feat: add auth email adapter

src/email.ts

@@ -0,0 +1,80 @@
+import type { Env } from "./env";
+
+export type AuthEmailKind = "verify-email" | "reset-password" | "email-otp" | "magic-link";
+
+export interface AuthEmailInput {
+  kind: AuthEmailKind;
+  to: string;
+  url?: string;
+  otp?: string;
+}
+
+export interface EmailMessage {
+  to: string;
+  subject: string;
+  text: string;
+}
+
+export function buildAuthEmail(input: AuthEmailInput): EmailMessage {
+  if (input.kind === "email-otp") {
+    return {
+      to: input.to,
+      subject: "Your sign-in code",
+      text: `Your sign-in code is ${input.otp ?? ""}.`,
+    };
+  }
+
+  if (input.kind === "reset-password") {
+    return {
+      to: input.to,
+      subject: "Reset your password",
+      text: `Use this link to reset your password: ${input.url ?? ""}`,
+    };
+  }
+
+  if (input.kind === "magic-link") {
+    return {
+      to: input.to,
+      subject: "Sign in to your account",
+      text: `Use this link to sign in: ${input.url ?? ""}`,
+    };
+  }
+
+  return {
+    to: input.to,
+    subject: "Verify your email",
+    text: `Use this link to verify your email: ${input.url ?? ""}`,
+  };
+}
+
+export async function sendEmail(env: Env, message: EmailMessage): Promise<void> {
+  if (!env.MAIL_PROVIDER) {
+    throw new Error("MAIL_PROVIDER is not configured");
+  }
+
+  if (env.MAIL_PROVIDER !== "resend") {
+    throw new Error(`Unsupported MAIL_PROVIDER: ${env.MAIL_PROVIDER}`);
+  }
+
+  if (!env.RESEND_API_KEY || !env.MAIL_FROM) {
+    throw new Error("RESEND_API_KEY and MAIL_FROM are required for resend");
+  }
+
+  const response = await fetch("https://api.resend.com/emails", {
+    method: "POST",
+    headers: {
+      Authorization: `Bearer ${env.RESEND_API_KEY}`,
+      "Content-Type": "application/json",
+    },
+    body: JSON.stringify({
+      from: env.MAIL_FROM,
+      to: message.to,
+      subject: message.subject,
+      text: message.text,
+    }),
+  });
+
+  if (!response.ok) {
+    throw new Error(`Email provider failed: ${response.status}`);
+  }
+}

tests/auth-config.test.ts

@@ -7,6 +7,7 @@ import {
   trustedOrigins,
   type Env,
 } from "../src/env";
+import { buildAuthEmail, sendEmail } from "../src/email";
 
 const env: Env = {
   BETTER_AUTH_URL: "http://localhost:8788",
@@ -40,3 +41,32 @@ describe("auth env helpers", () => {
     expect(optionalEnv({}, "MAIL_FROM")).toBeUndefined();
   });
 });
+
+describe("auth email adapter", () => {
+  it("builds a verification email", () => {
+    const message = buildAuthEmail({
+      kind: "verify-email",
+      to: "user@example.com",
+      url: "https://auth.example.com/verify",
+    });
+    expect(message.subject).toBe("Verify your email");
+    expect(message.to).toBe("user@example.com");
+    expect(message.text).toContain("https://auth.example.com/verify");
+  });
+
+  it("fails clearly when no mail provider is configured", async () => {
+    await expect(
+      sendEmail(
+        {
+          BETTER_AUTH_URL: "http://localhost:8788",
+          TRUSTED_ORIGINS: "http://localhost:8787",
+        },
+        {
+          to: "user@example.com",
+          subject: "Test",
+          text: "Test",
+        },
+      ),
+    ).rejects.toThrow("MAIL_PROVIDER is not configured");
+  });
+});