diff --git a/go/lib/session/chunked-iron-session.ts b/go/lib/session/chunked-iron-session.ts
new file mode 100644
index 0000000000..8a4ae0f419
--- /dev/null
+++ b/go/lib/session/chunked-iron-session.ts
@@ -0,0 +1,147 @@
+import {
+  IronSession,
+  SessionOptions,
+  sealData,
+  unsealData,
+} from "iron-session"
+
+// Minimal subset of next/headers' cookie store that we depend on.
+interface CookieStore {
+  get(name: string): { name: string; value: string } | undefined
+  set(name: string, value: string, options?: Record<string, unknown>): void
+}
+
+// Browsers reject Set-Cookie payloads larger than ~4096 bytes. We size each
+// chunk well below that to leave headroom for the cookie name and attributes.
+const DEFAULT_CHUNK_SIZE = 3500
+const MAX_COOKIE_SIZE = 4096
+
+const chunkName = (cookieName: string, index: number) => `${cookieName}.${index}`
+
+const readSealFromChunks = (cookieStore: CookieStore, cookieName: string): string => {
+  // Sessions that fit in a single cookie keep the original name, so prefer it.
+  const direct = cookieStore.get(cookieName)?.value
+  if (direct) return direct
+
+  const chunks: string[] = []
+  for (let i = 0; ; i++) {
+    const value = cookieStore.get(chunkName(cookieName, i))?.value
+    if (!value) break
+    chunks.push(value)
+  }
+  return chunks.join("")
+}
+
+const clearChunks = (
+  cookieStore: CookieStore,
+  cookieName: string,
+  cookieOptions: Record<string, unknown>
+) => {
+  for (let i = 0; ; i++) {
+    const name = chunkName(cookieName, i)
+    if (!cookieStore.get(name)?.value) break
+    cookieStore.set(name, "", { ...cookieOptions, maxAge: 0 })
+  }
+}
+
+const splitIntoChunks = (value: string, chunkSize: number): string[] => {
+  const chunks: string[] = []
+  for (let i = 0; i < value.length; i += chunkSize) {
+    chunks.push(value.slice(i, i + chunkSize))
+  }
+  return chunks
+}
+
+export interface ChunkingOptions {
+  chunkSize?: number
+}
+
+/**
+ * Drop-in replacement for iron-session's `getIronSession(cookieStore, …)` that
+ * transparently splits oversized seals across multiple cookies
+ * (`{cookieName}.0`, `{cookieName}.1`, …) and reassembles them on read.
+ *
+ * Mirrors PR vvo/iron-session#937 until upstream merges.
+ */
+export async function getChunkedIronSession<T extends object>(
+  cookieStore: CookieStore,
+  sessionOptions: SessionOptions,
+  chunking: ChunkingOptions = {}
+): Promise<IronSession<T>> {
+  const ttl = sessionOptions.ttl ?? 60 * 60 * 24 * 14
+  // Match iron-session: cookie expires 60s before the seal so the browser
+  // never sends a cookie we'd reject as expired.
+  const cookieOptions: Record<string, unknown> = {
+    httpOnly: true,
+    secure: true,
+    sameSite: "lax",
+    path: "/",
+    ...sessionOptions.cookieOptions,
+    ...(ttl === 0 ? {} : { maxAge: ttl - 60 }),
+  }
+  const chunkSize = chunking.chunkSize ?? DEFAULT_CHUNK_SIZE
+
+  const seal = readSealFromChunks(cookieStore, sessionOptions.cookieName)
+  let data: T = {} as T
+  if (seal) {
+    try {
+      data = await unsealData<T>(seal, {
+        password: sessionOptions.password,
+        ttl,
+      })
+    } catch {
+      // Corrupt/expired seal — start with an empty session, same as iron-session.
+      data = {} as T
+    }
+  }
+
+  const save = async () => {
+    const newSeal = await sealData(data, {
+      password: sessionOptions.password,
+      ttl,
+    })
+
+    // Always purge any existing chunks before deciding the new layout, so we
+    // never leak stale chunks across a chunked→single (or larger→smaller)
+    // transition.
+    clearChunks(cookieStore, sessionOptions.cookieName, cookieOptions)
+
+    const singleCookieSize =
+      sessionOptions.cookieName.length +
+      newSeal.length +
+      JSON.stringify(cookieOptions).length
+
+    if (singleCookieSize <= MAX_COOKIE_SIZE) {
+      cookieStore.set(sessionOptions.cookieName, newSeal, cookieOptions)
+      return
+    }
+
+    // Seal won't fit in one cookie — clear the main cookie and write chunks.
+    cookieStore.set(sessionOptions.cookieName, "", { ...cookieOptions, maxAge: 0 })
+    splitIntoChunks(newSeal, chunkSize).forEach((chunk, index) => {
+      cookieStore.set(chunkName(sessionOptions.cookieName, index), chunk, cookieOptions)
+    })
+  }
+
+  const destroy = () => {
+    for (const key of Object.keys(data)) {
+      delete (data as Record<string, unknown>)[key]
+    }
+    cookieStore.set(sessionOptions.cookieName, "", { ...cookieOptions, maxAge: 0 })
+    clearChunks(cookieStore, sessionOptions.cookieName, cookieOptions)
+  }
+
+  // Methods are non-enumerable so JSON.stringify/Object.assign/sealData
+  // don't accidentally serialize them into the cookie.
+  Object.defineProperties(data, {
+    save: { value: save },
+    destroy: { value: destroy },
+    updateConfig: {
+      value: () => {
+        // Not used in this codebase; left as a no-op for IronSession parity.
+      },
+    },
+  })
+
+  return data as IronSession<T>
+}
diff --git a/go/lib/session/session.ts b/go/lib/session/session.ts
index 34653a507d..d90412bd94 100644
--- a/go/lib/session/session.ts
+++ b/go/lib/session/session.ts
@@ -1,5 +1,5 @@
 import { add, isPast, sub } from "date-fns"
-import { IronSession, SessionOptions, getIronSession } from "iron-session"
+import { IronSession, SessionOptions } from "iron-session"
 import { unstable_rethrow } from "next/navigation"
 import { NextResponse, connection } from "next/server"
 
@@ -12,6 +12,7 @@ import { isBuildingGoApp } from "../helpers/next-phase"
 import { loadPatronServerSide } from "../helpers/service-layer"
 import { userIsAnonymous } from "../helpers/user"
 import { TSessionType, TUniloginTokenSet } from "../types/session"
+import { getChunkedIronSession } from "./chunked-iron-session"
 
 export const getSessionOptions = (): SessionOptions => {
   const sessionSecret = getServerEnv("GO_SESSION_SECRET")
@@ -76,7 +77,7 @@ export async function getSession(): Promise<IronSession<TSessionData>> {
     const { cookies } = await import("next/headers")
     const cookieStore = await cookies()
     const libraryToken = cookieStore.get(goConfig("library-token.cookie-name"))?.value
-    const session = await getIronSession<TSessionData>(cookieStore, sessionOptions)
+    const session = await getChunkedIronSession<TSessionData>(cookieStore, sessionOptions)
 
     if (!session?.isLoggedIn) {
       // Return the default session if the session is not logged in.