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

Annotations already declare readOnlyHint=true and destructiveHint=false, so safety is covered, but the description adds genuine context: it returns mermaid source and, importantly, discloses that the sibling update performs a full-source replacement rather than a patch. It stops short of noting error behavior for a missing id or workspace-resolution edge cases, but the replacement semantics are a meaningful non-obvious disclosure.

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 tight sentences with zero filler: the read purpose and id provenance come first, the editing workflow cue second. Nothing could be removed without losing information.

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 carries the return-value burden and does identify the return as the diagram's mermaid source, plus the id provenance and a mutation warning. It is essentially complete for a simple single-record read, though a hint about not-found behavior would close the last gap.

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 coverage is 100% and both parameters are already documented, including that diagramId comes from napkin_diagrams_list — which the description merely repeats. The workspace parameter's token/default rules exist only in the schema and are not echoed in the description, so no value is added beyond structured data. Baseline 3 applies.

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 (Fetch), resource (one Napkin diagram's mermaid source) and lookup key (by id), and explicitly differentiates from siblings by naming napkin_diagrams_list as the id source and napkin_diagrams_update as the mutation counterpart. An agent can distinguish this from the other napkin_diagrams_* tools without opening a 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?

Gives an explicit workflow condition — 'Read before editing — napkin_diagrams_update replaces the whole source' — naming the alternative and the reason it must precede it. This is exactly the when/why guidance that prevents a destructive blind overwrite.

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