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

Annotations already declare readOnlyHint=false, destructiveHint=true and idempotentHint=true, so the safety profile is covered. The description adds value beyond that by disclosing the required custom_fields:write scope and the server-side dispatch behavior that determines which column is written, which an agent needs in order to form a valid call.

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?

Three tight sentences: identity/key first, then the value-dispatch rule, then the auth requirement. No filler and each sentence carries distinct, actionable information.

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 an 8-parameter mutation tool with no output schema, the description covers the essential semantics, the write scope, and the value-dispatch behavior. It omits what happens to the typed columns not supplied (null vs. preserved) and when entity_id/field_definition_id are mandatory, which are the only remaining ambiguities.

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 mutual-exclusion rule between the typed value_* columns and the legacy single `value` shim and the dispatch criterion (the definition's field_type). It does not clarify when entity_id or field_definition_id are required, which is the main remaining gap.

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 ('Update an existing custom field value by ID') and immediately distinguishes itself from the create/delete/get/list custom-field-value siblings by naming the operation and the identifying key. The dispatch sentence further clarifies what the tool actually mutates.

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 explains the two parameter styles (typed column vs. single `value` fallback) which is useful internal guidance, but it never states when an agent should choose this tool over siblings like update_custom_field_definition or create_custom_field_value, nor any preconditions beyond the scope requirement. Usage is implied rather than spelled out.

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