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

With no output schema, the description carries the return contract itself ('compact JSON: the ids it created, changed or removed, the diagram revision, a one-line summary and the next step') and discloses an auth constraint ('A viewer-role account is refused') plus rendering semantics ('arrowhead at the target; bidirectional adds one at the source'). This is substantial disclosure beyond the annotations, which only cover the safety profile.

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 the 'When:' clause and then cleanly partitioned into Batch / Returns / Next sections. Every sentence carries actionable content — batching, return shape, follow-up step — with no filler.

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?

For a two-parameter mutation tool with no output schema, the description covers the missing pieces an agent needs: return shape, auth refusal, batch limits, and the prescribed next step (canvas_layout_canvas once the structure is complete). Nothing required to invoke it correctly is absent.

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 only 50%, and the description compensates by explaining the batch semantics of `edges` (up to 60, one call) and the intent behind `label` ('only when the reader could not guess what it does'). The nested item fields are already well documented in the schema, so this adds meaning where the schema was thin rather than duplicating it.

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 concrete verb+resource: 'Draw edges (arrows) between nodes to show how data flows,' and constrains scope with 'Operates on one Datadef diagram, named by diagram_id.' It is clear on its own, but it never differentiates itself from closely-named siblings like canvas_add_flow_arrow, canvas_add_lineage or canvas_update_edges, which an agent must disambiguate against.

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 explicit when-to-use ('flows that genuinely move data'), an exclusion ('Never connect a zone: connect a node inside it'), labelling guidance, and a batching rule ('put every item in one call rather than one call per item'). It stops short of naming an alternative tool, so it is strong context rather than full routing.

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