diff --git a/pkg/atproto/server_repo.go b/pkg/atproto/server_repo.go
index 725c8392d..287de1945 100644
--- a/pkg/atproto/server_repo.go
+++ b/pkg/atproto/server_repo.go
@@ -6,6 +6,8 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"sort"
+	"strings"
 	"sync"
 	"time"
 
@@ -406,6 +408,40 @@ func ServerRepoMerkleProof(ctx context.Context, collection string, rkey string)
 	return buf.Bytes(), nil
 }
 
+// ServerRepoListCollections walks the server repo's MST and returns
+// the distinct collection NSIDs currently holding at least one record.
+// Used by com.atproto.repo.describeRepo to advertise what's actually
+// in the repo rather than a hardcoded list. Returned collections are
+// sorted lexicographically for stable output.
+func ServerRepoListCollections(ctx context.Context) ([]string, error) {
+	serverRepoLock.Lock()
+	defer serverRepoLock.Unlock()
+
+	r, _, err := OpenServerRepo(ctx)
+	if err != nil {
+		return nil, fmt.Errorf("ServerRepoListCollections: failed to open repo: %w", err)
+	}
+	seen := map[string]struct{}{}
+	err = r.ForEach(ctx, "", func(rpath string, _ cid.Cid) error {
+		// rpath is "<collection>/<rkey>"; pull the prefix.
+		slash := strings.IndexByte(rpath, '/')
+		if slash <= 0 {
+			return nil
+		}
+		seen[rpath[:slash]] = struct{}{}
+		return nil
+	})
+	if err != nil {
+		return nil, fmt.Errorf("ServerRepoListCollections: error iterating records: %w", err)
+	}
+	out := make([]string, 0, len(seen))
+	for c := range seen {
+		out = append(out, c)
+	}
+	sort.Strings(out)
+	return out, nil
+}
+
 func ServerRepoListRecords(ctx context.Context, collection string, cursor string, limit int, repo string, reverse *bool) (*comatproto.RepoListRecords_Output, error) {
 	serverRepoLock.Lock()
 	defer serverRepoLock.Unlock()
diff --git a/pkg/atproto/server_repo_test.go b/pkg/atproto/server_repo_test.go
index 975f818c9..0b14a846c 100644
--- a/pkg/atproto/server_repo_test.go
+++ b/pkg/atproto/server_repo_test.go
@@ -66,6 +66,11 @@ func TestServerRepo(t *testing.T) {
 	require.NoError(t, err)
 	require.Len(t, listOut.Records, 1)
 
+	// ListCollections should report the collection we just wrote.
+	cols, err := ServerRepoListCollections(context.Background())
+	require.NoError(t, err)
+	require.Equal(t, []string{constants.PLACE_STREAM_LIVE_VIEWERCOUNT}, cols)
+
 	// Merkle proof
 	proof, err := ServerRepoMerkleProof(context.Background(), constants.PLACE_STREAM_LIVE_VIEWERCOUNT, "did:plc:abc123")
 	require.NoError(t, err)
@@ -93,5 +98,22 @@ func TestServerRepo(t *testing.T) {
 	require.NoError(t, err)
 	require.NotNil(t, out)
 
+	// After writing a record in a second collection, ListCollections
+	// should pick both up in sorted order.
+	origin := &streamplace.MediaOrigin{
+		LexiconTypeID: constants.PLACE_STREAM_MEDIA_ORIGIN,
+		Blob:          "babczxv...",
+		Size:          1234,
+		MimeType:      "video/mp4",
+	}
+	err = CommitServerRepoRecord(context.Background(), &cli, constants.PLACE_STREAM_MEDIA_ORIGIN, "babczxv1", origin)
+	require.NoError(t, err)
+	cols, err = ServerRepoListCollections(context.Background())
+	require.NoError(t, err)
+	require.Equal(t, []string{
+		constants.PLACE_STREAM_LIVE_VIEWERCOUNT,
+		constants.PLACE_STREAM_MEDIA_ORIGIN,
+	}, cols)
+
 	handle.Close()
 }
diff --git a/pkg/spxrpc/com_atproto_repo.go b/pkg/spxrpc/com_atproto_repo.go
index 5969cca41..e61c330b4 100644
--- a/pkg/spxrpc/com_atproto_repo.go
+++ b/pkg/spxrpc/com_atproto_repo.go
@@ -17,7 +17,6 @@ import (
 	"go.opentelemetry.io/otel"
 	"stream.place/streamplace/pkg/aqhttp"
 	"stream.place/streamplace/pkg/atproto"
-	"stream.place/streamplace/pkg/constants"
 	"stream.place/streamplace/pkg/log"
 )
 
@@ -92,13 +91,15 @@ func (s *Server) handleComAtprotoRepoDescribeRepo(ctx context.Context, repo stri
 	}
 
 	if s.isServerPDS(ctx) {
+		collections, err := atproto.ServerRepoListCollections(ctx)
+		if err != nil {
+			return nil, fmt.Errorf("list server repo collections: %w", err)
+		}
 		return &comatproto.RepoDescribeRepo_Output{
-			Handle: s.cli.ServerDID(),
-			Did:    s.cli.ServerDID(),
-			DidDoc: atproto.DIDDoc(s.cli.ServerHost, atproto.ServerPubMultibase),
-			Collections: []string{
-				constants.PLACE_STREAM_LIVE_VIEWERCOUNT,
-			},
+			Handle:          s.cli.ServerDID(),
+			Did:             s.cli.ServerDID(),
+			DidDoc:          atproto.DIDDoc(s.cli.ServerHost, atproto.ServerPubMultibase),
+			Collections:     collections,
 			HandleIsCorrect: true,
 		}, nil
 	}