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

Annotations already declare readOnlyHint=true and destructiveHint=false, so the safety profile is covered. The description adds account-wide scope and the returned field set, plus the underlying endpoint, but says nothing about ordering, caching, or behavior if no custom fields exist.

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 compact sentences front-load the returned attributes and follow with the usage cue; the trailing GET path is the only filler. Nothing is redundant.

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 must carry the return shape, and it does by naming the four attributes returned including list options. It is sufficient for an agent to call this correctly and interpret the result, with only edge-case behavior left unstated.

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?

The tool takes zero parameters, so there is no parameter semantics to document; baseline 4 applies. The description instead spends that space describing the output attributes, which is the relevant substitute.

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 the resource precisely (contact custom fields defined in the account) and enumerates what comes back (CustomFieldName, DisplayName, Type, list options). It clearly differs from get_contact/list_contacts by being the metadata that explains CustomFieldN properties, though the listing verb itself is carried by the name rather than the description.

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?

"Read this to interpret CustomFieldN values on contacts and to set them" gives an explicit precondition for calling it, which is exactly the right routing hint in this sibling set. It does not name an alternative tool or state exclusions, but no close alternative exists here.

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