diff --git a/packages/session-persistence/session-persistence/src/coordinator.ts b/packages/session-persistence/session-persistence/src/coordinator.ts
index beb5fca483..7dc0c9a0ba 100644
--- a/packages/session-persistence/session-persistence/src/coordinator.ts
+++ b/packages/session-persistence/session-persistence/src/coordinator.ts
@@ -125,6 +125,11 @@ function assertSupportedEvents(events: readonly SessionEvent[], id: SessionId):
   if (legacy !== undefined) {
     throw new Error(`session "${id}" contains unsupported legacy request/header-delta event at seq ${legacy.seq}`)
   }
+  const legacyModeType: string = 'mode/set'
+  const legacyMode = events.find(event => event.type === legacyModeType)
+  if (legacyMode !== undefined) {
+    throw new Error(`session "${id}" contains unsupported legacy mode/set event at seq ${legacyMode.seq}`)
+  }
   const fallback = events.find(event => event.type === 'request/header'
     && (event.data as { reason?: string }).reason === 'fallback')
   if (fallback !== undefined) {
diff --git a/packages/session-persistence/session-persistence/tests/persistence.spec.ts b/packages/session-persistence/session-persistence/tests/persistence.spec.ts
index e083ac3543..510ae38136 100644
--- a/packages/session-persistence/session-persistence/tests/persistence.spec.ts
+++ b/packages/session-persistence/session-persistence/tests/persistence.spec.ts
@@ -22,6 +22,16 @@ function legacyHeaderDelta(seq = 0): SessionEvent {
   } as unknown as SessionEvent
 }
 
+/** An unsupported named-mode fixture emulating an untyped producer. */
+function legacyModeSet(seq = 0): SessionEvent {
+  return {
+    type: 'mode/set',
+    seq,
+    time: 1,
+    data: { mode: 'plan' },
+  } as unknown as SessionEvent
+}
+
 /** An obsolete full-header reason fixture from the removed delta codec. */
 function legacyFallbackHeader(seq = 0): SessionEvent {
   return {
@@ -510,6 +520,19 @@ describe('SessionPersistence service registration', () => {
     await fiber.dispose()
   })
 
+  it('rejects a stored legacy named-mode event during load', async () => {
+    const id = SessionId('legacy-mode-load')
+    const m = meta(id, '/legacy')
+    const store: MemoryStore = new Map([[id, { meta: m, events: [legacyModeSet()] }]])
+    const ctx = new Context()
+    await ctx.plugin(SessionStore)
+    const fiber = await ctx.plugin(MemoryPersistence, { store })
+
+    await expect(ctx.sessionPersistence.load(id))
+      .rejects.toThrow('unsupported legacy mode/set event at seq 0')
+    await fiber.dispose()
+  })
+
   it('retires all coordinator bookkeeping for disposed sessions', async () => {
     const ctx = new Context()
     await ctx.plugin(SessionStore)