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

Annotations already cover read-only, idempotent, non-destructive behavior. The description adds non-obvious traits: it returns PNG/SVG links and a preview, auto-numbers arrows, resolves product logos/icons, omits nodes for known icon ids, and requires the full updated spec when editing.

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?

It is front-loaded with purpose, output behavior, and the key spec model. The dense semicolon-heavy enumeration of spec keys is mostly necessary reference material, but it is harder to scan than it could be.

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 complex renderer with no output schema, the description explains the returned artifacts and enough YAML structure to construct valid calls, while annotations cover safety. Remaining gaps are minor: error behavior is not described and the top-level theme parameter versus spec theme key is ambiguous.

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?

With 67% schema description coverage, the description compensates substantially by defining YAML spec keys, theme values, direction behavior, node/group/map shapes, edge syntax, and icon aliases. However, it does not reconcile the top-level `theme` schema parameter with the described `theme` spec key, leaving some ambiguity.

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: render a system design diagram. It explicitly scopes the tool to system design, architecture, data-flow, and infrastructure diagrams and contrasts it with generating an image or writing SVG/Mermaid. The sibling list_icons is clearly different.

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 first sentence tells the agent when to use this tool and what alternatives to avoid. Later instructions specify giving the user the PNG link, not generating another image, and calling again with the whole updated spec to edit.

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