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

Annotations already declare readOnlyHint=true and idempotentHint=true, lowering the burden. The description adds valuable behavioral context: return shape, concrete ASCII vs Unicode glyphs (+,-,| vs ┌,─,│), default behavior when useAscii is absent, and a typed error for impossible targetWidth bounds. There is no contradiction with annotations.

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?

Three sentences, front-loaded with the core purpose, and each subsequent sentence adds essential behavioral detail. No filler or repetition of schema content; it is efficiently structured.

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?

There is no output schema, so the description properly covers return shape and error behavior. It does not explain the extensive options object, but the schema already does, and the most terminal-relevant options (useAscii, targetWidth) are highlighted. This is complete enough for an agent to use the tool correctly.

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 the baseline is 3. The description adds meaningful details beyond the schema: it shows exact ASCII characters, specifies that absent useAscii defaults to Unicode, and reveals the error condition for targetWidth. These enrich the schema's brief descriptions without redundancy.

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 'Render a Mermaid source string to text,' clearly identifying the verb, resource, and output format. It distinguishes from sibling renderers (render_png/render_svg) by specifically targeting text output and even states the return shape ({ ok, text }).

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 does not explicitly name alternatives, but phrases like 'to text' and 'terminal display-cell bound' make the intended context (plain text/terminal rendering) very clear. It stops short of explicitly saying 'use this instead of render_png/render_svg,' so it is not a 5.

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