diff --git a/api/sync15/tree.go b/api/sync15/tree.go
index ff603358..3e58c302 100644
--- a/api/sync15/tree.go
+++ b/api/sync15/tree.go
@@ -153,10 +153,13 @@ func parseIndex(f io.Reader) ([]*Entry, string, error) {
 func (t *HashTree) IndexReader() (io.Reader, error) {
 	var w bytes.Buffer
 
-	schemaVersion := t.SchemaVersion
-	if schemaVersion == "" {
-		schemaVersion = SchemaVersionV3
-	}
+	// t.SchemaVersion records the format read from the server during Mirror().
+	// For writes, always emit v4 root indexes: current servers reject new
+	// v3-format root docSchema uploads even when the v3 HashEntries hash
+	// matches the body. Rehash() always SHA-256s the serialized output, so
+	// RMAPI_FORCE_SCHEMA_VERSION=3 changes the body schema but not the hash
+	// algorithm — use it only to inspect failure modes, not as a working mode.
+	schemaVersion := SchemaVersionV4
 
 	if envSchema := os.Getenv("RMAPI_FORCE_SCHEMA_VERSION"); envSchema != "" {
 		log.Info.Printf("forcing schema version to %s via RMAPI_FORCE_SCHEMA_VERSION", envSchema)
@@ -228,42 +231,19 @@ func (t *HashTree) Remove(id string) error {
 }
 
 func (t *HashTree) Rehash() error {
-	schemaVersion := t.SchemaVersion
-	if schemaVersion == "" {
-		schemaVersion = SchemaVersionV3
+	reader, err := t.IndexReader()
+	if err != nil {
+		return err
 	}
 
-	if envSchema := os.Getenv("RMAPI_FORCE_SCHEMA_VERSION"); envSchema != "" {
-		schemaVersion = envSchema
+	schemaBytes, err := io.ReadAll(reader)
+	if err != nil {
+		return err
 	}
 
-	var hash string
-	var err error
-
-	if schemaVersion == SchemaVersionV3 {
-		entries := []*Entry{}
-		for _, e := range t.Docs {
-			entries = append(entries, &e.Entry)
-		}
-		hash, err = HashEntries(entries)
-		if err != nil {
-			return err
-		}
-	} else {
-		reader, err := t.IndexReader()
-		if err != nil {
-			return err
-		}
-
-		schemaBytes, err := io.ReadAll(reader)
-		if err != nil {
-			return err
-		}
-
-		hasher := sha256.New()
-		hasher.Write(schemaBytes)
-		hash = hex.EncodeToString(hasher.Sum(nil))
-	}
+	hasher := sha256.New()
+	hasher.Write(schemaBytes)
+	hash := hex.EncodeToString(hasher.Sum(nil))
 
 	log.Info.Println("New root hash: ", hash)
 	t.Hash = hash
@@ -300,7 +280,6 @@ func (t *HashTree) Mirror(r RemoteStorage, maxconcurrent int) error {
 		return fmt.Errorf("cannot get root hash %v", err)
 	}
 	defer rootIndexReader.Close()
-
 	entries, schema, err := parseIndex(rootIndexReader)
 	if err != nil {
 		return fmt.Errorf("cannot parse rootIndex, %v", err)
diff --git a/api/sync15/tree_test.go b/api/sync15/tree_test.go
index ea3eba2b..c77a19d4 100644
--- a/api/sync15/tree_test.go
+++ b/api/sync15/tree_test.go
@@ -1,6 +1,8 @@
 package sync15
 
 import (
+	"crypto/sha256"
+	"encoding/hex"
 	"io"
 	"strings"
 	"testing"
@@ -97,6 +99,48 @@ blah:0:someid:0:10
 	}
 }
 
+// TestRootIndexWritesV4WhenMirroredV3 is the regression test for the bug
+// introduced in f5c7b9c: a tree whose SchemaVersion was read as v3 from the
+// server must still produce a v4 root docSchema on write.
+func TestRootIndexWritesV4WhenMirroredV3(t *testing.T) {
+	tree := HashTree{
+		SchemaVersion: SchemaVersionV3, // simulates a tree mirrored from a v3 root
+	}
+	doc := &BlobDoc{
+		Entry: Entry{Hash: "somehash", DocumentID: "someid"},
+	}
+	doc.AddFile(&Entry{Hash: "filehash", DocumentID: "someid.pdf", Size: 100})
+	tree.Add(doc)
+
+	reader, err := tree.IndexReader()
+	if err != nil {
+		t.Fatal(err)
+	}
+	body, err := io.ReadAll(reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, schema, err := parseIndex(strings.NewReader(string(body)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if schema != SchemaVersionV4 {
+		t.Errorf("IndexReader() produced schema %s for v3-mirrored tree; want v4", schema)
+	}
+
+	if err := tree.Rehash(); err != nil {
+		t.Fatal(err)
+	}
+	reader2, _ := tree.IndexReader()
+	body2, _ := io.ReadAll(reader2)
+	h := sha256.Sum256(body2)
+	expectedHash := hex.EncodeToString(h[:])
+	if tree.Hash != expectedHash {
+		t.Errorf("Rehash() = %s; want sha256(IndexReader) = %s", tree.Hash, expectedHash)
+	}
+}
+
 func TestCreateRootIndex(t *testing.T) {
 	tree := HashTree{
 		SchemaVersion: SchemaVersionV4,