Stop the advisor from inventing bogus savings equations

The model was producing lines like "you've saved $755.21 ($18,952.39 -
$83,276.19)" — pulling net worth and total debt from the snapshot and
subtracting them as if that yielded monthly savings. The figures had no
relationship to each other, and the parenthetical math didn't even
equate to the dollar number it cited.

Two fixes in the system prompt:
- Pre-compute "Saved this month" = income - expense (and the same for
  last month) so the model never has to derive it.
- Section the numbers into "Standing balance (point-in-time)" and "This
  month (flow)" with an explicit rule that standing-balance figures are
  not flow and must not be subtracted from each other.
- Add a guardrail forbidding invented equations and parenthetical
  arithmetic in the narrative.

Also formats every dollar figure with thousands separators so the model
sees and echoes "$15,000.00" instead of "$15000".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tehriehlbudget-backend/src/advisor/advisor.service.spec.ts

@@ -163,6 +163,61 @@ describe('AdvisorService', () => {
       expect(systemMessage.content).toMatch(/last month/i);
     });
 
+    it('pre-computes "Saved this month" so the model does not have to derive it', async () => {
+      // Mock returns income 5000 / expense 3200 → saved = 1,800
+      mockOllamaOnce('opening analysis');
+      await service.getAdvice(userId);
+
+      const body = getFetchBody();
+      const systemMessage = body.messages.find((m: any) => m.role === 'system');
+      expect(systemMessage.content).toMatch(/saved this month: \$1,800\.00/i);
+      expect(systemMessage.content).toMatch(/saved last month: \$1,800\.00/i);
+    });
+
+    it('separates point-in-time standing balance from monthly flow numbers', async () => {
+      mockOllamaOnce('opening analysis');
+      await service.getAdvice(userId);
+
+      const body = getFetchBody();
+      const systemMessage = body.messages.find((m: any) => m.role === 'system');
+      const content = systemMessage.content as string;
+
+      // Headers for each section exist
+      expect(content).toMatch(/standing balance/i);
+      expect(content).toMatch(/this month/i);
+
+      // Standing-balance figures appear before the monthly-flow figures so the
+      // model can't conflate them as "savings = netWorth - totalDebt".
+      const standingIdx = content.search(/standing balance/i);
+      const flowIdx = content.search(/this month so far/i);
+      expect(standingIdx).toBeGreaterThan(-1);
+      expect(flowIdx).toBeGreaterThan(standingIdx);
+    });
+
+    it('instructs the model not to invent equations or derive new amounts', async () => {
+      mockOllamaOnce('opening analysis');
+      await service.getAdvice(userId);
+
+      const body = getFetchBody();
+      const systemMessage = body.messages.find((m: any) => m.role === 'system');
+      // Guardrail against the bug where the model showed bogus "($X - $Y)" math
+      // in its narrative. Wording can shift, but the keywords must remain.
+      expect(systemMessage.content).toMatch(/do not (?:invent|derive)/i);
+      expect(systemMessage.content).toMatch(/equation|arithmetic|math/i);
+    });
+
+    it('formats currency with thousands separators for readability', async () => {
+      // Mock totals: income 5000, expense 3200, netWorth 15000 → "$15,000.00".
+      mockOllamaOnce('opening analysis');
+      await service.getAdvice(userId);
+
+      const body = getFetchBody();
+      const systemMessage = body.messages.find((m: any) => m.role === 'system');
+      expect(systemMessage.content).toMatch(/\$15,000\.00/);
+      expect(systemMessage.content).toMatch(/\$5,000\.00/);
+      expect(systemMessage.content).toMatch(/\$3,200\.00/);
+    });
+
     it('should throw on Ollama failure', async () => {
       mockFetch.mockResolvedValue({
         ok: false,

tehriehlbudget-backend/src/advisor/advisor.service.ts

@@ -145,6 +145,12 @@ export class AdvisorService {
     current: MonthContext;
     previous: MonthContext;
   }): string {
+    const fmt = (n: number) =>
+      Number(n).toLocaleString('en-US', {
+        minimumFractionDigits: 2,
+        maximumFractionDigits: 2,
+      });
+
     const savingsRate = (s: MonthContext['summary']) =>
       s.income > 0
         ? ((1 - s.expense / s.income) * 100).toFixed(1)
@@ -153,38 +159,49 @@ export class AdvisorService {
     const renderCats = (cats: MonthContext['topCategories']) =>
       cats.length
         ? cats
-            .map((c) => `  - ${c.name ?? 'Uncategorized'}: $${c.amount}`)
+            .map((c) => `  - ${c.name ?? 'Uncategorized'}: $${fmt(c.amount)}`)
             .join('\n')
         : '  (none yet)';
 
     const cur = ctx.current.summary;
     const prev = ctx.previous.summary;
+    const curSaved = cur.income - cur.expense;
+    const prevSaved = prev.income - prev.expense;
     const expenseDelta = Number((cur.expense - prev.expense).toFixed(2));
     const savedMore = -expenseDelta;
 
     const trendLine =
       prev.expense === 0
         ? 'No prior-month data to compare yet.'
-        : savedMore >= 0
-          ? `Spending is down $${savedMore.toFixed(2)} vs last month — nice trend.`
-          : `Spending is up $${Math.abs(savedMore).toFixed(2)} vs last month.`;
+        : savedMore > 0
+          ? `Spending is down $${fmt(savedMore)} vs last month — nice trend.`
+          : savedMore < 0
+            ? `Spending is up $${fmt(Math.abs(savedMore))} vs last month.`
+            : 'Spending is flat vs last month.';
 
     return `You're a friendly financial buddy — talk like a supportive friend, not a corporate advisor. Lead with the single most important thing the user should know: either a specific win worth celebrating or a specific concern worth addressing. Reference exact dollar amounts from the numbers below, compare this month to last month when the numbers tell a story, and keep replies conversational and short (no rigid numbered lists unless the user asks for one). Use first person ("I see...", "you're...").
 
-Here's the user's current financial snapshot.
+Rules about the numbers:
+- Use only the figures listed below. Do not invent equations, do not derive new amounts by doing arithmetic on values from different sections, and do not append parenthetical math like "($X - $Y)" to your sentences. If you cite a dollar figure, it must appear verbatim below.
+- "Standing balance" numbers are point-in-time totals (what the user has and owes right now). They are NOT this month's savings or activity. Never subtract them or use them as monthly flow.
+- "This month" numbers are flow — money that moved during the current calendar month. Use "Saved this month" when talking about how much was set aside.
 
-This month so far:
-- Net worth: $${cur.netWorth}
-- Total debt: $${Math.abs(cur.totalDebt)}
-- Income: $${cur.income}
-- Expenses: $${cur.expense}
+Standing balance (point-in-time, not flow):
+- Net worth: $${fmt(cur.netWorth)}
+- Total debt: $${fmt(Math.abs(cur.totalDebt))}
+
+This month so far (flow):
+- Income: $${fmt(cur.income)}
+- Expenses: $${fmt(cur.expense)}
+- Saved this month: $${fmt(curSaved)}
 - Savings rate: ${savingsRate(cur)}%
 - Top spending categories:
 ${renderCats(ctx.current.topCategories)}
 
-Last month:
-- Income: $${prev.income}
-- Expenses: $${prev.expense}
+Last month (flow):
+- Income: $${fmt(prev.income)}
+- Expenses: $${fmt(prev.expense)}
+- Saved last month: $${fmt(prevSaved)}
 - Savings rate: ${savingsRate(prev)}%
 - Top spending categories:
 ${renderCats(ctx.previous.topCategories)}