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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the bar is lower. The description adds genuine value by clarifying the semantic ceiling of a success result and restating that nothing is saved or published — important for an agent deciding whether validation is a safe, repeatable step.

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?

Four short sentences, front-loaded with the core action and zero filler. The closing 'Saves and publishes nothing' slightly overlaps the readOnly annotation but still reads as deliberate scoping.

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?

With no output schema, the description should convey what 'error details' look like (line numbers, messages), and it does not. The safety and semantic-scope story is complete, but the return-value shape and the meaning of the sole parameter are left open.

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?

The single parameter 'source' has 0% schema description coverage, so the description carries the full burden — but it never states that 'source' is the Mermaid diagram text, nor any format expectations. The agent can only infer this from 'Check Mermaid syntax'.

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: check Mermaid syntax and return error details. It implicitly distinguishes itself from render_mermaid via 'Saves and publishes nothing', but never names that sibling, so the differentiation requires inference.

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?

Explains what a valid result does and does not mean ('confirms syntax only', 'not that the diagram accurately describes a real system'), which helps an agent decide how much to trust the outcome. However, it never says when to call this versus render_mermaid, which is the only sibling and the obvious alternative.

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