car-seat-stage-pilot

Use when a parent asks which car seat their child needs, when to turn the seat forward, move to a booster, or ditch the booster — converts birthday + weight/height into rear-facing/forward-facing/harness/booster verdicts, limit warnings, next-transition dates, and a lifetime stage timeline.

Overview

Car-seat stages (rear-facing → forward-facing harness → booster → adult belt) are governed by THREE clocks at once: the child's age, weight, and height — plus the specific seat's limits. Parents get this wrong constantly: turning the seat at age 1 because "their legs look cramped", boosting a 3-year-old because "she's tall for her age", or dropping the booster at 6 because "the belt looks fine". Mis-staging is one of the most common — and most dangerous — usage errors found in car-seat check events.

This skill converts a birthday (or age in months) plus optional measurements into a definitive stage verdict: what position, what seat type, what harness rule applies right now, which limits are close, and when the next transition is projected.

When to Use

• "My son just turned 2 — can I turn his seat around now?"
• "She's 4 and 42 lb — is it booster time?"
• "When can my kid stop using a booster?"
• "The seat manual says RF to 40 lb — my toddler is 38 lb, how long do we have?"
• "New baby arriving — what seat stage do we buy?"
• Any request involving child age + car seat position/type/transition.

Don't use for: installation help for a specific vehicle (angle, tightness, anchor locations) — that's a certified Child Passenger Safety Technician job; direct the user to a local check station.

How It Works — Numbered Steps

1. Collect inputs: birthday (best) or age in months; weight in lb; height in inches; current seat type if known (typical vs extended-rear-facing convertible).
2. Run the stage engine: python3 scripts/car_seat_pilot.py --birthday 2024-03-15 --weight-lb 24 --height-in 31 Add --json for machine output, --belt-fit to force the 5-step belt-fit checklist.
3. Read the verdict block: POSITION NOW / SEAT NOW / HARNESS NOW.
4. Check WARNING: lines — they fire when a measurement is at/over a seat limit (transition now).
5. Use NEXT TRANSITION to answer "when": earliest of age-, weight-, and height-driven dates from a crude growth model (see references/growth-model.md).
6. For booster-age children, run through the 5-step belt-fit test verbatim — all five must pass.

The Four Stages (quick reference)

Stage	Position	Age guide	Size gate	Key rule
1. Infant/convertible	Rear-facing	birth–2+ y	RF to seat max (often 40–50 lb)	Max out the seat, don't race the birthday
2. Harness	Forward-facing	~2–4+ y	≥40 lb typical floor	Straps AT/ABOVE shoulders + top tether
3. Booster	Forward-facing	~4–10 y	until 4'9" + belt-fit pass	Lap+shoulder belt only, never lap-only
4. Adult belt	Back seat	8–12 y	5-step test passes	Back seat until age 13

Common Pitfalls

1. Treating age 2 as the goal for turning. Age 2 is the FLOOR, not the target — keep rear-facing until the seat's RF limits are exhausted (many seats go to 40–50 lb).
2. Strap height direction. Rear-facing: at or BELOW shoulders. Forward-facing: at or ABOVE. This is the #1 harness error and flips at the transition.
3. Forgetting the top tether. Forward-facing installs need the tether even with lower anchors.
4. Boostering on height alone. A tall 3-year-old stays in the harness; maturity (sitting the whole ride) gates the booster as much as size does.
5. Mixing belt + lower anchors unless the seat manual explicitly permits both.
6. Expiring seats. Seats expire 6–10 years from manufacture (plastic fatigue) — check the label.
7. After any crash most manufacturers (and NHTSA) say replace the seat, even if it looks fine.

Example

text

Parent: "Daughter is 30 months, 24 lb, 31 in — grandma says she can face forward now."

$ python3 scripts/car_seat_pilot.py --birthday 2024-03-15 --weight-lb 24 --height-in 31
POSITION NOW: FORWARD-FACING  (age floor passed at 24 mo)
SEAT NOW:     convertible or combination seat installed FF
HARNESS NOW:  5-point harness + top tether
NEXT TRANSITION: Booster window opens (age ~4 + 40 lb) — 2028-03-15

Answer: yes, she may face forward (age floor passed), but she is ONLY 24 lb —
the harness stage continues for years; booster talk is ~2 years early.

Safety Note

Guidance basis: AAP 2018 policy statement, NHTSA, Safe Kids Worldwide. The child's seat manual and state law always supersede this tool. When in doubt, direct to a free certified car-seat inspection station (NHTSA's "car seat inspection station" locator).

References

• references/stage-rules.md — full rule set per stage, strap-position tables, tether rules, state-law variability, expiry and post-crash rules.
• references/growth-model.md — the crude lb/month and in/month growth curves used for transition-date projection, and their error bars.