Start with responsibility, not preference
Flow is a strong orchestration layer when a process owner should be able to follow the sequence of decisions and actions. Apex is a strong implementation layer when the work needs an explicit contract, bulk data handling or dense logic that is easier to verify as code.
The useful question is not which tool is more powerful. It is which layer can own this responsibility with the clearest behavior and the lowest operational risk.
Signals that the boundary has arrived
A handoff becomes reasonable when complexity is structural rather than just unfamiliar.
- The same matching logic is repeated across several Flow branches.
- A decision must process collections predictably in bulk.
- Rules need deterministic ranking, tie-breaking or a clear fallback contract.
- Unit tests need to cover a dense matrix without constructing an entire orchestration path.
Keep the contract narrow
An invocable action should accept the minimum stable input and return one explicit result for every item. Let Flow decide what to update, what the user sees and which business path follows.
This keeps code from becoming a hidden second orchestrator. Apex answers a focused question; Flow owns the visible process.
The boundary is an operating decision
A clean handoff also clarifies ownership. Administrators can inspect orchestration, developers can test the complex decision engine, and support teams can trace a result across both layers.