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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), so the description only needed to add context beyond that — and it does, by disclosing that output is Mermaid source and that unknown participants cause rejection. It omits the size ceilings (60 messages, 15 participants) and any error shape, keeping it short of a 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?

Two sentences, capability front-loaded, optional features second, validation caveat last. No filler or restatement of the tool name.

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 formatting needn't be described, and the enums/patterns in the input schema are self-explanatory. The description covers purpose, core inputs, and optional constructs, but for a schema this intricate with zero property descriptions, the review-block semantics (else/end) and style variants are left to inference.

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 carry the semantics, and it partially does: it maps participants, ordered messages, activation, notes and blocks to concepts. It never mentions the required 'title', the sync/async/reply style enum, or the actor/participant kind, leaving several required fields explained only by their enum values.

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 and artifact ('Build a Mermaid sequence diagram'), names the exact inputs (participants and ordered messages), and enumerates optional features (activation, notes, alt/loop blocks). An agent can distinguish it from architecture_diagram or c4_model without opening any schema.

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 artifact type — nothing states when to pick this over the diagram-adjacent siblings (architecture_diagram, c4_model) or when not to. 'Rejects unknown participants' is a validation rule, not selection guidance.

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