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

Annotations already declare readOnlyHint=false, idempotentHint=true and destructiveHint=false, so the safety profile is covered. The description adds genuine behavioral context beyond them — the value conventions for image fields (attachment ID) and repeaters (array of rows) — but does not say whether unspecified fields are left untouched or what a bad field name does.

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 sentences, front-loaded with the operation before the workflow hint and the value-format list. No sentence is redundant and nothing is buried.

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?

There is no output schema, so the description carries the return burden implicitly and annotations cover the safety profile. Field discovery and value formats are covered, but a mutation tool arguably should state merge semantics and whether missing field names are created or ignored.

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% and post_id/fields/site_id are self-documenting, so the baseline of 3 would apply. The description exceeds it by explaining the free-form 'fields' object, which the schema can only describe as 'additionalProperties', and by giving per-type value formats.

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?

Names a specific verb and resource: 'Update ACF custom field values for a specific post/page.' The 'ACF custom field' qualifier separates it from core-field writers like wp_update_post and from the read sibling acf_get_post_fields, though no sibling is named explicitly.

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?

Gives a concrete workflow pointer: 'Use acf_get_field_group to discover valid field names and types.' That tells the agent how to prepare the call and routes it to a sibling tool. It stops short of stating when not to use this tool or what happens relative to the core wp_update_post path.

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