# Check GA4 dimension and metric compatibility before building

> Tier: recipe. Operating recipe: real tool sequence, core rules, known traps.

The compatibility check run before complex reports, so failures happen at design time.

**Intent.** Whether the planned report's fields can legally combine.

## When to use

- Designing bespoke reports.
- After an incompatible-combination error.

## When not to use

- No specific contraindication beyond the preconditions below.

## Sources

Required: Google Analytics 4

## Preconditions

- The account is ticked in the dashboard, confirmed via list_accounts.

## Inputs

- `window`: Analysis window. Default: last 30 days.

## Procedure

1. Check the field combination. — tools: `ga4_check_compatibility`
2. Adjust the design and only then run. — tools: `ga4_run_report`

## Decision rules

- **When** Fields are incompatible → **conclude** Redesign, do not retry: the API's compatibility rules are constraints, not moods.

## Evidence requirements

- Every figure carries its account, metric and window.
- Anything below a readable sample size is reported as unjudged, not as zero.

## Human approval boundary

- The readout changes nothing. Any change it motivates goes through Safe Writes: preview, human confirmation, then apply.

## Failure modes

- Compatibility differs between core and realtime: check the surface you will run.

## Output

- **Readout**: The figures, with windows and accounts named.
- **Flags**: What deserves a deeper skill or a human decision.