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

Annotations already declare readOnlyHint=false, idempotentHint=false, destructiveHint=false and openWorldHint=true, so the safety profile is largely covered. The description adds real value on top: that creation mutates the brand's data model and dispatches nothing, which is a meaningful side-effect disclosure not present in the annotations.

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 with the operation type front-loaded and no filler. Each sentence carries distinct information: operation, key semantics, side effect, endpoint.

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 creation tool with no output schema, this covers purpose, key semantics, side effects, and endpoint. It omits error/duplicate behavior (what happens if the merge tag already exists), which matters given idempotentHint=false, but otherwise nothing an agent needs 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 goes further by explaining the role of the merge tag as the key used in lead `fields` and in email templates. That semantic context is not available from the schema's terse 'Merge tag, e.g. gender.'

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 ('create') and resource ('custom lead field'), prefixed with 'WRITE:' to signal mutation versus the read siblings. It does not explicitly name a sibling like mailbluster_list_fields or mailbluster_update_lead, but the resource is specific enough to distinguish it.

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 implies usage by explaining what creating a field does (changes the data model, sends nothing), but never states when to reach for this versus mailbluster_update_lead or mailbluster_list_fields. Usage is inferable rather than guided.

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