibling tools.

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