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

Goes well past the annotations: discloses that generation spends the same AI cover allowance as the web app with no MCP-specific quota, that a failed generation costs nothing, that the image is attached rather than returned, and that the artwork is hard-wordless regardless of prompt. The one gap is that it doesn't say how long generation takes or whether the call blocks, which matters for a non-idempotent AI operation.

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-loaded with the verb and the attach behavior, then cost and constraint disclosures in decreasing order of importance. Every sentence carries content, though the prompt/no-text rule is stated twice (once in prose, once again via the schema echo of 'the no-text rule cannot be lifted by a prompt').

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 correctly closes the loop by stating the image is attached to the ebook and surfaces in the exported PDF rather than being returned. Cost model, failure semantics, overwrite safety and style-vs-layout separation are all covered, leaving nothing an agent needs before calling.

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 already 75%, so prompt, artStyle and replaceExisting are documented in the schema. The description adds real operational meaning on top: the consequence of omitting prompt (concept derived from section headings), that the no-text rule is un-liftable by prompt, and the artStyle-versus-cover.style distinction. It says nothing extra about projectId, but that parameter is self-evident.

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 ('Generate cover artwork with AI and attach it to one of the caller's ebooks') and scopes it to ebook covers versus content images. It also distinguishes the artwork from the cover LAYOUT, deflecting that concern to update_ebook's cover.style, so the agent can tell it apart from generate_content_image and update_ebook without opening a schema.

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?

Explicit sequencing guidance ('create the content first and this fits it'), explicit optionality ('Neither a prompt nor an art style is required'), an explicit handoff for layout to update_ebook, and an explicit overwrite condition via replaceExisting. When-to-use, when-not-to-use, and the alternative tool are all named.

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