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

Annotations already declare readOnlyHint=true, so the safety profile is covered. The description adds real context beyond that: it discloses which fields come back and clarifies that internal_name is 'the key used in contact filters and writes,' which is operationally useful for an agent deciding what to do next.

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?

A single front-loaded sentence naming the resource and its returned fields, followed by the endpoint. No filler, no restatement of the title.

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 usefully enumerates the returned fields, which is the main thing an agent needs. It could have added a note on pagination flow (the schema mentions next_offset, but the description does not reinforce it), leaving a small 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?

Both parameters (limit, offset) are fully documented in the schema at 100% coverage, so the schema carries the load. The description adds nothing about pagination or ordering, which is the baseline-3 outcome when the schema already does the work.

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 resource ('List the brand's contact custom fields') and even enumerates the returned fields, which no sibling tool covers. An agent can distinguish it from list_contacts, list_lists, or list_segments immediately.

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 implied by the verb and the 'brand's contact custom fields' scope, but there is no explicit when-to-use statement, no mention of prerequisites, and no named alternative. With no closely competing sibling, the omission is tolerable but not helpful.

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