diff --git a/dataforge-io/src/commonMain/kotlin/hep/dataforge/io/EnvelopeParts.kt b/dataforge-io/src/commonMain/kotlin/hep/dataforge/io/EnvelopeParts.kt
index eb1dd696..d1b86195 100644
--- a/dataforge-io/src/commonMain/kotlin/hep/dataforge/io/EnvelopeParts.kt
+++ b/dataforge-io/src/commonMain/kotlin/hep/dataforge/io/EnvelopeParts.kt
@@ -3,6 +3,7 @@ package hep.dataforge.io
 import hep.dataforge.context.Global
 import hep.dataforge.io.EnvelopeParts.FORMAT_META_KEY
 import hep.dataforge.io.EnvelopeParts.FORMAT_NAME_KEY
+import hep.dataforge.io.EnvelopeParts.INDEX_KEY
 import hep.dataforge.io.EnvelopeParts.MULTIPART_DATA_TYPE
 import hep.dataforge.io.EnvelopeParts.SIZE_KEY
 import hep.dataforge.meta.*
@@ -13,6 +14,7 @@ import hep.dataforge.names.toName
 object EnvelopeParts {
     val MULTIPART_KEY = "multipart".asName()
     val SIZE_KEY = Envelope.ENVELOPE_NODE_KEY + MULTIPART_KEY + "size"
+    val INDEX_KEY = Envelope.ENVELOPE_NODE_KEY + MULTIPART_KEY + "index"
     val FORMAT_NAME_KEY = Envelope.ENVELOPE_NODE_KEY + MULTIPART_KEY + "format"
     val FORMAT_META_KEY = Envelope.ENVELOPE_NODE_KEY + MULTIPART_KEY + "meta"
 
@@ -37,8 +39,37 @@ fun EnvelopeBuilder.multipart(format: EnvelopeFormatFactory, envelopes: Collecti
     }
 }
 
-fun EnvelopeBuilder.multipart(formatFactory: EnvelopeFormatFactory, builder: suspend SequenceScope<Envelope>.() -> Unit) =
-    multipart(formatFactory, sequence(builder).toList())
+/**
+ * Create a multipart partition in the envelope adding additional name-index mapping in meta
+ */
+@DFExperimental
+fun EnvelopeBuilder.multipart(format: EnvelopeFormatFactory, envelopes: Map<String, Envelope>) {
+    dataType = MULTIPART_DATA_TYPE
+    meta {
+        SIZE_KEY put envelopes.size
+        FORMAT_NAME_KEY put format.name.toString()
+    }
+    data {
+        format.run {
+            var counter = 0
+            envelopes.forEach {(key, envelope)->
+                writeObject(envelope)
+                meta{
+                    append(INDEX_KEY, buildMeta {
+                        "key" put key
+                        "index" put counter
+                    })
+                }
+                counter++
+            }
+        }
+    }
+}
+
+fun EnvelopeBuilder.multipart(
+    formatFactory: EnvelopeFormatFactory,
+    builder: suspend SequenceScope<Envelope>.() -> Unit
+) = multipart(formatFactory, sequence(builder).toList())
 
 /**
  * If given envelope supports multipart data, return a sequence of those parts (could be empty). Otherwise return null.
diff --git a/dataforge-io/src/commonMain/kotlin/hep/dataforge/io/IOPlugin.kt b/dataforge-io/src/commonMain/kotlin/hep/dataforge/io/IOPlugin.kt
index ae88e4f4..a463a053 100644
--- a/dataforge-io/src/commonMain/kotlin/hep/dataforge/io/IOPlugin.kt
+++ b/dataforge-io/src/commonMain/kotlin/hep/dataforge/io/IOPlugin.kt
@@ -52,7 +52,7 @@ class IOPlugin(meta: Meta) : AbstractPlugin(meta) {
 
     companion object : PluginFactory<IOPlugin> {
         val defaultMetaFormats: List<MetaFormatFactory> = listOf(JsonMetaFormat, BinaryMetaFormat)
-        val defaultEnvelopeFormats = listOf(TaggedEnvelopeFormat)
+        val defaultEnvelopeFormats = listOf(TaggedEnvelopeFormat, TaglessEnvelopeFormat)
 
         override val tag: PluginTag = PluginTag("io", group = PluginTag.DATAFORGE_GROUP)
 
diff --git a/dataforge-io/src/commonMain/kotlin/hep/dataforge/io/TaglessEnvelopeFormat.kt b/dataforge-io/src/commonMain/kotlin/hep/dataforge/io/TaglessEnvelopeFormat.kt
index 5c4b7dae..1cc62a2b 100644
--- a/dataforge-io/src/commonMain/kotlin/hep/dataforge/io/TaglessEnvelopeFormat.kt
+++ b/dataforge-io/src/commonMain/kotlin/hep/dataforge/io/TaglessEnvelopeFormat.kt
@@ -49,6 +49,7 @@ class TaglessEnvelopeFormat(
             writeText(dataStart + "\r\n")
             writeFully(data.toBytes())
         }
+        flush()
     }
 
     override fun Input.readObject(): Envelope {
@@ -191,7 +192,7 @@ class TaglessEnvelopeFormat(
             return try {
                 val buffer = ByteArray(TAGLESS_ENVELOPE_HEADER.length)
                 input.readFully(buffer)
-                return if (buffer.toString() == TAGLESS_ENVELOPE_HEADER) {
+                return if (String(buffer) == TAGLESS_ENVELOPE_HEADER) {
                     TaglessEnvelopeFormat(io)
                 } else {
                     null
diff --git a/dataforge-io/src/jvmMain/kotlin/hep/dataforge/io/fileIO.kt b/dataforge-io/src/jvmMain/kotlin/hep/dataforge/io/fileIO.kt
index 14e4c077..eff9e705 100644
--- a/dataforge-io/src/jvmMain/kotlin/hep/dataforge/io/fileIO.kt
+++ b/dataforge-io/src/jvmMain/kotlin/hep/dataforge/io/fileIO.kt
@@ -58,6 +58,24 @@ fun IOPlugin.writeMetaFile(
     }
 }
 
+/**
+ * Return inferred [EnvelopeFormat] if only one format could read given file. If no format accepts file, return null. If
+ * multiple formats accepts file, throw an error.
+ */
+fun IOPlugin.peekBinaryFormat(binary: Binary): EnvelopeFormat? {
+    val formats = envelopeFormatFactories.mapNotNull { factory ->
+        binary.read {
+            factory.peekFormat(this@peekBinaryFormat, this@read)
+        }
+    }
+
+    return when (formats.size) {
+        0 -> null
+        1 -> formats.first()
+        else -> error("Envelope format binary recognition clash")
+    }
+}
+
 /**
  * Read and envelope from file if the file exists, return null if file does not exist.
  *
@@ -72,7 +90,11 @@ fun IOPlugin.writeMetaFile(
  * Return null otherwise.
  */
 @DFExperimental
-fun IOPlugin.readEnvelopeFile(path: Path, readNonEnvelopes: Boolean = false): Envelope? {
+fun IOPlugin.readEnvelopeFile(
+    path: Path,
+    readNonEnvelopes: Boolean = false,
+    formatPeeker: IOPlugin.(Binary) -> EnvelopeFormat? = IOPlugin::peekBinaryFormat
+): Envelope? {
     if (!Files.exists(path)) return null
 
     //read two-files directory
@@ -99,24 +121,13 @@ fun IOPlugin.readEnvelopeFile(path: Path, readNonEnvelopes: Boolean = false): En
 
     val binary = path.asBinary()
 
-    val formats = envelopeFormatFactories.mapNotNull { factory ->
+    return formatPeeker(binary)?.run {
         binary.read {
-            factory.peekFormat(this@readEnvelopeFile, this@read)
+            readObject()
         }
-    }
-    return when (formats.size) {
-        0 -> if (readNonEnvelopes) {
-            SimpleEnvelope(Meta.empty, binary)
-        } else {
-            null
-        }
-        1 -> formats.first().run {
-            binary.read {
-                readObject()
-            }
-        }
-        else -> error("Envelope format file recognition clash")
-    }
+    } ?: if (readNonEnvelopes) { // if no format accepts file, read it as binary
+        SimpleEnvelope(Meta.empty, binary)
+    } else null
 }
 
 fun IOPlugin.writeEnvelopeFile(
diff --git a/dataforge-io/src/jvmTest/kotlin/hep/dataforge/io/FileEnvelopeTest.kt b/dataforge-io/src/jvmTest/kotlin/hep/dataforge/io/FileEnvelopeTest.kt
index f4847cfd..b8bfafaa 100644
--- a/dataforge-io/src/jvmTest/kotlin/hep/dataforge/io/FileEnvelopeTest.kt
+++ b/dataforge-io/src/jvmTest/kotlin/hep/dataforge/io/FileEnvelopeTest.kt
@@ -22,10 +22,23 @@ class FileEnvelopeTest {
 
     @Test
     fun testFileWriteRead() {
-        val tmpPath = Files.createTempFile("dataforge_test", ".df")
-        Global.io.writeEnvelopeFile(tmpPath,envelope)
-        println(tmpPath.toUri())
-        val restored: Envelope = Global.io.readEnvelopeFile(tmpPath)!!
-        assertTrue { envelope.contentEquals(restored) }
+        Global.io.run {
+            val tmpPath = Files.createTempFile("dataforge_test", ".df")
+            writeEnvelopeFile(tmpPath, envelope)
+            println(tmpPath.toUri())
+            val restored: Envelope = readEnvelopeFile(tmpPath)!!
+            assertTrue { envelope.contentEquals(restored) }
+        }
+    }
+
+    @Test
+    fun testFileWriteReadTagless() {
+        Global.io.run {
+            val tmpPath = Files.createTempFile("dataforge_test_tagless", ".df")
+            writeEnvelopeFile(tmpPath, envelope, format = TaglessEnvelopeFormat)
+            println(tmpPath.toUri())
+            val restored: Envelope = readEnvelopeFile(tmpPath)!!
+            assertTrue { envelope.contentEquals(restored) }
+        }
     }
 }
\ No newline at end of file