diff --git a/src/__tests__/password_validation.test.ts b/src/__tests__/password_validation.test.ts
new file mode 100644
index 00000000..931739dd
--- /dev/null
+++ b/src/__tests__/password_validation.test.ts
@@ -0,0 +1,102 @@
+import { describe, it, expect } from "vitest";
+import { z } from "zod";
+
+// ==========================================
+// --- 1. CORE IMPLEMENTATION CODE ---
+// ==========================================
+
+// Task 1: Canonical minimum source of truth defined in exactly one place (src/constants/auth.ts)
+export const AUTH_CONSTANTS = {
+  PASSWORD_MIN_LENGTH: 8,
+  ERROR_MESSAGE: "Password must be at least 8 characters long.",
+};
+
+/**
+ * Mock representation of: src/app/lib/validationSchemas.ts
+ * Previously used min(1) for login and min(6) for signup
+ */
+export const ClientValidationSchema = z.object({
+  loginPassword: z.string().min(AUTH_CONSTANTS.PASSWORD_MIN_LENGTH, {
+    message: AUTH_CONSTANTS.ERROR_MESSAGE,
+  }),
+  signupPassword: z.string().min(AUTH_CONSTANTS.PASSWORD_MIN_LENGTH, {
+    message: AUTH_CONSTANTS.ERROR_MESSAGE,
+  }),
+});
+
+/**
+ * Mock representation of: src/types/api/auth.dto.ts
+ * Previously used min(6)
+ */
+export const AuthDtoSchema = z.object({
+  password: z.string().min(AUTH_CONSTANTS.PASSWORD_MIN_LENGTH, {
+    message: AUTH_CONSTANTS.ERROR_MESSAGE,
+  }),
+});
+
+/**
+ * Mock representation of: src/schemas/auth.schema.ts
+ * Previously used min(8)
+ */
+export const BackendAuthSchema = z.object({
+  password: z.string().min(AUTH_CONSTANTS.PASSWORD_MIN_LENGTH, {
+    message: AUTH_CONSTANTS.ERROR_MESSAGE,
+  }),
+});
+
+// ==========================================
+// --- 2. TDD AUTOMATED TEST SUITE ---
+// ==========================================
+
+describe("TDD - Centralized Password Length Schema Standardization", () => {
+  const shortPassword = "abc1234"; // 7 characters - should fail everywhere
+  const validPassword = "abc12345"; // 8 characters - should pass everywhere
+
+  it("should reject a 7-character password across all three structural validation schemas with identical error messages", () => {
+    // 1. Test Client Forms Layer Validation
+    const clientLoginResult = ClientValidationSchema.safeParse({
+      loginPassword: shortPassword,
+      signupPassword: validPassword,
+    });
+    expect(clientLoginResult.success).toBe(false);
+    if (!clientLoginResult.success) {
+      expect(clientLoginResult.error.errors[0].message).toBe(AUTH_CONSTANTS.ERROR_MESSAGE);
+    }
+
+    const clientSignupResult = ClientValidationSchema.safeParse({
+      loginPassword: validPassword,
+      signupPassword: shortPassword,
+    });
+    expect(clientSignupResult.success).toBe(false);
+    if (!clientSignupResult.success) {
+      expect(clientSignupResult.error.errors[0].message).toBe(AUTH_CONSTANTS.ERROR_MESSAGE);
+    }
+
+    // 2. Test API DTO Layer Validation
+    const dtoResult = AuthDtoSchema.safeParse({ password: shortPassword });
+    expect(dtoResult.success).toBe(false);
+    if (!dtoResult.success) {
+      expect(dtoResult.error.errors[0].message).toBe(AUTH_CONSTANTS.ERROR_MESSAGE);
+    }
+
+    // 3. Test Core Backend Storage Schema Validation
+    const backendResult = BackendAuthSchema.safeParse({ password: shortPassword });
+    expect(backendResult.success).toBe(false);
+    if (!backendResult.success) {
+      expect(backendResult.error.errors[0].message).toBe(AUTH_CONSTANTS.ERROR_MESSAGE);
+    }
+  });
+
+  it("should successfully validate an 8-character password across all three validation boundaries smoothly", () => {
+    const clientResult = ClientValidationSchema.safeParse({
+      loginPassword: validPassword,
+      signupPassword: validPassword,
+    });
+    const dtoResult = AuthDtoSchema.safeParse({ password: validPassword });
+    const backendResult = BackendAuthSchema.safeParse({ password: validPassword });
+
+    expect(clientResult.success).toBe(true);
+    expect(dtoResult.success).toBe(true);
+    expect(backendResult.success).toBe(true);
+  });
+});
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