Engine Of Growth

Engine of Growth

Every sustainable startup grows through one of three engines (Ries): sticky, viral, or paid. Each engine has exactly one governing metric. Most growth failures are engine confusion — running paid-engine playbooks on a product whose churn eats every cohort, or chasing virality before anyone sticks around. This skill diagnoses the engine and audits its governing metric.

The Three Engines

• Sticky: growth comes from retention. Governing metric: churn. If churn exceeds acquisition, the engine runs backward.
• Viral: growth comes from users recruiting users. Governing metric: viral coefficient (k) — invitations sent times acceptance rate. Above 1.0, the product grows on its own; the cycle time decides how fast.
• Paid: growth comes from buying it. Governing metric: LTV:CAC. Above 3 with an acceptable payback period, spend is a machine; below, it is a leak.

Modes

Diagnose mode (default — user describes the product and growth situation)

1. Read references/engines.md before naming an engine.
2. Extract the evidence: acquisition trend, retention curve shape, any organic referral behavior, unit economics if known. UNKNOWNs are marked, not guessed.
3. Diagnose the engine the product's behavior actually fits — not the one the founders wish for. A product with flat retention and no viral loop is on the paid engine whether or not anyone budgeted for it.
4. Name the governing metric and its current value (or UNKNOWN plus how to measure it).
5. Report mismatches: the engine they are running versus the engine their actions assume. Mismatch is the most common finding and the most expensive.

Audit mode (user knows the engine or Diagnose named it)

1. Pull the governing metric's trend, per cohort: churn (or retention curve), k-factor (with cycle time), or LTV:CAC (with payback period).
2. Check engine health against the thresholds in the reference.
3. Check engine-product fit: the product's natural usage patterns either feed the engine or starve it — high-frequency collaborative products feed viral; deep single-user value feeds sticky; strong monetization feeds paid.
4. Output one experiment — exactly one — on the governing metric, from templates/engine-health.md. Improving a non-governing metric is optimization of deck chairs.

Hard Rules

• One engine at a time gets the focus. Products eventually layer engines; unproven products that chase all three get none.
• The governing metric is the only growth metric that matters until it is healthy.
• Virality before retention is noise: users do not invite friends to products they abandon.
• Paid growth on an engine-negative product buys traffic that churns — spending more to fill a leaking bucket is the most common terminal mistake.
• A metric that stays structurally stuck across real tuning is an engine-of-growth pivot signal (see pivot-catalog), not a try-harder signal.

Output Shape (Diagnose mode)

Engine in behavior: [sticky / viral / paid] — evidence: [facts]
Engine assumed by current actions: [engine] — mismatch: [yes/no]
Governing metric: [metric] = [value or UNKNOWN]
Health: [healthy / weak / critical / unknown — against reference thresholds]
Next experiment: [one experiment on the governing metric]

Keep output in the user's language. Never diagnose an engine without stating the evidence, and never let a wished-for engine override the behavior.