diff --git a/src/app/api/auth/register/__tests__/route.test.ts b/src/app/api/auth/register/__tests__/route.test.ts
index 566261e..00f41fa 100644
--- a/src/app/api/auth/register/__tests__/route.test.ts
+++ b/src/app/api/auth/register/__tests__/route.test.ts
@@ -9,7 +9,7 @@ import { describe, it, expect, vi, beforeEach } from 'vitest';
 const { prismaMock, bcryptMock } = vi.hoisted(() => ({
   prismaMock: {
     user: {
-      findUnique: vi.fn(),
+      findFirst: vi.fn(),
       create: vi.fn(),
     },
   },
@@ -27,6 +27,7 @@ vi.mock('bcryptjs', () => ({
 }));
 
 import { POST } from '../route';
+import { Prisma } from '@/generated/prisma/client';
 
 function makeRequest(body: unknown): Request {
   return new Request('http://localhost/api/auth/register', {
@@ -38,7 +39,7 @@ function makeRequest(body: unknown): Request {
 
 beforeEach(() => {
   vi.clearAllMocks();
-  prismaMock.user.findUnique.mockResolvedValue(null);
+  prismaMock.user.findFirst.mockResolvedValue(null);
   prismaMock.user.create.mockResolvedValue({ id: 'user-1' });
   bcryptMock.hash.mockResolvedValue('hashed');
 });
@@ -61,4 +62,47 @@ describe('POST /api/auth/register', () => {
     expect(res.status).toBe(201);
     expect(prismaMock.user.create).toHaveBeenCalledTimes(1);
   });
+
+  it('normalizes email casing/whitespace before checking for an existing user', async () => {
+    await POST(makeRequest({ email: '  Foo@Example.COM  ', password: 'longenough' }));
+    expect(prismaMock.user.findFirst).toHaveBeenCalledWith({
+      where: { email: { equals: 'foo@example.com', mode: 'insensitive' } },
+    });
+  });
+
+  it('stores the normalized email, not the raw input casing', async () => {
+    await POST(makeRequest({ email: 'Foo@Example.COM', password: 'longenough' }));
+    const [args] = prismaMock.user.create.mock.calls[0] as [{ data: { email: string } }];
+    expect(args.data.email).toBe('foo@example.com');
+  });
+
+  it('treats a case-variant of an existing email as a duplicate', async () => {
+    prismaMock.user.findFirst.mockResolvedValueOnce({ id: 'existing-user' });
+    const res = await POST(makeRequest({ email: 'FOO@example.com', password: 'longenough' }));
+    expect(res.status).toBe(400);
+    expect(prismaMock.user.create).not.toHaveBeenCalled();
+  });
+
+  it('treats a concurrent duplicate registration (unique constraint race) as "already exists", not a 500', async () => {
+    // findFirst didn't see it yet, but create() hits the DB's unique
+    // constraint because another request won the race in between.
+    prismaMock.user.create.mockRejectedValueOnce(
+      new Prisma.PrismaClientKnownRequestError('Unique constraint failed on the fields: (`email`)', {
+        code: 'P2002',
+        clientVersion: 'test',
+      }),
+    );
+    const res = await POST(makeRequest({ email: 'a@b.com', password: 'longenough' }));
+    expect(res.status).toBe(400);
+    const body = await res.json();
+    expect(body.error).toMatch(/already exists/i);
+  });
+
+  it('still returns 500 for a create() failure that is not a unique constraint violation', async () => {
+    prismaMock.user.create.mockRejectedValueOnce(new Error('connection reset'));
+    const consoleSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
+    const res = await POST(makeRequest({ email: 'a@b.com', password: 'longenough' }));
+    expect(res.status).toBe(500);
+    consoleSpy.mockRestore();
+  });
 });
diff --git a/src/app/api/auth/register/route.ts b/src/app/api/auth/register/route.ts
index 2945c6c..5ecceef 100644
--- a/src/app/api/auth/register/route.ts
+++ b/src/app/api/auth/register/route.ts
@@ -1,6 +1,8 @@
 import { NextResponse } from 'next/server';
 import bcrypt from 'bcryptjs';
 import { prisma } from '@/lib/prisma';
+import { normalizeEmail } from '@/lib/email';
+import { Prisma } from '@/generated/prisma/client';
 
 export async function POST(request: Request) {
   try {
@@ -13,7 +15,16 @@ export async function POST(request: Request) {
       );
     }
 
-    if (typeof email !== 'string' || !/^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email)) {
+    if (typeof email !== 'string') {
+      return NextResponse.json(
+        { error: 'Enter a valid email address' },
+        { status: 400 }
+      );
+    }
+
+    const normalizedEmail = normalizeEmail(email);
+
+    if (!/^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(normalizedEmail)) {
       return NextResponse.json(
         { error: 'Enter a valid email address' },
         { status: 400 }
@@ -27,8 +38,11 @@ export async function POST(request: Request) {
       );
     }
 
-    const existingUser = await prisma.user.findUnique({
-      where: { email },
+    // Case-insensitive check so a legacy mixed-case row (from before email
+    // normalization) still counts as a duplicate — the DB's unique
+    // constraint is case-sensitive and wouldn't catch it on its own.
+    const existingUser = await prisma.user.findFirst({
+      where: { email: { equals: normalizedEmail, mode: 'insensitive' } },
     });
 
     if (existingUser) {
@@ -40,18 +54,36 @@ export async function POST(request: Request) {
 
     const passwordHash = await bcrypt.hash(password, 10);
 
-    const user = await prisma.user.create({
-      data: {
-        email,
-        name: name || email.split('@')[0],
-        passwordHash,
-      },
-    });
+    // The findFirst check above and this create() aren't atomic, so a
+    // concurrent registration with the same normalized email can still
+    // slip past it and hit the DB's unique constraint here. Treat that
+    // race the same as the check finding it first, rather than letting it
+    // fall through to the generic 500 below.
+    try {
+      const user = await prisma.user.create({
+        data: {
+          email: normalizedEmail,
+          name: name || normalizedEmail.split('@')[0],
+          passwordHash,
+        },
+      });
 
-    return NextResponse.json(
-      { message: 'User created', userId: user.id },
-      { status: 201 }
-    );
+      return NextResponse.json(
+        { message: 'User created', userId: user.id },
+        { status: 201 }
+      );
+    } catch (createError) {
+      if (
+        createError instanceof Prisma.PrismaClientKnownRequestError &&
+        createError.code === 'P2002'
+      ) {
+        return NextResponse.json(
+          { error: 'User already exists' },
+          { status: 400 }
+        );
+      }
+      throw createError;
+    }
   } catch (error) {
     console.error('Registration error:', error);
     return NextResponse.json(
diff --git a/src/lib/__tests__/authorize-credentials.test.ts b/src/lib/__tests__/authorize-credentials.test.ts
new file mode 100644
index 0000000..e487f64
--- /dev/null
+++ b/src/lib/__tests__/authorize-credentials.test.ts
@@ -0,0 +1,83 @@
+/**
+ * Tests for the extracted authorizeCredentials login logic.
+ *
+ * Covers: case-insensitive email matching (so accounts registered with
+ * mixed-case emails before normalization can still log in), fail-closed
+ * behavior on an ambiguous case-insensitive match, and runtime type
+ * guards on the credentials NextAuth hands in (not compile-time enforced).
+ */
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+const { prismaMock, bcryptMock } = vi.hoisted(() => ({
+  prismaMock: {
+    user: {
+      findMany: vi.fn(),
+    },
+  },
+  bcryptMock: {
+    compare: vi.fn(),
+  },
+}));
+
+vi.mock('../prisma', () => ({
+  prisma: prismaMock,
+}));
+
+vi.mock('bcryptjs', () => ({
+  default: bcryptMock,
+}));
+
+import { authorizeCredentials } from '../authorize-credentials';
+
+beforeEach(() => {
+  vi.clearAllMocks();
+  prismaMock.user.findMany.mockResolvedValue([]);
+  bcryptMock.compare.mockResolvedValue(false);
+});
+
+describe('authorizeCredentials', () => {
+  it('returns null when email or password is missing or not a string', async () => {
+    expect(await authorizeCredentials(undefined)).toBeNull();
+    expect(await authorizeCredentials({})).toBeNull();
+    expect(await authorizeCredentials({ email: 'a@b.com' })).toBeNull();
+    expect(await authorizeCredentials({ email: 12345 as unknown as string, password: 'x' })).toBeNull();
+  });
+
+  it('looks up the user case-insensitively with a normalized email', async () => {
+    await authorizeCredentials({ email: '  Foo@Example.COM  ', password: 'x' });
+    expect(prismaMock.user.findMany).toHaveBeenCalledWith({
+      where: { email: { equals: 'foo@example.com', mode: 'insensitive' } },
+    });
+  });
+
+  it('returns null when no user matches', async () => {
+    prismaMock.user.findMany.mockResolvedValueOnce([]);
+    expect(await authorizeCredentials({ email: 'a@b.com', password: 'x' })).toBeNull();
+  });
+
+  it('fails closed instead of picking one when multiple users match case-insensitively', async () => {
+    prismaMock.user.findMany.mockResolvedValueOnce([
+      { id: 'u1', email: 'Foo@example.com', passwordHash: 'h1' },
+      { id: 'u2', email: 'foo@example.com', passwordHash: 'h2' },
+    ]);
+    const consoleSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
+    const result = await authorizeCredentials({ email: 'foo@example.com', password: 'x' });
+    expect(result).toBeNull();
+    expect(bcryptMock.compare).not.toHaveBeenCalled();
+    expect(consoleSpy).toHaveBeenCalled();
+    consoleSpy.mockRestore();
+  });
+
+  it('returns null when the password does not match', async () => {
+    prismaMock.user.findMany.mockResolvedValueOnce([{ id: 'u1', email: 'a@b.com', passwordHash: 'hashed' }]);
+    bcryptMock.compare.mockResolvedValueOnce(false);
+    expect(await authorizeCredentials({ email: 'a@b.com', password: 'wrong' })).toBeNull();
+  });
+
+  it('returns the user on a successful match, matching a legacy mixed-case row', async () => {
+    prismaMock.user.findMany.mockResolvedValueOnce([{ id: 'u1', email: 'Foo@Example.com', name: 'Foo', passwordHash: 'hashed' }]);
+    bcryptMock.compare.mockResolvedValueOnce(true);
+    const result = await authorizeCredentials({ email: 'foo@example.com', password: 'correct' });
+    expect(result).toEqual({ id: 'u1', email: 'Foo@Example.com', name: 'Foo' });
+  });
+});
diff --git a/src/lib/__tests__/email.test.ts b/src/lib/__tests__/email.test.ts
new file mode 100644
index 0000000..107ef8e
--- /dev/null
+++ b/src/lib/__tests__/email.test.ts
@@ -0,0 +1,16 @@
+import { describe, it, expect } from 'vitest';
+import { normalizeEmail } from '../email';
+
+describe('normalizeEmail', () => {
+  it('lowercases the email', () => {
+    expect(normalizeEmail('Foo@Example.com')).toBe('foo@example.com');
+  });
+
+  it('trims surrounding whitespace', () => {
+    expect(normalizeEmail('  foo@example.com  ')).toBe('foo@example.com');
+  });
+
+  it('is idempotent for an already-normalized email', () => {
+    expect(normalizeEmail('foo@example.com')).toBe('foo@example.com');
+  });
+});
diff --git a/src/lib/auth.ts b/src/lib/auth.ts
index 1037527..f687986 100644
--- a/src/lib/auth.ts
+++ b/src/lib/auth.ts
@@ -1,7 +1,6 @@
 import NextAuth from 'next-auth';
 import Credentials from 'next-auth/providers/credentials';
-import bcrypt from 'bcryptjs';
-import { prisma } from './prisma';
+import { authorizeCredentials } from './authorize-credentials';
 
 export const { handlers, signIn, signOut, auth } = NextAuth({
   providers: [
@@ -11,34 +10,7 @@ export const { handlers, signIn, signOut, auth } = NextAuth({
         email: { label: 'Email', type: 'email' },
         password: { label: 'Password', type: 'password' },
       },
-      async authorize(credentials) {
-        if (!credentials?.email || !credentials?.password) {
-          return null;
-        }
-
-        const user = await prisma.user.findUnique({
-          where: { email: credentials.email as string },
-        });
-
-        if (!user) {
-          return null;
-        }
-
-        const isValid = await bcrypt.compare(
-          credentials.password as string,
-          user.passwordHash
-        );
-
-        if (!isValid) {
-          return null;
-        }
-
-        return {
-          id: user.id,
-          email: user.email,
-          name: user.name,
-        };
-      },
+      authorize: authorizeCredentials,
     }),
   ],
   session: {
diff --git a/src/lib/authorize-credentials.ts b/src/lib/authorize-credentials.ts
new file mode 100644
index 0000000..3ee7a9d
--- /dev/null
+++ b/src/lib/authorize-credentials.ts
@@ -0,0 +1,55 @@
+/**
+ * Credentials-login lookup logic, kept free of any next-auth import so it
+ * can be unit tested directly — importing next-auth itself pulls in
+ * next/server, which isn't available in the Vitest/jsdom environment.
+ */
+import bcrypt from 'bcryptjs';
+import { prisma } from './prisma';
+import { normalizeEmail } from './email';
+
+export async function authorizeCredentials(
+  credentials: Partial<Record<'email' | 'password', unknown>> | undefined,
+) {
+  if (
+    typeof credentials?.email !== 'string' ||
+    typeof credentials?.password !== 'string'
+  ) {
+    return null;
+  }
+
+  const normalizedEmail = normalizeEmail(credentials.email);
+
+  // Case-insensitive lookup so accounts registered before email
+  // normalization (potentially stored with mixed casing) can still log in.
+  // If more than one row matches case-insensitively — a legacy data
+  // anomaly the case-sensitive unique constraint didn't prevent — fail
+  // closed rather than silently picking one account.
+  const matches = await prisma.user.findMany({
+    where: { email: { equals: normalizedEmail, mode: 'insensitive' } },
+  });
+
+  if (matches.length === 0) {
+    return null;
+  }
+  if (matches.length > 1) {
+    console.error(
+      '[auth] multiple users matched email case-insensitively; refusing to pick one',
+      { userIds: matches.map((u) => u.id) },
+    );
+    return null;
+  }
+
+  const [user] = matches;
+
+  const isValid = await bcrypt.compare(credentials.password, user.passwordHash);
+
+  if (!isValid) {
+    return null;
+  }
+
+  return {
+    id: user.id,
+    email: user.email,
+    name: user.name,
+  };
+}
diff --git a/src/lib/email.ts b/src/lib/email.ts
new file mode 100644
index 0000000..dc421bf
--- /dev/null
+++ b/src/lib/email.ts
@@ -0,0 +1,11 @@
+/**
+ * Shared email normalization for auth. Postgres's unique constraint on
+ * User.email is case-sensitive, and NextAuth's Credentials `authorize`
+ * looks users up by exact string — without normalizing both write
+ * (registration) and read (login) paths the same way, a user who signs up
+ * as "Foo@Example.com" can fail to log back in with different casing or
+ * incidental whitespace.
+ */
+export function normalizeEmail(email: string): string {
+  return email.trim().toLowerCase();
+}