Centre Marks (ISO 128-23)

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

Annotations only declare destructiveHint=false, so the description carries nearly the whole load and does: it frames the operation as a write ('before the first entity is written'), lists six distinct validity refusals, notes the ocs_tilted_plane capability limitation, and warns against supplying remembered radii. That is unusually rich behavioral disclosure beyond structured fields.

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?

It is dense but front-loaded with the purpose before the guidance, and the refusal list is scannable. Idiosyncratic asides like 'the classic silent wrong number' cost some space, but every sentence still carries actionable 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 an output schema present, return values need not be described, and the description covers inputs, validation, layer behavior, and the ISO standard. Minor gaps remain (default layer when `layer` is null, what specifically gets created), but nothing critical for correct invocation is missing.

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%, so the baseline is 3, but the description adds genuine semantics: handles are preferred over centers because real centre/radius are read back, and style='lines' requires a radius. The mark-vs-lines and extension meanings are already in the schema, so it does not double up unnecessarily.

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?

It states a specific action and target: placing centre marks on the CENTER layer matching ISO 128-23's dash pattern. The verb is implied rather than stated outright ('Centre marks on the CENTER layer'), but the resource and standard are unambiguous and no sibling tool does the same thing.

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 gives real selection guidance: prefer `handles` so radii come from the drawing via entity_get rather than being restated, and it enumerates the preconditions that cause refusal (neither/both input sets, non-CIRCLE/ARC handles, tilted planes, bad style, non-positive extension). It stops short of naming an alternative tool for the explicit-coordinate case, but the when-to-use logic is present.

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