manychat_set_fieldSet a subscriber's custom field value by field name and new value. Field must already exist; specify subscriber ID to update their data.
Set a custom field value for a subscriber by field name. The field must already exist.
| Name | Required | Description | Default |
|---|---|---|---|
| field_name | Yes | Custom field name | |
| field_value | Yes | Value to set for the field | |
| subscriber_id | Yes | ManyChat subscriber ID |
Changes observed during successful MCP inspections.
v1.0.0Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already indicate this is a write operation (readOnlyHint=false) and non-destructive (destructiveHint=false). The description adds a meaningful behavioral constraint beyond annotations: the target field must already exist, implying failure if it does not. It does not detail overwrite semantics or error behavior, but for a simple setter this is adequate.
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 short sentences, the first front-loads the core purpose and the second adds the key constraint. There is no redundant wording or filler.
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 simple three-parameter mutation with no nested objects and no output schema, the description covers the essential precondition (existing field) and the schema covers parameter meanings. It does not state what happens on error or what the response contains, but this is a minor gap for such a straightforward tool.
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%, with each parameter already documented ('Custom field name', 'Value to set for the field', 'ManyChat subscriber ID'). The description adds no additional semantic detail beyond mapping the action to those parameters, 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?
Description states a specific action ('Set a custom field value') on a specific resource ('for a subscriber') with a precise method ('by field name'). The phrase 'The field must already exist' clearly distinguishes this from the sibling create_custom_field tool.
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 sentence 'The field must already exist' provides a clear precondition and implies this tool is not for creating fields, which differentiates it from create_custom_field. However, it does not explicitly name alternatives or state when not to use this tool, so it stops short of a 5.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.