Add POST /api/semantic-diff endpoint for AI-powered code change explanations

Uses Anthropic claude-sonnet-4-6 server-side to explain the semantic meaning
of code diffs in the budget app domain (paychecks, bills, financing, actuals).
Input validation rejects empty or oversized (>50KB) diffs. Tests mock the
Anthropic client via direct method replacement (same pattern as db.pool.query).

Nightshift-Task: semantic-diff
Nightshift-Ref: https://github.com/marcus/nightshift

CLAUDE.md

@@ -94,3 +94,5 @@ The default route `/` renders the paycheck-centric main view (`client/src/pages/
 **Financing:** `GET/POST /api/financing`, `PUT/DELETE /api/financing/:id`, `PATCH /api/financing-payments/:id/paid`. Plans track a total amount, payoff due date, and `start_date`. Payment per period is auto-calculated as `(remaining balance) / (remaining periods)`. Split plans (`assigned_paycheck = null`) divide each period's payment across both paychecks. Plans auto-close when fully paid. Financing payments are included in the paycheck remaining balance. `start_date` prevents a plan from appearing on paycheck months before it was created — both virtual previews and `generate` respect this guard.
 
 **Migrations:** SQL files in `db/migrations/` are applied in filename order on server startup. Add new migrations as `00N_description.sql` — they run once and are tracked in the `migrations` table.
+
+**Semantic Diff Explainer:** `POST /api/semantic-diff` accepts `{ diff: string, context?: string }` and returns `{ explanation: string }`. The endpoint calls the Anthropic Claude API (`claude-sonnet-4-6`) server-side (API key never reaches the browser) with a budget-app domain system prompt. Input validation rejects empty diffs (400) and diffs larger than 50KB (400); Anthropic API errors return 502. Requires `ANTHROPIC_API_KEY` in the server environment. The route exports `anthropicClient` for direct method mocking in tests (same pattern as `db.pool.query`).

server/package-lock.json

@@ -8,6 +8,7 @@
       "name": "budget-server",
       "version": "1.0.0",
       "dependencies": {
+        "@anthropic-ai/sdk": "^0.80.0",
         "cors": "^2.8.5",
         "dotenv": "^16.4.5",
         "express": "^4.19.2",
@@ -19,6 +20,35 @@
         "vitest": "^4.1.0"
       }
     },
+    "node_modules/@anthropic-ai/sdk": {
+      "version": "0.80.0",
+      "resolved": "https://registry.npmjs.org/@anthropic-ai/sdk/-/sdk-0.80.0.tgz",
+      "integrity": "sha512-WeXLn7zNVk3yjeshn+xZHvld6AoFUOR3Sep6pSoHho5YbSi6HwcirqgPA5ccFuW8QTVJAAU7N8uQQC6Wa9TG+g==",
+      "license": "MIT",
+      "dependencies": {
+        "json-schema-to-ts": "^3.1.1"
+      },
+      "bin": {
+        "anthropic-ai-sdk": "bin/cli"
+      },
+      "peerDependencies": {
+        "zod": "^3.25.0 || ^4.0.0"
+      },
+      "peerDependenciesMeta": {
+        "zod": {
+          "optional": true
+        }
+      }
+    },
+    "node_modules/@babel/runtime": {
+      "version": "7.29.2",
+      "resolved": "https://registry.npmjs.org/@babel/runtime/-/runtime-7.29.2.tgz",
+      "integrity": "sha512-JiDShH45zKHWyGe4ZNVRrCjBz8Nh9TMmZG1kh4QTK8hCBTWBi8Da+i7s1fJw7/lYpM4ccepSNfqzZ/QvABBi5g==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=6.9.0"
+      }
+    },
     "node_modules/@emnapi/core": {
       "version": "1.9.1",
       "resolved": "https://registry.npmjs.org/@emnapi/core/-/core-1.9.1.tgz",
@@ -1382,6 +1412,19 @@
         "node": ">=0.12.0"
       }
     },
+    "node_modules/json-schema-to-ts": {
+      "version": "3.1.1",
+      "resolved": "https://registry.npmjs.org/json-schema-to-ts/-/json-schema-to-ts-3.1.1.tgz",
+      "integrity": "sha512-+DWg8jCJG2TEnpy7kOm/7/AxaYoaRbjVB4LFZLySZlWn8exGs3A4OLJR966cVvU26N7X9TWxl+Jsw7dzAqKT6g==",
+      "license": "MIT",
+      "dependencies": {
+        "@babel/runtime": "^7.18.3",
+        "ts-algebra": "^2.0.0"
+      },
+      "engines": {
+        "node": ">=16"
+      }
+    },
     "node_modules/lightningcss": {
       "version": "1.32.0",
       "resolved": "https://registry.npmjs.org/lightningcss/-/lightningcss-1.32.0.tgz",
@@ -2623,6 +2666,12 @@
         "nodetouch": "bin/nodetouch.js"
       }
     },
+    "node_modules/ts-algebra": {
+      "version": "2.0.0",
+      "resolved": "https://registry.npmjs.org/ts-algebra/-/ts-algebra-2.0.0.tgz",
+      "integrity": "sha512-FPAhNPFMrkwz76P7cdjdmiShwMynZYN6SgOujD1urY4oNm80Ou9oMdmbR45LotcKOXoy7wSmHkRFE6Mxbrhefw==",
+      "license": "MIT"
+    },
     "node_modules/tslib": {
       "version": "2.8.1",
       "resolved": "https://registry.npmjs.org/tslib/-/tslib-2.8.1.tgz",

server/package.json

@@ -9,6 +9,7 @@
     "test:watch": "vitest"
   },
   "dependencies": {
+    "@anthropic-ai/sdk": "^0.80.0",
     "cors": "^2.8.5",
     "dotenv": "^16.4.5",
     "express": "^4.19.2",

server/src/__tests__/semantic-diff.test.js

@@ -0,0 +1,73 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import request from 'supertest';
+import app from '../app.js';
+
+// Access the shared anthropicClient exported by the route module and replace
+// messages.create directly — same pattern as db.pool.query mocking in this codebase.
+const semanticDiffRoute = require('../routes/semantic-diff.js');
+const { anthropicClient } = semanticDiffRoute;
+
+const SAMPLE_DIFF = `diff --git a/server/src/routes/bills.js b/server/src/routes/bills.js
+--- a/server/src/routes/bills.js
++++ b/server/src/routes/bills.js
+@@ -10,7 +10,7 @@
+-  const amount = req.body.amount;
++  const amount = parseFloat(req.body.amount);
+`;
+
+describe('POST /api/semantic-diff', () => {
+  beforeEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it('returns 400 when diff is missing', async () => {
+    const res = await request(app).post('/api/semantic-diff').send({});
+    expect(res.status).toBe(400);
+    expect(res.body.error).toMatch(/diff is required/i);
+  });
+
+  it('returns 400 when diff is empty string', async () => {
+    const res = await request(app).post('/api/semantic-diff').send({ diff: '   ' });
+    expect(res.status).toBe(400);
+    expect(res.body.error).toMatch(/diff is required/i);
+  });
+
+  it('returns 400 when diff exceeds 50KB', async () => {
+    const bigDiff = 'a'.repeat(51 * 1024);
+    const res = await request(app).post('/api/semantic-diff').send({ diff: bigDiff });
+    expect(res.status).toBe(400);
+    expect(res.body.error).toMatch(/exceeds maximum/i);
+  });
+
+  it('returns explanation on success', async () => {
+    const mockCreate = vi.spyOn(anthropicClient.messages, 'create').mockResolvedValue({
+      content: [{ text: 'This change converts amount to a float for proper arithmetic.' }],
+    });
+
+    const res = await request(app).post('/api/semantic-diff').send({ diff: SAMPLE_DIFF });
+    expect(res.status).toBe(200);
+    expect(res.body.explanation).toBe('This change converts amount to a float for proper arithmetic.');
+    expect(mockCreate).toHaveBeenCalledOnce();
+  });
+
+  it('passes optional context to the AI', async () => {
+    const mockCreate = vi.spyOn(anthropicClient.messages, 'create').mockResolvedValue({
+      content: [{ text: 'Explanation with context.' }],
+    });
+
+    await request(app)
+      .post('/api/semantic-diff')
+      .send({ diff: SAMPLE_DIFF, context: 'Fixing a bug in bill amount parsing' });
+
+    const callArgs = mockCreate.mock.calls[0][0];
+    expect(callArgs.messages[0].content).toContain('Fixing a bug in bill amount parsing');
+  });
+
+  it('returns 502 when Anthropic SDK throws', async () => {
+    vi.spyOn(anthropicClient.messages, 'create').mockRejectedValue(new Error('API unavailable'));
+
+    const res = await request(app).post('/api/semantic-diff').send({ diff: SAMPLE_DIFF });
+    expect(res.status).toBe(502);
+    expect(res.body.error).toMatch(/failed to get explanation/i);
+  });
+});

server/src/app.js

@@ -9,6 +9,7 @@ const actualsRouter = require('./routes/actuals');
 const oneTimeExpensesRouter = require('./routes/one-time-expenses');
 const summaryRouter = require('./routes/summary');
 const { router: financingRouter } = require('./routes/financing');
+const semanticDiffRouter = require('./routes/semantic-diff');
 
 const app = express();
 
@@ -24,6 +25,7 @@ app.use('/api', actualsRouter);
 app.use('/api', oneTimeExpensesRouter);
 app.use('/api', summaryRouter);
 app.use('/api', financingRouter);
+app.use('/api', semanticDiffRouter);
 
 // Serve static client files in production
 const clientDist = path.join(__dirname, '../../client/dist');

server/src/routes/semantic-diff.js

@@ -0,0 +1,56 @@
+const express = require('express');
+const Anthropic = require('@anthropic-ai/sdk');
+
+const router = express.Router();
+
+// Exported so tests can replace client.messages.create without real API calls
+const anthropicClient = new Anthropic({ apiKey: process.env.ANTHROPIC_API_KEY || 'test' });
+
+const MAX_DIFF_BYTES = 50 * 1024; // 50KB
+
+const SYSTEM_PROMPT = `You are a code change analyst for a personal budget web application.
+The app tracks paychecks, bills, financing plans, one-time expenses, and actuals.
+Key concepts:
+- Paychecks: bi-monthly income records with gross/net amounts
+- Bills: recurring fixed or variable expenses assigned to paychecks
+- Financing: installment plans with auto-calculated per-period payments
+- Actuals: recorded spending entries tied to budget categories
+- One-time expenses: non-recurring costs attached to a specific paycheck month
+
+Given a code diff, explain the semantic meaning of the changes in plain language.
+Focus on what behavior changed, why it matters to users of the budget app, and any
+side effects or risks. Be concise but thorough.`;
+
+router.post('/semantic-diff', async (req, res) => {
+  const { diff, context } = req.body;
+
+  if (!diff || typeof diff !== 'string' || diff.trim().length === 0) {
+    return res.status(400).json({ error: 'diff is required and must be a non-empty string' });
+  }
+
+  if (Buffer.byteLength(diff, 'utf8') > MAX_DIFF_BYTES) {
+    return res.status(400).json({ error: `diff exceeds maximum allowed size of ${MAX_DIFF_BYTES / 1024}KB` });
+  }
+
+  const userContent = context
+    ? `Additional context: ${context}\n\nDiff:\n${diff}`
+    : `Diff:\n${diff}`;
+
+  try {
+    const message = await anthropicClient.messages.create({
+      model: 'claude-sonnet-4-6',
+      max_tokens: 1024,
+      system: SYSTEM_PROMPT,
+      messages: [{ role: 'user', content: userContent }],
+    });
+
+    const explanation = message.content[0].text;
+    return res.json({ explanation });
+  } catch (err) {
+    console.error('Anthropic API error:', err);
+    return res.status(502).json({ error: 'Failed to get explanation from AI service' });
+  }
+});
+
+module.exports = router;
+module.exports.anthropicClient = anthropicClient;