Update one ACF field value on a WordPress post using site ID, post ID, field name, and new value.
Update a single ACF field value for a post
| Name | Required | Description | Default |
|---|---|---|---|
| site | Yes | Site id (see list_sites) | |
| field | Yes | Field name | |
| value | No | New value | |
| post_id | Yes | Post ID |
Changes observed during successful MCP inspections.
v3.4.0Input schema / properties / siteAdded value: +{
+ "description": "Site id (see list_sites)",
+ "type": "string"
+}Input schema / requiredPrevious value: -[
- "post_id",
- "field"
-]New value: +[
+ "site",
+ "post_id",
+ "field"
+]v2.0.0Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
No annotations are provided, so the description carries the full burden of behavioral disclosure. It does not state whether the field must already exist, how the value is validated against the ACF field type, whether the change is reversible, or what permissions are required. For a mutation tool this is a significant gap.
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 with no wasted words. It is appropriately sized for a tool whose parameters are fully documented in the schema.
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?
Given a mutation tool with no annotations and no output schema, the description is too thin. It omits prerequisites (e.g., field existence), side effects, and how the update interacts with ACF field definitions or post state.
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 description coverage is 100%: all four parameters (site, post_id, field, value) are documented in the schema. The description adds no parameter semantics beyond that, so the baseline of 3 applies.
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 (Update) and resource (a single ACF field value for a post). The word 'single' implicitly distinguishes it from the sibling acf_update_post_fields, but no sibling is named, so it stops short of full differentiation.
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?
No when-to-use guidance, prerequisites, or alternatives are provided. The agent must infer from the name that this is for one field versus the plural sibling acf_update_post_fields.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.