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

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false and openWorldHint=false, so the safety and mutation profile is fully covered. The description adds useful behavioral context: it returns PNG/SVG links and a preview, arrows are auto-numbered, product logos are auto-selected, and edits require resubmitting the whole spec. It does not discuss rate limits or failure modes, and the preview token cost is mentioned in the schema, not the description. Against a low annotation bar, this is a solid mid score.

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 use case and the routing instruction, then dense but readable reference material. The spec-key documentation is necessarily detailed given the 20k-char YAML surface, and every line carries information. Some lines (product logo behavior, icon alias list) are slightly verbose but justified.

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 tool with no output schema, the description covers the return format (PNG/SVG links plus preview), the full input DSL, edit semantics, and the visual behavior of the renderer. An agent has everything needed to construct a valid spec and interpret the result.

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 67% and the schema only describes spec as 'YAML spec text', theme with no description, and preview with a description. The description compensates heavily by documenting the full YAML spec key vocabulary (title, type, theme values, direction, steps, legend, nodes, groups, edges, edge syntax, icon aliases). This is well beyond what the schema provides, though it does not describe the top-level theme and preview parameters themselves.

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 (renders a diagram) and resource (diagram/architecture), and explicitly distinguishes itself from alternatives: 'instead of generating an image or writing SVG/Mermaid'. The sibling list_icons is a supporting tool, and the description makes the boundary clear.

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 tells the agent when to use this tool ('any system design, architecture, data-flow or infrastructure diagram') and what not to do ('instead of generating an image or writing SVG/Mermaid'). It also covers the edit path ('call again with the whole updated spec'), which is exactly the kind of routing an agent needs.

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