diff --git a/apps/api/src/lib/external-auth-resolver.ts b/apps/api/src/lib/external-auth-resolver.ts
new file mode 100644
index 00000000..2cd7744a
--- /dev/null
+++ b/apps/api/src/lib/external-auth-resolver.ts
@@ -0,0 +1,214 @@
+import { randomUUID } from "node:crypto";
+import { eq, sql } from "drizzle-orm";
+import type { FastifyBaseLogger } from "fastify";
+import { env } from "../config.js";
+import { db, schema } from "../db/index.js";
+import { auditLog, sanitizeAuditInput } from "./audit.js";
+
+// ── Types ─────────────────────────────────────────────────────────
+
+export interface ExternalAuthParams {
+  provider: string; // "oidc" or "saml"
+  externalId: string; // OIDC sub or SAML NameID
+  email?: string;
+  emailVerified?: boolean;
+  username: string; // derived/sanitized username
+  autoCreate: boolean;
+  autoLink: boolean;
+  defaultRole: string;
+  logger: FastifyBaseLogger;
+  ip: string;
+  requestId: string;
+}
+
+export interface ExternalAuthResult {
+  user: { id: string; username: string; role: string; team: string } | null;
+  action: "matched" | "linked" | "created" | "denied";
+  deniedReason?: "user_not_authorized" | "user_limit_reached";
+}
+
+// ── Username helpers ──────────────────────────────────────────────
+
+export function sanitizeUsername(raw: string): string {
+  let sanitized = raw
+    .toLowerCase()
+    .replace(/[^a-z0-9_.-]/g, "_")
+    .replace(/_{2,}/g, "_")
+    .replace(/^[_.-]+|[_.-]+$/g, "");
+
+  // Enforce 3-50 char limit (truncate to 46 to leave room for collision suffix)
+  if (sanitized.length > 46) {
+    sanitized = sanitized.slice(0, 46);
+  }
+  if (sanitized.length < 3) {
+    sanitized = sanitized.padEnd(3, "_");
+  }
+
+  return sanitized;
+}
+
+export async function findUniqueUsername(base: string): Promise<string> {
+  const [existing] = await db
+    .select({ username: schema.users.username })
+    .from(schema.users)
+    .where(eq(schema.users.username, base));
+
+  if (!existing) return base;
+
+  for (let i = 2; i <= 1000; i++) {
+    const candidate = `${base}_${i}`;
+    const [taken] = await db
+      .select({ username: schema.users.username })
+      .from(schema.users)
+      .where(eq(schema.users.username, candidate));
+    if (!taken) return candidate;
+  }
+
+  // Extremely unlikely fallback
+  return `${base}_${Date.now()}`;
+}
+
+// ── Resolver ─────────────────────────────────────────────────────
+
+export async function resolveExternalUser(params: ExternalAuthParams): Promise<ExternalAuthResult> {
+  const {
+    provider,
+    externalId,
+    email,
+    emailVerified,
+    username,
+    autoCreate,
+    autoLink,
+    defaultRole,
+    logger,
+    ip,
+    requestId,
+  } = params;
+
+  const providerUpper = provider.toUpperCase();
+
+  const audit = (event: string, details: Record<string, unknown> = {}) =>
+    auditLog(logger, event, details, ip, requestId);
+
+  // 1. Match by externalId
+  const [existingByExtId] = await db
+    .select()
+    .from(schema.users)
+    .where(eq(schema.users.externalId, externalId))
+    .limit(1);
+
+  if (existingByExtId) {
+    // Update email if changed
+    if (email && email !== existingByExtId.email) {
+      await db
+        .update(schema.users)
+        .set({ email, updatedAt: new Date() })
+        .where(eq(schema.users.id, existingByExtId.id));
+    }
+    return {
+      user: {
+        id: existingByExtId.id,
+        username: existingByExtId.username,
+        role: existingByExtId.role,
+        team: existingByExtId.team,
+      },
+      action: "matched",
+    };
+  }
+
+  // 2. Auto-link by email
+  if (autoLink && email && emailVerified) {
+    const [existingByEmail] = await db
+      .select()
+      .from(schema.users)
+      .where(eq(schema.users.email, email))
+      .limit(1);
+
+    if (existingByEmail) {
+      await db
+        .update(schema.users)
+        .set({
+          externalId,
+          authProvider: provider,
+          updatedAt: new Date(),
+        })
+        .where(eq(schema.users.id, existingByEmail.id));
+
+      await audit(`${providerUpper}_USER_LINKED`, {
+        userId: existingByEmail.id,
+        username: existingByEmail.username,
+        email,
+      });
+
+      return {
+        user: {
+          id: existingByEmail.id,
+          username: existingByEmail.username,
+          role: existingByEmail.role,
+          team: existingByEmail.team,
+        },
+        action: "linked",
+      };
+    }
+  }
+
+  // 3. Auto-create
+  if (autoCreate) {
+    // Check user limit
+    if (env.MAX_USERS > 0) {
+      const [countResult] = await db.select({ count: sql<number>`COUNT(*)` }).from(schema.users);
+      if (countResult && countResult.count >= env.MAX_USERS) {
+        logger.warn(`${provider} auto-create blocked: user limit reached`);
+        return { user: null, action: "denied", deniedReason: "user_limit_reached" };
+      }
+    }
+
+    const uniqueUsername = await findUniqueUsername(username);
+    const newUserId = randomUUID();
+
+    // Look up the default team
+    const [defaultTeam] = await db
+      .select()
+      .from(schema.teams)
+      .where(eq(schema.teams.name, "Default"));
+    const teamId = defaultTeam?.id ?? "default-team-00000000";
+
+    await db.insert(schema.users).values({
+      id: newUserId,
+      username: uniqueUsername,
+      passwordHash: null,
+      role: defaultRole,
+      team: teamId,
+      mustChangePassword: false,
+      authProvider: provider,
+      externalId,
+      email: email ?? null,
+    });
+
+    await audit(`${providerUpper}_USER_CREATED`, {
+      userId: newUserId,
+      username: uniqueUsername,
+      email,
+      role: defaultRole,
+    });
+
+    return {
+      user: {
+        id: newUserId,
+        username: uniqueUsername,
+        role: defaultRole,
+        team: teamId,
+      },
+      action: "created",
+    };
+  }
+
+  // 4. Denied: no matching user, auto-link did not match, auto-create disabled
+  logger.warn({ externalId, email }, `${provider} user not authorized`);
+  await audit(`${providerUpper}_LOGIN_FAILED`, {
+    reason: "user_not_authorized",
+    externalId: sanitizeAuditInput(String(externalId)),
+  });
+
+  return { user: null, action: "denied", deniedReason: "user_not_authorized" };
+}
diff --git a/apps/api/src/plugins/oidc.ts b/apps/api/src/plugins/oidc.ts
index 0ef5d2fc..77782389 100644
--- a/apps/api/src/plugins/oidc.ts
+++ b/apps/api/src/plugins/oidc.ts
@@ -1,11 +1,10 @@
-import { randomUUID } from "node:crypto";
 import type {} from "@fastify/cookie";
-import { eq, sql } from "drizzle-orm";
 import type { FastifyInstance, FastifyReply, FastifyRequest } from "fastify";
 import * as oidc from "openid-client";
 import { env } from "../config.js";
 import { db, schema } from "../db/index.js";
 import { auditFromRequest, sanitizeAuditInput } from "../lib/audit.js";
+import { resolveExternalUser, sanitizeUsername } from "../lib/external-auth-resolver.js";
 import { createSessionToken } from "./auth.js";
 
 // ── Types ─────────────────────────────────────────────────────────
@@ -85,45 +84,6 @@ function deriveUsername(claims: Record<string, unknown>): string {
   return claims.sub as string;
 }
 
-function sanitizeUsername(raw: string): string {
-  let sanitized = raw
-    .toLowerCase()
-    .replace(/[^a-z0-9_.-]/g, "_")
-    .replace(/_{2,}/g, "_")
-    .replace(/^[_.-]+|[_.-]+$/g, "");
-
-  // Enforce 3-50 char limit (truncate to 46 to leave room for collision suffix)
-  if (sanitized.length > 46) {
-    sanitized = sanitized.slice(0, 46);
-  }
-  if (sanitized.length < 3) {
-    sanitized = sanitized.padEnd(3, "_");
-  }
-
-  return sanitized;
-}
-
-async function findUniqueUsername(base: string): Promise<string> {
-  const [existing] = await db
-    .select({ username: schema.users.username })
-    .from(schema.users)
-    .where(eq(schema.users.username, base));
-
-  if (!existing) return base;
-
-  for (let i = 2; i <= 1000; i++) {
-    const candidate = `${base}_${i}`;
-    const [taken] = await db
-      .select({ username: schema.users.username })
-      .from(schema.users)
-      .where(eq(schema.users.username, candidate));
-    if (!taken) return candidate;
-  }
-
-  // Extremely unlikely fallback
-  return `${base}_${Date.now()}`;
-}
-
 // ── Helpers ───────────────────────────────────────────────────────
 
 function isSecure(): boolean {
@@ -275,124 +235,47 @@ export async function oidcRoutes(app: FastifyInstance): Promise<void> {
     const email = typeof claims.email === "string" ? claims.email : undefined;
     const emailVerified = claims.email_verified === true;
     const rawUsername = deriveUsername(claims as Record<string, unknown>);
-    const username = sanitizeUsername(rawUsername);
+    const derivedUsername = sanitizeUsername(rawUsername);
     const idToken = tokenResponse.id_token ?? null;
 
-    // 4. User resolution
-    let userId: string | null = null;
+    // 4. User resolution (delegated to shared resolver)
+    const result = await resolveExternalUser({
+      provider: "oidc",
+      externalId: sub,
+      email,
+      emailVerified,
+      username: derivedUsername,
+      autoCreate: env.OIDC_AUTO_CREATE_USERS,
+      autoLink: env.OIDC_AUTO_LINK_USERS,
+      defaultRole: env.OIDC_DEFAULT_ROLE,
+      logger: request.log,
+      ip: request.ip,
+      requestId: request.id,
+    });
 
-    // 4a. Find by externalId (OIDC subject)
-    const [existingByExtId] = await db
-      .select()
-      .from(schema.users)
-      .where(eq(schema.users.externalId, sub))
-      .limit(1);
-
-    if (existingByExtId) {
-      userId = existingByExtId.id;
-      // Update email if changed
-      if (email && email !== existingByExtId.email) {
-        await db
-          .update(schema.users)
-          .set({ email, updatedAt: new Date() })
-          .where(eq(schema.users.id, existingByExtId.id));
+    if (result.action === "denied" || !result.user) {
+      if (result.deniedReason === "user_limit_reached") {
+        return redirectToLogin(reply, "oidc_user_limit_reached");
       }
-    }
-
-    // 4b. Auto-link: match by email
-    if (!userId && env.OIDC_AUTO_LINK_USERS && email && emailVerified) {
-      const [existingByEmail] = await db
-        .select()
-        .from(schema.users)
-        .where(eq(schema.users.email, email))
-        .limit(1);
-
-      if (existingByEmail) {
-        await db
-          .update(schema.users)
-          .set({
-            externalId: sub,
-            updatedAt: new Date(),
-          })
-          .where(eq(schema.users.id, existingByEmail.id));
-        userId = existingByEmail.id;
-        await audit("OIDC_USER_LINKED", {
-          userId: existingByEmail.id,
-          username: existingByEmail.username,
-          email,
-        });
-      }
-    }
-
-    // 4c. Auto-create
-    if (!userId && env.OIDC_AUTO_CREATE_USERS) {
-      // Check user limit
-      if (env.MAX_USERS > 0) {
-        const [countResult] = await db.select({ count: sql<number>`COUNT(*)` }).from(schema.users);
-        if (countResult && countResult.count >= env.MAX_USERS) {
-          request.log.warn("OIDC auto-create blocked: user limit reached");
-          return redirectToLogin(reply, "oidc_user_limit_reached");
-        }
-      }
-
-      const uniqueUsername = await findUniqueUsername(username);
-      const newUserId = randomUUID();
-
-      // Look up the default team
-      const [defaultTeam] = await db
-        .select()
-        .from(schema.teams)
-        .where(eq(schema.teams.name, "Default"));
-      const teamId = defaultTeam?.id ?? "default-team-00000000";
-
-      await db.insert(schema.users).values({
-        id: newUserId,
-        username: uniqueUsername,
-        passwordHash: null,
-        role: env.OIDC_DEFAULT_ROLE,
-        team: teamId,
-        mustChangePassword: false,
-        authProvider: "oidc",
-        externalId: sub,
-        email: email ?? null,
-      });
-
-      userId = newUserId;
-      await audit("OIDC_USER_CREATED", {
-        userId: newUserId,
-        username: uniqueUsername,
-        email,
-        role: env.OIDC_DEFAULT_ROLE,
-      });
-    }
-
-    // 4d. No user found and no auto-create
-    if (!userId) {
-      request.log.warn({ sub, email }, "OIDC user not authorized");
-      await audit("OIDC_LOGIN_FAILED", {
-        reason: "user_not_authorized",
-        sub: sanitizeAuditInput(String(sub)),
-      });
       return redirectToLogin(reply, "oidc_user_not_authorized");
     }
 
+    const resolvedUser = result.user;
+
     // 5. Create session
     const token = createSessionToken();
     const expiresAt = new Date(Date.now() + SESSION_DURATION_MS);
 
     await db.insert(schema.sessions).values({
       id: token,
-      userId,
+      userId: resolvedUser.id,
       expiresAt,
       idToken,
     });
 
-    // Fetch the user for audit logging
-    const [user] = await db.select().from(schema.users).where(eq(schema.users.id, userId));
-
     await audit("OIDC_LOGIN_SUCCESS", {
-      userId,
-      username: user?.username ?? username,
+      userId: resolvedUser.id,
+      username: resolvedUser.username,
     });
 
     // 6. Set session cookie
diff --git a/tests/unit/api/external-auth-resolver.test.ts b/tests/unit/api/external-auth-resolver.test.ts
new file mode 100644
index 00000000..19d3e815
--- /dev/null
+++ b/tests/unit/api/external-auth-resolver.test.ts
@@ -0,0 +1,72 @@
+import { describe, expect, it } from "vitest";
+
+// ── Pure function tests (no DB required) ─────────────────────────
+
+describe("external auth resolver", () => {
+  it("module exports resolveExternalUser", async () => {
+    const mod = await import("../../../apps/api/src/lib/external-auth-resolver.js");
+    expect(typeof mod.resolveExternalUser).toBe("function");
+  });
+
+  it("module exports sanitizeUsername", async () => {
+    const mod = await import("../../../apps/api/src/lib/external-auth-resolver.js");
+    expect(typeof mod.sanitizeUsername).toBe("function");
+  });
+
+  it("module exports findUniqueUsername", async () => {
+    const mod = await import("../../../apps/api/src/lib/external-auth-resolver.js");
+    expect(typeof mod.findUniqueUsername).toBe("function");
+  });
+});
+
+describe("sanitizeUsername", () => {
+  let sanitizeUsername: (raw: string) => string;
+
+  beforeAll(async () => {
+    const mod = await import("../../../apps/api/src/lib/external-auth-resolver.js");
+    sanitizeUsername = mod.sanitizeUsername;
+  });
+
+  it("lowercases input", () => {
+    expect(sanitizeUsername("JohnDoe")).toBe("johndoe");
+  });
+
+  it("replaces non-alphanumeric characters with underscores", () => {
+    expect(sanitizeUsername("john doe!")).toBe("john_doe");
+  });
+
+  it("collapses multiple underscores", () => {
+    expect(sanitizeUsername("john___doe")).toBe("john_doe");
+  });
+
+  it("strips leading and trailing separators", () => {
+    expect(sanitizeUsername("_john_")).toBe("john");
+    expect(sanitizeUsername(".john.")).toBe("john");
+    expect(sanitizeUsername("-john-")).toBe("john");
+  });
+
+  it("truncates to 46 characters", () => {
+    const long = "a".repeat(60);
+    expect(sanitizeUsername(long).length).toBe(46);
+  });
+
+  it("pads short usernames to 3 characters", () => {
+    expect(sanitizeUsername("ab").length).toBe(3);
+    expect(sanitizeUsername("ab")).toBe("ab_");
+  });
+
+  it("preserves dots and hyphens", () => {
+    expect(sanitizeUsername("john.doe")).toBe("john.doe");
+    expect(sanitizeUsername("john-doe")).toBe("john-doe");
+  });
+
+  it("handles email addresses as input", () => {
+    expect(sanitizeUsername("user@example.com")).toBe("user_example.com");
+  });
+
+  it("handles empty-after-strip edge case", () => {
+    // All characters stripped, then padded
+    const result = sanitizeUsername("___");
+    expect(result.length).toBeGreaterThanOrEqual(3);
+  });
+});