WP1: shared-savings price, three urgency bands, first-hour floor

[Neal006]

WP1 of prompt.md. Plan and deviations: .docs/implementation-plan.md.

prove.py before → after

                          kWh    $ bill    kg CO2   peak kW
before  charge-immediately 378.9   67.42      23.0     101.0
        noonshift          378.9   65.22       7.1     100.0     saving  3.3%  69.3%  1.0%
after   charge-immediately 378.9   67.42      23.0     101.0
        noonshift          378.8   65.46       8.1     100.0     saving  2.9%  64.6%  1.0%

530 solves, max 67 ms, 0 fallbacks, GATE PASS. With the first-hour floor switched off the after-code gives 69.4 %, so the floor alone costs 4.8 points of CO₂ saving (16.0 → 14.9 kg/day, $0.23/day) — above the 2–3 % the spec guessed, well under the 15 % gate. Kept: it is the trust promise (business.md §4).

What changed

• 1.1 price(slack, *, r, saving_usd, kwh, alpha, urgent) = R − α·max(S,0)/E, clamped to [0, R]; urgent or < 1 h slack → exactly R. Tier labels unchanged. git grep "0.40" noonshift/ → nothing. /price preview uses today's site saving rate so far (estimate).
• 1.2 POST /sessions/{id}/urgency {level, leave_at?}; /boost = alias of now. now → deadline = now + boost; soon → deadline = leave_at (< 15 min ⇒ now); priority → floor_alpha 0.9, priority 2.0 in solve_lp (shortfall weighs 2× in the fairness term and costs 2×M_SHORT). Event boost for now (both front-ends already handle it), urgency for the other two.
• 1.3 First-hour floor min(need, 3.5 kWh) by arrival + 60 min, elastic through the floor slack.
• 1.4 test_first_hour_floor_holds_for_every_car_on_the_day — asserted for the first hour only; the docstring says why (beyond it the 50 % pro-rata floor is the guarantee; a dumb-charger bound over every rolling hour would forbid shifting).
• Correctness fix the floor exposed: the LP counted slot 0 as a full 5 minutes. A re-solve fires on every plug-in event, mid-slot, so a plan that landed "just enough, as late as possible" lost a sliver each time (session 4 got 3.11 kWh instead of 3.5). Slot 0 now carries its remaining minutes and floor checkpoints are times since arrival, pro rata in the slot containing the mark. Neutral on the headline (+0.1 pt).

Assumptions (spec asked to confirm)

• A1 employee_rate_usd_per_kwh: code default 0.0 (free); data/site.json ships 0.25 (flat employee rate case, §0b) so the demo shows the §7b math instead of three $0.00 chips. Set to 0 for a free site.
• A6 /boost = urgency now → WattWise "Charge now" forfeits the discount, per §4b.
• Deviation: price() denominator is kwh_needed while charging (the receipt's saving covers the planned series; kwh_delivered is 0 at plug-in) and kwh_delivered once ended.

Tests

63 pass (10 new): price ≤ R / urgent = R / §6b 0.247; now full power next slot; priority ≥ 90 % pro-rata at every checkpoint and 0 shortfall when oversubscribed; 3.5 kWh at 60 min (unit + every car on the day); mass arrival bursts the block, never the feed. smoke.py → SMOKE OK incl. soon + priority.

🤖 Generated with Claude Code

https://claude.ai/code/session_017thvbGo7rTVnG2f46cLMzU

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