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

Annotations already establish read-only, idempotent, non-destructive behavior. The description adds valuable context: hosted rendering uses resvg-wasm with bundled fonts, bytes may differ from the local napi renderer, and the return shape is { ok, png_base64 }. This goes beyond the annotations without contradicting them.

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?

The description is three sentences with no fluff. The first sentence states the core purpose and return format, the second adds a critical determinism caveat, and the third points to an alternative for different artifact types. Every sentence earns its place.

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?

Given the absence of an output schema, the description usefully states the return shape. It also covers the hosted/local distinction and determinism caveat. The schema is very rich and already documents all parameters. Minor gaps remain (e.g., error behavior, size limits), but overall the description is complete enough for an agent to invoke and interpret results.

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 100%, so each parameter already has a description. The tool description adds little new parameter-level meaning beyond confirming the source is a string and mentioning a distinction for file/URL artifacts. Since the schema carries the parameter burden, a baseline of 3 is appropriate.

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 description opens with a specific verb+resource: 'Rasterize a Mermaid source string to PNG.' It also states the return shape '{ ok, png_base64 }' and explicitly distinguishes this tool from image-byte determinism and the local stdio server, making its scope clear relative to sibling tools like render_svg and render_ascii.

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 description gives clear context: this is the hosted PNG renderer for Mermaid source strings, and it warns that hosted output is not part of the byte-determinism contract. It also provides an exclusion: 'For file/URL artifacts use the local stdio server.' It doesn't explicitly name sibling tools like render_svg, but the guidance is sufficient for most selection decisions.

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