diff --git a/app/api/routesF/roman-numeral-validator/route.test.ts b/app/api/routesF/roman-numeral-validator/route.test.ts
new file mode 100644
index 00000000..b409a54e
--- /dev/null
+++ b/app/api/routesF/roman-numeral-validator/route.test.ts
@@ -0,0 +1,103 @@
+import { POST } from './route';
+
+async function validate(roman: unknown) {
+  const req = new Request('http://localhost/api/routesF/roman-numeral-validator', {
+    method: 'POST',
+    body: JSON.stringify({ roman }),
+  });
+
+  const res = await POST(req);
+  return {
+    status: res.status,
+    data: await res.json(),
+  };
+}
+
+describe('Roman numeral validator API', () => {
+  it('accepts legal strict Roman numerals', async () => {
+    await expect(validate('I')).resolves.toEqual({
+      status: 200,
+      data: { valid: true, value: 1 },
+    });
+
+    await expect(validate('IV')).resolves.toEqual({
+      status: 200,
+      data: { valid: true, value: 4 },
+    });
+
+    await expect(validate('XLII')).resolves.toEqual({
+      status: 200,
+      data: { valid: true, value: 42 },
+    });
+
+    await expect(validate('MCMXCIV')).resolves.toEqual({
+      status: 200,
+      data: { valid: true, value: 1994 },
+    });
+
+    await expect(validate('MMMCMXCIX')).resolves.toEqual({
+      status: 200,
+      data: { valid: true, value: 3999 },
+    });
+  });
+
+  it('rejects illegal additive and repeated forms', async () => {
+    for (const roman of ['IIII', 'VV', 'XXXX', 'LL', 'CCCC', 'DD']) {
+      const { status, data } = await validate(roman);
+
+      expect(status).toBe(200);
+      expect(data.valid).toBe(false);
+      expect(data.reason).toBe('Roman numeral is not in strict subtractive notation');
+    }
+  });
+
+  it('rejects illegal subtractive forms', async () => {
+    for (const roman of ['IC', 'IL', 'XD', 'XM', 'VX', 'LC']) {
+      const { status, data } = await validate(roman);
+
+      expect(status).toBe(200);
+      expect(data.valid).toBe(false);
+      expect(data.reason).toBe('Roman numeral is not in strict subtractive notation');
+    }
+  });
+
+  it('rejects malformed input', async () => {
+    await expect(validate('')).resolves.toMatchObject({
+      status: 200,
+      data: { valid: false, reason: 'Roman numeral cannot be empty' },
+    });
+
+    await expect(validate(' ix')).resolves.toMatchObject({
+      status: 200,
+      data: { valid: false, reason: 'Roman numeral cannot include whitespace' },
+    });
+
+    await expect(validate('ix')).resolves.toMatchObject({
+      status: 200,
+      data: { valid: false, reason: 'Roman numeral must use uppercase letters' },
+    });
+
+    await expect(validate('ABC')).resolves.toMatchObject({
+      status: 200,
+      data: { valid: false, reason: 'Roman numeral contains invalid characters' },
+    });
+
+    await expect(validate(42)).resolves.toMatchObject({
+      status: 200,
+      data: { valid: false, reason: 'Roman numeral must be a string' },
+    });
+  });
+
+  it('returns 400 for invalid JSON', async () => {
+    const req = new Request('http://localhost/api/routesF/roman-numeral-validator', {
+      method: 'POST',
+      body: '{',
+    });
+
+    const res = await POST(req);
+    const data = await res.json();
+
+    expect(res.status).toBe(400);
+    expect(data).toEqual({ error: 'Invalid JSON body' });
+  });
+});
diff --git a/app/api/routesF/roman-numeral-validator/route.ts b/app/api/routesF/roman-numeral-validator/route.ts
new file mode 100644
index 00000000..8e38e5c9
--- /dev/null
+++ b/app/api/routesF/roman-numeral-validator/route.ts
@@ -0,0 +1,79 @@
+import { NextResponse } from 'next/server';
+
+type RomanValidationResult = {
+  valid: boolean;
+  value?: number;
+  reason?: string;
+};
+
+const ROMAN_VALUES: Record<string, number> = {
+  I: 1,
+  V: 5,
+  X: 10,
+  L: 50,
+  C: 100,
+  D: 500,
+  M: 1000,
+};
+
+const ROMAN_CHARACTERS = /^[IVXLCDM]+$/;
+const STRICT_ROMAN_NUMERAL =
+  /^(?=.)M{0,3}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})$/;
+
+function romanToNumber(roman: string): number {
+  let total = 0;
+
+  for (let index = 0; index < roman.length; index++) {
+    const current = ROMAN_VALUES[roman[index]];
+    const next = ROMAN_VALUES[roman[index + 1]] ?? 0;
+
+    total += current < next ? -current : current;
+  }
+
+  return total;
+}
+
+function validateRoman(roman: unknown): RomanValidationResult {
+  if (typeof roman !== 'string') {
+    return { valid: false, reason: 'Roman numeral must be a string' };
+  }
+
+  if (roman.length === 0) {
+    return { valid: false, reason: 'Roman numeral cannot be empty' };
+  }
+
+  if (roman.trim() !== roman) {
+    return { valid: false, reason: 'Roman numeral cannot include whitespace' };
+  }
+
+  if (roman !== roman.toUpperCase()) {
+    return { valid: false, reason: 'Roman numeral must use uppercase letters' };
+  }
+
+  if (!ROMAN_CHARACTERS.test(roman)) {
+    return { valid: false, reason: 'Roman numeral contains invalid characters' };
+  }
+
+  if (!STRICT_ROMAN_NUMERAL.test(roman)) {
+    return { valid: false, reason: 'Roman numeral is not in strict subtractive notation' };
+  }
+
+  const value = romanToNumber(roman);
+
+  if (value < 1 || value > 3999) {
+    return { valid: false, reason: 'Roman numeral must be in the range 1-3999' };
+  }
+
+  return { valid: true, value };
+}
+
+export async function POST(request: Request) {
+  try {
+    const body = await request.json();
+    const result = validateRoman(body?.roman);
+
+    return NextResponse.json(result);
+  } catch {
+    return NextResponse.json({ error: 'Invalid JSON body' }, { status: 400 });
+  }
+}