diff --git a/src/server/routes/route-utils.ts b/src/server/routes/route-utils.ts
index f234e10..a4edefa 100644
--- a/src/server/routes/route-utils.ts
+++ b/src/server/routes/route-utils.ts
@@ -264,11 +264,15 @@ export function handleError(
 
 export async function getUser(req: express.Request) {
   function notConnected(): never {
-    req.logout((error) => {
-      if (error) {
-        logger.error("logout failed", serializeError(error));
-      }
-    });
+    // Anonymous sessions can hold an in-progress OAuth flow. Passport's
+    // logout regenerates the session, so only clear an invalid identity.
+    if (req.user) {
+      req.logout((error) => {
+        if (error) {
+          logger.error("logout failed", serializeError(error));
+        }
+      });
+    }
     throw new AnonymousError("not_connected", {
       httpStatus: 401,
     });
diff --git a/test/oauth-session-preservation.test.js b/test/oauth-session-preservation.test.js
new file mode 100644
index 0000000..409dbbd
--- /dev/null
+++ b/test/oauth-session-preservation.test.js
@@ -0,0 +1,81 @@
+/* global fetch */
+const { expect } = require("chai");
+const express = require("express");
+const session = require("express-session");
+const { Passport } = require("passport");
+require("ts-node/register/transpile-only");
+const { getUser } = require("../src/server/routes/route-utils");
+
+describe("anonymous OAuth session preservation", function () {
+  let server;
+  let base;
+
+  before(async function () {
+    const app = express();
+    const passport = new Passport();
+    app.use(session({ secret: "test-session-secret", resave: false, saveUninitialized: false }));
+    app.use(passport.initialize());
+    app.use(passport.session());
+    app.get("/start", (req, res) => {
+      req.session["oauth2:github.com"] = { state: "legacy-state" };
+      req.session.githubAppFlow = { state: "app-state" };
+      res.json({ id: req.sessionID });
+    });
+    app.get("/callback", async (req, res) => {
+      const pendingLogouts = [];
+      const logout = req.logout.bind(req);
+      req.logout = done => {
+        pendingLogouts.push(new Promise((resolve, reject) => {
+          logout(error => {
+            done(error);
+            if (error) reject(error);
+            else resolve();
+          });
+        }));
+      };
+      // Both the rate limiter and anonymous API requests check getUser.
+      for (let i = 0; i < 2; i++) {
+        try {
+          await getUser(req);
+          return res.status(500).json({ error: "anonymous-user-authenticated" });
+        } catch (error) {
+          if (error.message !== "not_connected") throw error;
+        }
+      }
+      await Promise.all(pendingLogouts);
+      res.json({ id: req.sessionID, legacy: req.session["oauth2:github.com"], app: req.session.githubAppFlow });
+    });
+    await new Promise(resolve => { server = app.listen(0, "127.0.0.1", resolve); });
+    base = `http://127.0.0.1:${server.address().port}`;
+  });
+
+  after(async function () {
+    server.closeAllConnections();
+    await new Promise(resolve => server.close(resolve));
+  });
+
+  it("retains legacy and App state when checking an anonymous user", async function () {
+    const start = await fetch(`${base}/start`);
+    const cookie = start.headers.get("set-cookie").split(";")[0];
+    const initial = await start.json();
+    const callback = await fetch(`${base}/callback`, { headers: { Cookie: cookie } });
+    expect(callback.status).to.equal(200);
+    expect(await callback.json()).to.deep.equal({
+      id: initial.id,
+      legacy: { state: "legacy-state" },
+      app: { state: "app-state" },
+    });
+  });
+
+  it("still logs out an invalid authenticated identity", async function () {
+    let loggedOut = false;
+    try {
+      await getUser({ user: {}, logout(done) { loggedOut = true; done(); } });
+      throw new Error("Expected authentication rejection");
+    } catch (error) {
+      expect(error.message).to.equal("not_connected");
+      expect(error.httpStatus).to.equal(401);
+    }
+    expect(loggedOut).to.equal(true);
+  });
+});