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

With no annotations, the description carries the full burden and does well: it discloses the sandboxed Chromium execution model, blocked network access, securityLevel 'strict' and the resulting injection protection, plus a per-diagram cost of €0.002 charged to the Bearer API key. It does not cover failure modes (invalid Mermaid syntax, oversized source) or latency, which keeps it below 5.

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 purpose, then security/cost behavior, then a compact labeled parameter rundown. Dense but every clause carries information; the parameter run-on is the only mild readability cost.

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?

An output schema exists, so return values need not be explained, and security, cost, and all five parameters are covered. The one omission is that code is functionally required despite every schema field being nullable — the description never states that code must be supplied, leaving an agent to guess.

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 0%, so the description must compensate and it does: it gives the 20000-char limit on code, the svg|png options with the svg default, the full theme enum with the default, PNG-specific meaning of scale and background, and the #rrggbb/'transparent' syntax. This supplies enums and defaults the schema entirely lacks.

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+resource: Mermaid diagram source rendered to SVG or PNG, with the supported diagram families enumerated. The 'Mermaid' qualifier distinguishes it from the sibling chart_render, though that sibling is never named explicitly.

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?

Usage is only implied by the input type — an agent infers 'use this when you have Mermaid source.' There is no explicit when-to-use/when-not guidance, and no mention of chart_render or md_convert as alternatives for other diagram or document needs.

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