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

With no annotations, the description carries the burden and does well: it discloses a per-call cost of €0.012 and that billing is tied to the API key in the Authorization: Bearer header, which are the two facts an agent most needs before calling. It stops short of stating the return shape (URL vs. binary), retention, or failure modes, though an output schema exists.

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?

Output format precedes cost, auth, and parameters, so the most decision-relevant facts are front-loaded. The trailing parameter block is a single dense run-on sentence separated by dashes and semicolons, which is compact but slightly harder to scan than a list.

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 zero-annotation image generator with an output schema, the essentials are present: what is produced, resolution, cost, auth, template choices, and per-parameter meaning. Gaps are secondary — no indication of which template fields are required, no color value format, and no note on what happens with invalid or conflicting template/parameter combinations.

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 0%, so the description must compensate, and it lists all ten parameters with meaning, including the template enum (post|minimal|product|html) and the cross-parameter constraint tying image_url to template=product and html to template=html. Color parameters (bg, fg, accent) are not given a format such as hex, and no field is marked required or optional.

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 first sentence names the concrete artifact (a 1200×630 PNG social preview image) and the two production modes (built-in template or custom HTML), which is more specific than the tool name alone. It does not contrast itself with adjacent siblings such as screenshot, html_pdf, or seo_meta, so an agent must infer which image-generation route to take.

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?

Usage context is implied by 'for social previews' plus the note that image_url applies only when template=product, which gives some conditional guidance. There is no explicit statement of when-not-to-use this and no pointer to a sibling alternative, so routing between this and html_pdf/screenshot is left to inference.

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