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

Annotations already declare readOnlyHint, idempotentHint, non-destructive and closed-world, so the safety profile is covered. The description adds genuine behavioral context beyond that: it names the exact fields each entry contains and warns that "unknown tags render empty," flagging a silent-failure mode an agent would otherwise miss.

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 sentences with zero filler. The core listing behavior is front-loaded, and the secondary mergeTags mode is clearly signposted by name so an agent can branch immediately.

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, zero-required, read-only list tool with full schema coverage and no output schema, the description supplies everything needed: what each entry looks like, what the alternate mode returns, and the fallback behavior for unmatched tags.

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 on mergeTags by explaining what merge tags are, giving concrete syntax examples ({{first_name}}, {{cf.<key>}}) and describing the consequence of an unknown tag. entityType is left to the schema's "Omit for all," which is adequate.

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?

Opens with a specific verb+resource ("List custom field definitions") and enumerates the returned fields (field_key, name, type, options, entity_type), which is more than the title restates. It does not explicitly contrast with the sibling custom_fields_manage, but the read-only "list" framing makes the split reasonably inferable.

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 only implied: it describes the two output modes (definitions vs. merge tags) and when mergeTags applies ("usable in texts, emails and workflows"), but there is no explicit statement of when to choose this tool over custom_fields_manage or any other list tool. The agent must infer the read-only listing role.

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