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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so safety is covered. The description adds non-obvious behavioral context beyond that: the response includes example images and enumerates the available looks, telling the agent this call produces visual reference material rather than a list of text keys.

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 with no filler: the first defines the output, the second gives the routing rule. The purpose and the next-step guidance are front-loaded and immediately actionable.

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?

With no output schema, the description carries the return-value burden and does so by indicating the response contains example images and the enumerated style options. For a zero-parameter, read-only listing tool, nothing an agent needs to select or invoke it 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?

The tool takes zero parameters, so there is no argument syntax for the description to clarify. Baseline for a parameterless tool is 4, and nothing here misrepresents the input contract.

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 ('Shows the looks of a Collager.ai photo mosaic') and names the concrete style variants (soft classic, clean subject, stronger mosaic, uniform/mixed tile sizes). The parenthetical definition of a photo mosaic also disambiguates it from the shape-collage siblings (list_collage_shapes, get_shape_collage).

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?

Explicitly says to use it 'before start_photo_mosaic when the user wants to choose a style,' which names the downstream sibling and the triggering condition. It stops short of noting when this call is unnecessary (e.g., style already chosen), so it is clear context rather than full when/when-not coverage.

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