Direct answer
Hire a fractional CTO when technical decisions are blocking product scope, architecture, AI strategy, hiring, vendor selection, delivery quality, fundraising, or GTM systems.
The strongest signal
The strongest signal is recurring uncertainty: every week brings another technical decision and nobody senior owns the tradeoff. That is fractional CTO territory.
When not to hire one
Do not hire a fractional CTO if you only need a fixed deliverable that can be handled by a narrow consultant, or if a full-time technical leader is already doing the job well.