diff --git a/lib/auth/discord-user-core.test.ts b/lib/auth/discord-user-core.test.ts
new file mode 100644
index 0000000..a5b85ee
--- /dev/null
+++ b/lib/auth/discord-user-core.test.ts
@@ -0,0 +1,131 @@
+import { describe, expect, test } from "bun:test";
+import {
+  reconcileDiscordUser,
+  type DiscordIdentity,
+  type DiscordUserOperations,
+} from "@/lib/auth/discord-user-core";
+
+type MemoryUser = {
+  id: string;
+  discordId: string;
+  name: string;
+  image: string | null;
+  email: string | null;
+};
+
+function createMemoryStore() {
+  const users = new Map<string, MemoryUser>();
+  const userIdByDiscord = new Map<string, string>();
+  const accounts = new Map<string, { userId: string; fields: Record<string, unknown> }>();
+  let nextUserId = 1;
+
+  const operations: DiscordUserOperations<MemoryUser> = {
+    async findLinkedUserId(discordId) {
+      return accounts.get(discordId)?.userId ?? null;
+    },
+    async upsertUser(identity) {
+      let userId = userIdByDiscord.get(identity.discordId);
+      if (!userId) {
+        userId = `user-${nextUserId++}`;
+        userIdByDiscord.set(identity.discordId, userId);
+        users.set(userId, {
+          id: userId,
+          discordId: identity.discordId,
+          name: identity.displayName,
+          image: identity.avatarUrl,
+          email: identity.email ?? null,
+        });
+      }
+      return userId;
+    },
+    async insertAccount(userId, discordId, account) {
+      if (!accounts.has(discordId)) {
+        accounts.set(discordId, {
+          userId,
+          fields: { type: account?.type ?? "oauth" },
+        });
+      }
+    },
+    async updateUser(userId, identity) {
+      const user = users.get(userId);
+      if (!user) return null;
+      const updated = {
+        ...user,
+        discordId: identity.discordId,
+        name: identity.displayName,
+        image: identity.avatarUrl,
+        ...(identity.email !== undefined ? { email: identity.email } : {}),
+      };
+      users.set(userId, updated);
+      return updated;
+    },
+    async updateAccount(discordId, update) {
+      const account = accounts.get(discordId);
+      if (account) account.fields = { ...account.fields, ...update };
+    },
+  };
+
+  return { users, accounts, operations };
+}
+
+const botIdentity: DiscordIdentity = {
+  discordId: "123456789012345678",
+  displayName: "Bot-created user",
+  avatarUrl: "https://cdn.discordapp.com/avatar.png",
+};
+
+describe("Discord account reconciliation", () => {
+  test("creates one user and one account for a new bot sender", async () => {
+    const store = createMemoryStore();
+    const user = await reconcileDiscordUser(botIdentity, store.operations);
+
+    expect(user.id).toBe("user-1");
+    expect(store.users.size).toBe(1);
+    expect(store.accounts.size).toBe(1);
+    expect(store.accounts.get(botIdentity.discordId)?.userId).toBe(user.id);
+  });
+
+  test("concurrent first uploads resolve to the same database winner", async () => {
+    const store = createMemoryStore();
+    const [first, second] = await Promise.all([
+      reconcileDiscordUser(botIdentity, store.operations),
+      reconcileDiscordUser(botIdentity, store.operations),
+    ]);
+
+    expect(first.id).toBe(second.id);
+    expect(store.users.size).toBe(1);
+    expect(store.accounts.size).toBe(1);
+  });
+
+  test("OAuth reuses a bot-created user and bot refreshes preserve email", async () => {
+    const store = createMemoryStore();
+    const botUser = await reconcileDiscordUser(botIdentity, store.operations);
+    const oauthUser = await reconcileDiscordUser(
+      {
+        ...botIdentity,
+        displayName: "OAuth name",
+        email: "member@example.com",
+        account: {
+          type: "oauth",
+          access_token: "access-token",
+          refresh_token: "refresh-token",
+        },
+      },
+      store.operations,
+    );
+    const refreshedByBot = await reconcileDiscordUser(
+      { ...botIdentity, displayName: "Latest Discord name" },
+      store.operations,
+    );
+
+    expect(oauthUser.id).toBe(botUser.id);
+    expect(refreshedByBot.id).toBe(botUser.id);
+    expect(refreshedByBot.email).toBe("member@example.com");
+    expect(store.users.size).toBe(1);
+    expect(store.accounts.size).toBe(1);
+    expect(store.accounts.get(botIdentity.discordId)?.fields).toMatchObject({
+      access_token: "access-token",
+      refresh_token: "refresh-token",
+    });
+  });
+});
diff --git a/lib/auth/discord-user-core.ts b/lib/auth/discord-user-core.ts
new file mode 100644
index 0000000..af0a3cf
--- /dev/null
+++ b/lib/auth/discord-user-core.ts
@@ -0,0 +1,78 @@
+import type { AdapterAccount } from "next-auth/adapters";
+
+export type DiscordIdentity = {
+  discordId: string;
+  displayName: string;
+  avatarUrl: string | null;
+  email?: string | null;
+  account?: Partial<AdapterAccount>;
+};
+
+export function discordAccountUpdate(
+  account: Partial<AdapterAccount> | undefined,
+) {
+  if (!account) return {};
+  return {
+    ...(account.type !== undefined ? { type: account.type } : {}),
+    ...(account.refresh_token !== undefined
+      ? { refresh_token: account.refresh_token }
+      : {}),
+    ...(account.access_token !== undefined
+      ? { access_token: account.access_token }
+      : {}),
+    ...(account.expires_at !== undefined
+      ? { expires_at: account.expires_at }
+      : {}),
+    ...(account.token_type !== undefined
+      ? { token_type: account.token_type }
+      : {}),
+    ...(account.scope !== undefined ? { scope: account.scope } : {}),
+    ...(account.id_token !== undefined ? { id_token: account.id_token } : {}),
+    ...(account.session_state !== undefined
+      ? { session_state: account.session_state as string | null }
+      : {}),
+  };
+}
+
+export type DiscordAccountUpdate = ReturnType<typeof discordAccountUpdate>;
+
+export type DiscordUserOperations<TUser> = {
+  findLinkedUserId(discordId: string): Promise<string | null>;
+  upsertUser(identity: DiscordIdentity): Promise<string>;
+  insertAccount(
+    userId: string,
+    discordId: string,
+    account: Partial<AdapterAccount> | undefined,
+  ): Promise<void>;
+  updateUser(userId: string, identity: DiscordIdentity): Promise<TUser | null>;
+  updateAccount(
+    discordId: string,
+    update: DiscordAccountUpdate,
+  ): Promise<void>;
+};
+
+export async function reconcileDiscordUser<TUser>(
+  identity: DiscordIdentity,
+  operations: DiscordUserOperations<TUser>,
+) {
+  const { discordId, account } = identity;
+  let userId = await operations.findLinkedUserId(discordId);
+
+  if (!userId) {
+    userId = await operations.upsertUser(identity);
+    await operations.insertAccount(userId, discordId, account);
+
+    const winner = await operations.findLinkedUserId(discordId);
+    if (!winner) throw new Error("Failed to link Discord account");
+    userId = winner;
+  }
+
+  const user = await operations.updateUser(userId, identity);
+  const nextAccount = discordAccountUpdate(account);
+  if (Object.keys(nextAccount).length > 0) {
+    await operations.updateAccount(discordId, nextAccount);
+  }
+
+  if (!user) throw new Error("Failed to reconcile Discord user");
+  return user;
+}
diff --git a/lib/auth/discord-user.ts b/lib/auth/discord-user.ts
index 7f99cd0..976dbeb 100644
--- a/lib/auth/discord-user.ts
+++ b/lib/auth/discord-user.ts
@@ -1,132 +1,94 @@
 import "server-only";
 
-import type { AdapterAccount } from "next-auth/adapters";
 import { and, eq } from "drizzle-orm";
 import { db } from "@/db";
 import { accounts, users } from "@/db/schema";
+import {
+  discordAccountUpdate,
+  reconcileDiscordUser,
+  type DiscordIdentity,
+} from "@/lib/auth/discord-user-core";
 
-export type DiscordIdentity = {
-  discordId: string;
-  displayName: string;
-  avatarUrl: string | null;
-  email?: string | null;
-  account?: Partial<AdapterAccount>;
-};
-
-function accountUpdate(account: Partial<AdapterAccount> | undefined) {
-  if (!account) return {};
-  return {
-    ...(account.type !== undefined ? { type: account.type } : {}),
-    ...(account.refresh_token !== undefined
-      ? { refresh_token: account.refresh_token }
-      : {}),
-    ...(account.access_token !== undefined
-      ? { access_token: account.access_token }
-      : {}),
-    ...(account.expires_at !== undefined
-      ? { expires_at: account.expires_at }
-      : {}),
-    ...(account.token_type !== undefined
-      ? { token_type: account.token_type }
-      : {}),
-    ...(account.scope !== undefined ? { scope: account.scope } : {}),
-    ...(account.id_token !== undefined ? { id_token: account.id_token } : {}),
-    ...(account.session_state !== undefined
-      ? { session_state: account.session_state as string | null }
-      : {}),
-  };
-}
+export type { DiscordIdentity } from "@/lib/auth/discord-user-core";
 
 export async function ensureDiscordUser(
   identity: DiscordIdentity
 ): Promise<typeof users.$inferSelect> {
-  const { discordId, displayName, avatarUrl, email, account } = identity;
-
   return db.transaction(async (tx) => {
-    const [linked] = await tx
-      .select({ userId: accounts.userId })
-      .from(accounts)
-      .where(
-        and(
-          eq(accounts.provider, "discord"),
-          eq(accounts.providerAccountId, discordId)
-        )
-      )
-      .limit(1);
-
-    let userId = linked?.userId;
-    if (!userId) {
-      const [user] = await tx
-        .insert(users)
-        .values({
-          discordId,
-          name: displayName,
-          image: avatarUrl,
-          ...(email !== undefined ? { email } : {}),
-        })
-        .onConflictDoUpdate({
-          target: users.discordId,
-          set: {
-            name: displayName,
-            image: avatarUrl,
-            ...(email !== undefined ? { email } : {}),
-          },
-        })
-        .returning({ id: users.id });
-      userId = user.id;
-
-      await tx
-        .insert(accounts)
-        .values({
-          userId,
-          provider: "discord",
-          providerAccountId: discordId,
-          type: account?.type ?? "oauth",
-          ...accountUpdate(account),
-        })
-        .onConflictDoNothing({
-          target: [accounts.provider, accounts.providerAccountId],
-        });
-
-      const [winner] = await tx
+    const findLinkedUserId = async (discordId: string) => {
+      const [linked] = await tx
         .select({ userId: accounts.userId })
         .from(accounts)
         .where(
           and(
             eq(accounts.provider, "discord"),
-            eq(accounts.providerAccountId, discordId)
-          )
+            eq(accounts.providerAccountId, discordId),
+          ),
         )
         .limit(1);
-      if (!winner) throw new Error("Failed to link Discord account");
-      userId = winner.userId;
-    }
+      return linked?.userId ?? null;
+    };
 
-    const [user] = await tx
-      .update(users)
-      .set({
-        discordId,
-        name: displayName,
-        image: avatarUrl,
-        ...(email !== undefined ? { email } : {}),
-      })
-      .where(eq(users.id, userId))
-      .returning();
-
-    const nextAccount = accountUpdate(account);
-    if (Object.keys(nextAccount).length > 0) {
-      await tx
-        .update(accounts)
-        .set(nextAccount)
-        .where(
-          and(
-            eq(accounts.provider, "discord"),
-            eq(accounts.providerAccountId, discordId)
-          )
-        );
-    }
-
-    if (!user) throw new Error("Failed to reconcile Discord user");
-    return user;
+    return reconcileDiscordUser(identity, {
+      findLinkedUserId,
+      async upsertUser({ discordId, displayName, avatarUrl, email }) {
+        const [user] = await tx
+          .insert(users)
+          .values({
+            discordId,
+            name: displayName,
+            image: avatarUrl,
+            ...(email !== undefined ? { email } : {}),
+          })
+          .onConflictDoUpdate({
+            target: users.discordId,
+            set: {
+              name: displayName,
+              image: avatarUrl,
+              ...(email !== undefined ? { email } : {}),
+            },
+          })
+          .returning({ id: users.id });
+        return user.id;
+      },
+      async insertAccount(userId, discordId, account) {
+        await tx
+          .insert(accounts)
+          .values({
+            userId,
+            provider: "discord",
+            providerAccountId: discordId,
+            type: account?.type ?? "oauth",
+            ...discordAccountUpdate(account),
+          })
+          .onConflictDoNothing({
+            target: [accounts.provider, accounts.providerAccountId],
+          });
+      },
+      async updateUser(userId, { discordId, displayName, avatarUrl, email }) {
+        const [user] = await tx
+          .update(users)
+          .set({
+            discordId,
+            name: displayName,
+            image: avatarUrl,
+            ...(email !== undefined ? { email } : {}),
+          })
+          .where(eq(users.id, userId))
+          .returning();
+        return user ?? null;
+      },
+      async updateAccount(discordId, update) {
+        await tx
+          .update(accounts)
+          .set(update)
+          .where(
+            and(
+              eq(accounts.provider, "discord"),
+              eq(accounts.providerAccountId, discordId),
+            ),
+          );
+      },
+    });
   });
 }