diff --git a/cli/command_blob_list.go b/cli/command_blob_list.go
index fea6ffcac..0e460fcd1 100644
--- a/cli/command_blob_list.go
+++ b/cli/command_blob_list.go
@@ -57,7 +57,7 @@ func (c *commandBlobList) run(ctx context.Context, rep repo.DirectRepository) er
 
 func (c *commandBlobList) shouldInclude(b blob.Metadata) bool {
 	if c.dataOnly {
-		if strings.HasPrefix(string(b.BlobID), content.IndexBlobPrefix) {
+		if strings.HasPrefix(string(b.BlobID), content.LegacyIndexBlobPrefix) {
 			return false
 		}
 
diff --git a/cli/command_index_recover.go b/cli/command_index_recover.go
index 41c886bf0..16ec4b27a 100644
--- a/cli/command_index_recover.go
+++ b/cli/command_index_recover.go
@@ -58,7 +58,7 @@ func (c *commandIndexRecover) run(ctx context.Context, rep repo.DirectRepository
 	}()
 
 	if c.deleteIndexes {
-		if err := rep.BlobReader().ListBlobs(ctx, content.IndexBlobPrefix, func(bm blob.Metadata) error {
+		if err := rep.BlobReader().ListBlobs(ctx, content.LegacyIndexBlobPrefix, func(bm blob.Metadata) error {
 			if c.commit {
 				log(ctx).Infof("deleting old index blob: %v", bm.BlobID)
 				return errors.Wrap(rep.BlobStorage().DeleteBlob(ctx, bm.BlobID), "error deleting index blob")
diff --git a/cli/command_repository_set_parameters.go b/cli/command_repository_set_parameters.go
index 435eb4aeb..ca940bae6 100644
--- a/cli/command_repository_set_parameters.go
+++ b/cli/command_repository_set_parameters.go
@@ -172,6 +172,12 @@ func (c *commandRepositorySetParameters) run(ctx context.Context, rep repo.Direc
 		return errors.Wrap(err, "error setting parameters")
 	}
 
+	if upgradeToEpochManager {
+		if err := content.WriteLegacyIndexPoisonBlob(ctx, rep.BlobStorage()); err != nil {
+			log(ctx).Errorf("unable to write legacy index poison blob: %v", err)
+		}
+	}
+
 	log(ctx).Infof("NOTE: Repository parameters updated, you must disconnect and re-connect all other Kopia clients.")
 
 	return nil
diff --git a/cli/command_repository_upgrade.go b/cli/command_repository_upgrade.go
index 49c7779b6..10ab2a892 100644
--- a/cli/command_repository_upgrade.go
+++ b/cli/command_repository_upgrade.go
@@ -269,6 +269,12 @@ func (c *commandRepositoryUpgrade) upgrade(ctx context.Context, rep repo.DirectR
 	if err := rep.SetParameters(ctx, mp, rep.BlobCfg()); err != nil {
 		return errors.Wrap(err, "error setting parameters")
 	}
+
+	// poison V0 index so that old readers won't be able to open it.
+	if err := content.WriteLegacyIndexPoisonBlob(ctx, rep.BlobStorage()); err != nil {
+		log(ctx).Errorf("unable to write legacy index poison blob: %v", err)
+	}
+
 	// we need to reopen the repository after this point
 
 	log(ctx).Infof("Repository indices have been upgraded.")
diff --git a/repo/content/committed_read_manager.go b/repo/content/committed_read_manager.go
index 188d776fa..9e8d9337f 100644
--- a/repo/content/committed_read_manager.go
+++ b/repo/content/committed_read_manager.go
@@ -42,7 +42,7 @@ const (
 
 // nolint:gochecknoglobals
 var cachedIndexBlobPrefixes = []blob.ID{
-	IndexBlobPrefix,
+	LegacyIndexBlobPrefix,
 	compactionLogBlobPrefix,
 	cleanupBlobPrefix,
 
@@ -54,7 +54,7 @@ var cachedIndexBlobPrefixes = []blob.ID{
 
 // nolint:gochecknoglobals
 var allIndexBlobPrefixes = []blob.ID{
-	IndexBlobPrefix,
+	LegacyIndexBlobPrefix,
 	epoch.UncompactedIndexBlobPrefix,
 	epoch.SingleEpochCompactionBlobPrefix,
 	epoch.RangeCheckpointIndexBlobPrefix,
diff --git a/repo/content/content_index_recovery_test.go b/repo/content/content_index_recovery_test.go
index c52769052..accb3c775 100644
--- a/repo/content/content_index_recovery_test.go
+++ b/repo/content/content_index_recovery_test.go
@@ -28,7 +28,7 @@ func (s *contentManagerSuite) TestContentIndexRecovery(t *testing.T) {
 	}
 
 	// delete all index blobs
-	require.NoError(t, bm.st.ListBlobs(ctx, IndexBlobPrefix, func(bi blob.Metadata) error {
+	require.NoError(t, bm.st.ListBlobs(ctx, LegacyIndexBlobPrefix, func(bi blob.Metadata) error {
 		t.Logf("deleting %v", bi.BlobID)
 		return bm.st.DeleteBlob(ctx, bi.BlobID)
 	}))
diff --git a/repo/content/content_manager.go b/repo/content/content_manager.go
index 26e7208a6..ce3af3847 100644
--- a/repo/content/content_manager.go
+++ b/repo/content/content_manager.go
@@ -50,9 +50,6 @@ var PackBlobIDPrefixes = []blob.ID{
 	PackBlobIDPrefixSpecial,
 }
 
-// IndexBlobPrefix is the prefix for all index blobs.
-const IndexBlobPrefix = "n"
-
 const (
 	parallelFetches          = 5                // number of parallel reads goroutines
 	flushPackIndexTimeout    = 10 * time.Minute // time after which all pending indexes are flushes
diff --git a/repo/content/content_manager_test.go b/repo/content/content_manager_test.go
index 945420770..c45bd7471 100644
--- a/repo/content/content_manager_test.go
+++ b/repo/content/content_manager_test.go
@@ -550,7 +550,7 @@ func validateIndexCount(t *testing.T, data map[blob.ID][]byte, wantIndexCount, w
 	var indexCnt, compactionLogCnt int
 
 	for blobID := range data {
-		if strings.HasPrefix(string(blobID), IndexBlobPrefix) || strings.HasPrefix(string(blobID), "x") {
+		if strings.HasPrefix(string(blobID), LegacyIndexBlobPrefix) || strings.HasPrefix(string(blobID), "x") {
 			indexCnt++
 		}
 
@@ -1202,7 +1202,7 @@ func (s *contentManagerSuite) TestFlushWaitsForAllPendingWriters(t *testing.T) {
 	bm.Flush(ctx)
 	t.Logf("<<< end of flushing")
 
-	indexBlobPrefix := blob.ID(IndexBlobPrefix)
+	indexBlobPrefix := blob.ID(LegacyIndexBlobPrefix)
 	if s.mutableParameters.EpochParameters.Enabled {
 		indexBlobPrefix = "x"
 	}
diff --git a/repo/content/index_blob_manager_v0.go b/repo/content/index_blob_manager_v0.go
index b817e95d4..a73d52bbe 100644
--- a/repo/content/index_blob_manager_v0.go
+++ b/repo/content/index_blob_manager_v0.go
@@ -14,7 +14,12 @@ import (
 	"github.com/kopia/kopia/repo/logging"
 )
 
+// LegacyIndexBlobPrefix is the prefix for all legacy (v0) index blobs.
+const LegacyIndexBlobPrefix = "n"
+
 const (
+	legacyIndexPoisonBlobID = "n00000000000000000000000000000000-repository_unreadable_by_this_kopia_version_upgrade_required"
+
 	defaultEventualConsistencySettleTime = 1 * time.Hour
 	compactionLogBlobPrefix              = "m"
 	cleanupBlobPrefix                    = "l"
@@ -75,7 +80,7 @@ func (m *indexBlobManagerV0) listActiveIndexBlobs(ctx context.Context) ([]IndexB
 	})
 
 	eg.Go(func() error {
-		v, err := blob.ListAllBlobs(ctx, m.st, IndexBlobPrefix)
+		v, err := blob.ListAllBlobs(ctx, m.st, LegacyIndexBlobPrefix)
 		storageIndexBlobs = v
 
 		return errors.Wrap(err, "error listing index blobs")
@@ -183,7 +188,7 @@ func (m *indexBlobManagerV0) writeIndexBlobs(ctx context.Context, dataShards []g
 	var result []blob.Metadata
 
 	for _, data := range dataShards {
-		bm, err := m.enc.encryptAndWriteBlob(ctx, data, IndexBlobPrefix, sessionID)
+		bm, err := m.enc.encryptAndWriteBlob(ctx, data, LegacyIndexBlobPrefix, sessionID)
 		if err != nil {
 			return nil, errors.Wrap(err, "error writing index blbo")
 		}
@@ -517,6 +522,18 @@ func (m *indexBlobManagerV0) dropContentsFromBuilder(bld index.Builder, opt Comp
 	}
 }
 
+// WriteLegacyIndexPoisonBlob writes a "poison blob" that will prevent old kopia clients
+// that have not been upgraded from being able to open the repository after its format
+// has been upgraded.
+func WriteLegacyIndexPoisonBlob(ctx context.Context, st blob.Storage) error {
+	// nolint:wrapcheck
+	return st.PutBlob(
+		ctx,
+		legacyIndexPoisonBlobID,
+		gather.FromSlice([]byte("The format of this repository has been upgraded and cannot be read by old clients")),
+		blob.PutOptions{})
+}
+
 func addIndexBlobsToBuilder(ctx context.Context, enc *encryptedBlobMgr, bld index.Builder, indexBlobID blob.ID) error {
 	var data gather.WriteBuffer
 	defer data.Close()
diff --git a/repo/content/index_blob_manager_v0_test.go b/repo/content/index_blob_manager_v0_test.go
index 99e30686b..131e8fece 100644
--- a/repo/content/index_blob_manager_v0_test.go
+++ b/repo/content/index_blob_manager_v0_test.go
@@ -699,7 +699,7 @@ func getAllFakeContentsInternal(ctx context.Context, t *testing.T, m *indexBlobM
 func assertBlobCounts(t *testing.T, data blobtesting.DataMap, wantN, wantM, wantL int) {
 	t.Helper()
 	require.Len(t, keysWithPrefix(data, compactionLogBlobPrefix), wantM)
-	require.Len(t, keysWithPrefix(data, IndexBlobPrefix), wantN)
+	require.Len(t, keysWithPrefix(data, LegacyIndexBlobPrefix), wantN)
 	require.Len(t, keysWithPrefix(data, "l"), wantL)
 }
 
@@ -783,7 +783,7 @@ func newIndexBlobManagerForTesting(t *testing.T, st blob.Storage, localTimeNow f
 	st = ownwrites.NewWrapper(
 		st,
 		blobtesting.NewMapStorage(blobtesting.DataMap{}, nil, nil),
-		[]blob.ID{IndexBlobPrefix, compactionLogBlobPrefix, cleanupBlobPrefix},
+		[]blob.ID{LegacyIndexBlobPrefix, compactionLogBlobPrefix, cleanupBlobPrefix},
 		15*time.Minute,
 	)
 
diff --git a/repo/open.go b/repo/open.go
index 010e1e446..8b876644d 100644
--- a/repo/open.go
+++ b/repo/open.go
@@ -389,7 +389,7 @@ func wrapLockingStorage(st blob.Storage, r content.BlobCfgBlob) blob.Storage {
 		prefixes = append(prefixes, string(prefix))
 	}
 
-	prefixes = append(prefixes, content.IndexBlobPrefix, epoch.EpochManagerIndexUberPrefix, FormatBlobID,
+	prefixes = append(prefixes, content.LegacyIndexBlobPrefix, epoch.EpochManagerIndexUberPrefix, FormatBlobID,
 		BlobCfgBlobID)
 
 	return beforeop.NewWrapper(st, nil, nil, nil, func(ctx context.Context, id blob.ID, opts *blob.PutOptions) error {
diff --git a/repo/repository_test.go b/repo/repository_test.go
index fcce64cea..26bcf5537 100644
--- a/repo/repository_test.go
+++ b/repo/repository_test.go
@@ -565,7 +565,7 @@ func TestObjectWritesWithRetention(t *testing.T) {
 		prefixesWithRetention = append(prefixesWithRetention, string(prefix))
 	}
 
-	prefixesWithRetention = append(prefixesWithRetention, content.IndexBlobPrefix, epoch.EpochManagerIndexUberPrefix,
+	prefixesWithRetention = append(prefixesWithRetention, content.LegacyIndexBlobPrefix, epoch.EpochManagerIndexUberPrefix,
 		repo.FormatBlobID, repo.BlobCfgBlobID)
 
 	// make sure that we cannot set mtime on the kopia objects created due to the
diff --git a/tests/compat_test/compat_test.go b/tests/compat_test/compat_test.go
index 7a28de244..6d355cb68 100644
--- a/tests/compat_test/compat_test.go
+++ b/tests/compat_test/compat_test.go
@@ -44,9 +44,13 @@ func TestRepoCreatedWith08CanBeOpenedWithCurrent(t *testing.T) {
 		"--io-drain-timeout", "1s", "--allow-unsafe-upgrade",
 		"--status-poll-interval", "1s")
 
-	// now 0.8 can't open it anymore
+	// now 0.8 client can't open it anymore because they won't understand format V2
 	e3 := testenv.NewCLITest(t, testenv.RepoFormatNotImportant, runner08)
 	e3.RunAndExpectFailure(t, "repo", "connect", "filesystem", "--path", e1.RepoDir)
+
+	// old 0.8 client who has cached the format blob and never disconnected
+	// can't open the repository because of the poison blob
+	e1.RunAndExpectFailure(t, "snap", "ls")
 }
 
 func TestRepoCreatedWithCurrentWithFormatVersion1CanBeOpenedWith08(t *testing.T) {