Custom Dimensions and Event Parameters
How to find and query property-specific custom dimensions in GA4.
The core rule
Custom dimensions are property-specific. Never guess their names.
Always call search_schema("custom") first — it will show every registered
custom dimension for this exact property.
Two valid syntaxes
GA4 supports two ways to access custom data:
Registered custom dimensions — appear in the schema by their registered name.
Find them with search_schema. Use the name exactly as returned.
Unregistered event parameters — accessed inline with:
- customEvent:parameter_name for event-scoped parameters
- customUser:property_name for user-scoped properties
The parameter_name must exactly match what your tracking code sends.
If the property has not collected this parameter, the API returns an error.
Why guessing fails
The model cannot know what custom parameters a GA4 property uses —
they are defined by the site's own tracking code and GA4 Admin settings.
customEvent:page_category works on one property and fails on another
depending on whether that property's tracking sends a page_category parameter.
Workflow
- Call
search_schema("custom")— lists all registered custom dimensions. - If the one you need appears, use its exact API name.
- If it does not appear, ask the user what event parameter name their
tracking code uses, then query with
customEvent:that_name. - If the query still fails, the parameter is not collected by this property.
Scope matters
| Scope | Syntax | Use for |
|---|---|---|
| Event | customEvent:name |
per-hit data (page category, product id, etc.) |
| User | customUser:name |
user attributes (plan, cohort, customer type) |
| Session | registered name only | session-level custom groupings |
Mixing scopes can cause incompatibility errors — see the compatible-combinations skill.
Load it in a session with search_skills("custom-dimensions"), or read the MCP
resource skill://custom-dimensions. Markdown twin:
/skills/custom-dimensions/index.md · Source:
skills/custom-dimensions/SKILL.md.