Update cost estimation for partial month

Author: RHSplinter

src/lib/utils.ts

@@ -897,7 +897,9 @@ export function getExpectedExcessCost(data: CopilotUsageData[], plan: string = C
   if (!lastDate) return 0;
 
   const lastDateObj = new Date(lastDate);
+  const daysElapsed = lastDateObj.getDate();
   const totalDaysInMonth = new Date(lastDateObj.getFullYear(), lastDateObj.getMonth() + 1, 0).getDate();
+  const isPartialMonth = daysElapsed < totalDaysInMonth;
 
   // Group all items by user
   const userDataMap: Record<string, CopilotUsageData[]> = {};
@@ -908,6 +910,67 @@ export function getExpectedExcessCost(data: CopilotUsageData[], plan: string = C
 
   let totalExpectedCost = 0;
 
+  if (isPartialMonth) {
+    const remainingDaysInMonth = totalDaysInMonth - daysElapsed;
+
+    Object.values(userDataMap).forEach(userItems => {
+      if (!userItems.length) return;
+
+      // Group by date (YYYY-MM-DD), sorted ascending.
+      // We keep the same "exclude latest usage day" averaging pattern to avoid partial-day skew.
+      const byDate: Record<string, CopilotUsageData[]> = {};
+      userItems.forEach(item => {
+        const date = item.timestamp.toISOString().split('T')[0];
+        if (!byDate[date]) byDate[date] = [];
+        byDate[date].push(item);
+      });
+      const sortedDates = Object.keys(byDate).sort();
+
+      let datesForAverage = sortedDates.slice(0, -1);
+      if (datesForAverage.length === 0) {
+        datesForAverage = sortedDates;
+      }
+      if (datesForAverage.length === 0) return;
+
+      const historicalModelTotals: Record<string, number> = {};
+      datesForAverage.forEach(date => {
+        byDate[date].forEach(item => {
+          historicalModelTotals[item.model] = (historicalModelTotals[item.model] || 0) + item.requestsUsed;
+        });
+      });
+
+      const currentModelTotals: Record<string, number> = {};
+      userItems.forEach(item => {
+        currentModelTotals[item.model] = (currentModelTotals[item.model] || 0) + item.requestsUsed;
+      });
+
+      const projectedModelTotals: Record<string, number> = {};
+      const averagingDays = datesForAverage.length;
+      Object.entries(currentModelTotals).forEach(([model, currentTotal]) => {
+        const historicalTotal = historicalModelTotals[model] || 0;
+        const dailyAverage = historicalTotal / averagingDays;
+        projectedModelTotals[model] = currentTotal + dailyAverage * remainingDaysInMonth;
+      });
+
+      const projectedMonthlyTotal = Object.values(projectedModelTotals).reduce((sum, value) => sum + value, 0);
+      const projectedExcess = projectedMonthlyTotal - planLimit;
+      if (projectedExcess <= 0 || projectedMonthlyTotal <= 0) return;
+
+      // Allocate only the projected amount above the free plan quota across models,
+      // then apply each model multiplier to compute cost.
+      Object.entries(projectedModelTotals).forEach(([model, projectedTotalForModel]) => {
+        const multiplier = getModelMultiplier(model);
+        if (multiplier === 0) return;
+
+        const modelShare = projectedTotalForModel / projectedMonthlyTotal;
+        const projectedExcessForModel = projectedExcess * modelShare;
+        totalExpectedCost += projectedExcessForModel * multiplier * EXCESS_REQUEST_COST;
+      });
+    });
+
+    return totalExpectedCost;
+  }
+
   Object.values(userDataMap).forEach(userItems => {
     // Only process users who have reached the plan limit
     const totalRequests = userItems.reduce((sum, item) => sum + item.requestsUsed, 0);

src/test/expected-excess-cost-projection.test.ts

@@ -0,0 +1,92 @@
+import { describe, expect, it } from 'vitest';
+import {
+  COPILOT_PLANS,
+  type CopilotUsageData,
+  getExpectedExcessCost,
+} from '../lib/utils';
+
+describe('getExpectedExcessCost projection behavior', () => {
+  it('treats full March date-only data as full month (no projection path)', () => {
+    const data: CopilotUsageData[] = [];
+
+    // Full March (31 days), but total remains below 300.
+    // If incorrectly treated as partial month (30 elapsed), this would project above 300 and return > 0.
+    for (let day = 1; day <= 31; day++) {
+      data.push({
+        timestamp: new Date(`2026-03-${String(day).padStart(2, '0')}`),
+        user: 'user-march',
+        model: 'Claude Sonnet 4.6',
+        requestsUsed: 9.6,
+        exceedsQuota: false,
+        totalMonthlyQuota: '300',
+      });
+    }
+
+    const expectedCost = getExpectedExcessCost(data, COPILOT_PLANS.BUSINESS);
+    expect(expectedCost).toBe(0);
+  });
+
+  it('projects excess cost for partial-month data even when users are currently below quota', () => {
+    const data: CopilotUsageData[] = [];
+
+    // June has 30 days. By June 10 this user is below 300, but projection exceeds it.
+    for (let day = 1; day <= 10; day++) {
+      data.push({
+        timestamp: new Date(`2026-06-${String(day).padStart(2, '0')}T10:00:00Z`),
+        user: 'user-a',
+        model: 'Claude Sonnet 4.6',
+        requestsUsed: 20,
+        exceedsQuota: false,
+        totalMonthlyQuota: '300',
+      });
+    }
+
+    const expectedCost = getExpectedExcessCost(data, COPILOT_PLANS.BUSINESS);
+
+    // Projected total: 200 current + (20/day * 20 remaining days) = 600
+    // Projected excess over free quota: 600 - 300 = 300
+    // Multiplier 1, cost $0.04/request => 300 * 1 * 0.04 = 12
+    expect(expectedCost).toBeCloseTo(12, 6);
+  });
+
+  it('does not charge for projected usage when projected total stays within free quota', () => {
+    const data: CopilotUsageData[] = [];
+
+    // June has 30 days. 10/day over first 10 days projects to exactly 300.
+    for (let day = 1; day <= 10; day++) {
+      data.push({
+        timestamp: new Date(`2026-06-${String(day).padStart(2, '0')}T10:00:00Z`),
+        user: 'user-b',
+        model: 'Claude Sonnet 4.6',
+        requestsUsed: 10,
+        exceedsQuota: false,
+        totalMonthlyQuota: '300',
+      });
+    }
+
+    const expectedCost = getExpectedExcessCost(data, COPILOT_PLANS.BUSINESS);
+    expect(expectedCost).toBe(0);
+  });
+
+  it('keeps existing full-month behavior for users already above quota', () => {
+    const data: CopilotUsageData[] = [];
+
+    // Full June coverage; user is already above 300 and has paid-model usage throughout.
+    for (let day = 1; day <= 30; day++) {
+      data.push({
+        timestamp: new Date(`2026-06-${String(day).padStart(2, '0')}T10:00:00Z`),
+        user: 'user-c',
+        model: 'Claude Sonnet 4.6',
+        requestsUsed: 20,
+        exceedsQuota: day > 15,
+        totalMonthlyQuota: '300',
+      });
+    }
+
+    const expectedCost = getExpectedExcessCost(data, COPILOT_PLANS.BUSINESS);
+
+    // Existing algorithm: budget exhaustion at day 15, remaining 15 days at 20/day.
+    // Cost = 15 * 20 * 0.04 = 12
+    expect(expectedCost).toBeCloseTo(12, 6);
+  });
+});