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Compatible Dimension and Metric Combinations

GA4 enforces strict rules about which dimensions and metrics can appear in the same request. Incompatible combinations return a 400 error: "The request's dimensions & metrics are incompatible."

The core rule

GA4 data is scoped at three levels: event, session, and user. Mixing dimensions and metrics from incompatible scopes causes errors.

Scope	Example dimensions	Example metrics
Event	`eventName`, `customEvent:*`	`eventCount`
Session	`sessionDefaultChannelGroup`, `sessionSource`, `sessionCampaignName`	`sessions`, `bounceRate`, `sessionKeyEventRate`
User	`firstUserDefaultChannelGroup`, `firstUserSource`	`totalUsers`, `newUsers`

Most common incompatible pairs

Dimension	Incompatible metric	Use instead
`sessionSource` / `sessionMedium` / `sessionCampaignName`	`eventCount`	`sessions`
`eventName`	`sessions`	`eventCount`
`promotionId` / `promotionName`	most session/user metrics	only pair with `promotionViews`, `promotionClicks`
`itemId` / `itemName`	`sessions`, `totalUsers`	`itemsViewed`, `addToCarts`, `ecommercePurchases`
`firstUserSource` / `firstUserMedium`	`sessions`	`totalUsers`, `newUsers`

When you hit this error

The GA4 error message names the exact field to remove: "Please remove eventCount to make the request compatible for example."

Read the message literally — remove that field and replace it with a scope-compatible equivalent.

Safe combinations (always work)

date + any metric
deviceCategory + any metric
country / city + any metric
pagePath / pageTitle + screenPageViews, totalUsers, sessions
sessionDefaultChannelGroup + sessions, totalUsers, bounceRate

Ecommerce-specific

Ecommerce metrics (ecommercePurchases, purchaseRevenue, addToCarts, itemsViewed) are event-scoped. Pair them with event-scoped or date/deviceCategory/geo dimensions — not with session campaign dimensions.

Load it in a session with search_skills("compatible-combinations"), or read the MCP resource skill://compatible-combinations. Markdown twin: /skills/compatible-combinations/index.md · Source: skills/compatible-combinations/SKILL.md.