diff --git a/amber/src/test/scala/org/apache/texera/amber/engine/architecture/sendsemantics/partitioners/NetworkOutputBufferSpec.scala b/amber/src/test/scala/org/apache/texera/amber/engine/architecture/sendsemantics/partitioners/NetworkOutputBufferSpec.scala
new file mode 100644
index 00000000000..765dd386b89
--- /dev/null
+++ b/amber/src/test/scala/org/apache/texera/amber/engine/architecture/sendsemantics/partitioners/NetworkOutputBufferSpec.scala
@@ -0,0 +1,274 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.texera.amber.engine.architecture.sendsemantics.partitioners
+
+import org.apache.texera.amber.config.ApplicationConfig
+import org.apache.texera.amber.core.state.State
+import org.apache.texera.amber.core.tuple.{Attribute, AttributeType, Schema, Tuple}
+import org.apache.texera.amber.core.virtualidentity.ActorVirtualIdentity
+import org.apache.texera.amber.engine.architecture.messaginglayer.NetworkOutputGateway
+import org.apache.texera.amber.engine.common.ambermessage.{
+  DataFrame,
+  StateFrame,
+  WorkflowFIFOMessage
+}
+import org.scalatest.flatspec.AnyFlatSpec
+
+import scala.collection.mutable.ArrayBuffer
+
+class NetworkOutputBufferSpec extends AnyFlatSpec {
+
+  // --- fixtures --------------------------------------------------------------
+
+  private val sender = ActorVirtualIdentity("sender")
+  private val receiver = ActorVirtualIdentity("receiver-1")
+
+  private val intAttr = new Attribute("v", AttributeType.INTEGER)
+  private val schema: Schema = Schema().add(intAttr)
+  private def tuple(value: Int): Tuple =
+    Tuple.builder(schema).add(intAttr, value).build()
+
+  /** Recording wrapper around a real `NetworkOutputGateway`. */
+  private class Capture {
+    val messages: ArrayBuffer[WorkflowFIFOMessage] = ArrayBuffer.empty
+    val gateway: NetworkOutputGateway =
+      new NetworkOutputGateway(sender, m => messages += m)
+  }
+
+  private def newBuffer(batchSize: Int = 4): (NetworkOutputBuffer, Capture) = {
+    val cap = new Capture
+    val buf = new NetworkOutputBuffer(receiver, cap.gateway, batchSize = batchSize)
+    (buf, cap)
+  }
+
+  // --- construction defaults -------------------------------------------------
+
+  "NetworkOutputBuffer" should "default batchSize to ApplicationConfig.defaultDataTransferBatchSize" in {
+    val cap = new Capture
+    val buf = new NetworkOutputBuffer(receiver, cap.gateway)
+    assert(buf.batchSize == ApplicationConfig.defaultDataTransferBatchSize)
+  }
+
+  it should "expose `to` and `dataOutputPort` as immutable accessors" in {
+    val cap = new Capture
+    val buf = new NetworkOutputBuffer(receiver, cap.gateway, batchSize = 4)
+    assert(buf.to == receiver)
+    assert(buf.dataOutputPort eq cap.gateway)
+  }
+
+  it should "start with an empty buffer (no implicit auto-flush at construction)" in {
+    val (_, cap) = newBuffer()
+    assert(cap.messages.isEmpty)
+  }
+
+  // --- addTuple buffering / auto-flush --------------------------------------
+
+  "NetworkOutputBuffer.addTuple" should "NOT flush while the buffer is below batchSize" in {
+    val (buf, cap) = newBuffer(batchSize = 4)
+    buf.addTuple(tuple(0))
+    buf.addTuple(tuple(1))
+    buf.addTuple(tuple(2))
+    assert(cap.messages.isEmpty, "no DataFrame should be sent until batchSize is reached")
+  }
+
+  it should "auto-flush when the buffer exactly reaches batchSize" in {
+    val (buf, cap) = newBuffer(batchSize = 3)
+    buf.addTuple(tuple(0))
+    buf.addTuple(tuple(1))
+    buf.addTuple(tuple(2)) // boundary: now size == batchSize
+    assert(cap.messages.size == 1, "exactly one DataFrame should be auto-flushed at the boundary")
+    val frame = cap.messages.head.payload.asInstanceOf[DataFrame]
+    assert(frame.frame.toList == List(tuple(0), tuple(1), tuple(2)))
+  }
+
+  it should "produce a separate DataFrame for each successive batch" in {
+    val (buf, cap) = newBuffer(batchSize = 2)
+    (0 until 6).foreach(i => buf.addTuple(tuple(i)))
+    assert(cap.messages.size == 3, "three full batches → three DataFrames")
+    val payloads = cap.messages.map(_.payload.asInstanceOf[DataFrame].frame.toList)
+    assert(payloads.head == List(tuple(0), tuple(1)))
+    assert(payloads(1) == List(tuple(2), tuple(3)))
+    assert(payloads(2) == List(tuple(4), tuple(5)))
+  }
+
+  it should "send DataFrames to the configured receiver only" in {
+    val (buf, cap) = newBuffer(batchSize = 2)
+    buf.addTuple(tuple(0))
+    buf.addTuple(tuple(1))
+    assert(cap.messages.size == 1)
+    val msg = cap.messages.head
+    assert(msg.channelId.fromWorkerId == sender)
+    assert(msg.channelId.toWorkerId == receiver)
+    assert(!msg.channelId.isControl, "data path must not use the control channel")
+  }
+
+  // --- flush() ----------------------------------------------------------------
+
+  "NetworkOutputBuffer.flush" should "send a DataFrame and reset the buffer when the buffer is non-empty" in {
+    val (buf, cap) = newBuffer(batchSize = 100) // never auto-flushes
+    buf.addTuple(tuple(7))
+    buf.addTuple(tuple(8))
+    buf.flush()
+    assert(cap.messages.size == 1)
+    val frame = cap.messages.head.payload.asInstanceOf[DataFrame]
+    assert(frame.frame.toList == List(tuple(7), tuple(8)))
+    // A second flush() with nothing buffered must not send another frame.
+    buf.flush()
+    assert(cap.messages.size == 1, "flush() on an empty buffer must be a no-op")
+  }
+
+  it should "be a no-op when called on an empty buffer (no DataFrame, no StateFrame)" in {
+    val (buf, cap) = newBuffer()
+    buf.flush()
+    buf.flush()
+    buf.flush()
+    assert(cap.messages.isEmpty)
+  }
+
+  it should "assign monotonically increasing sequence numbers across multiple flushes" in {
+    // The gateway tracks sequence numbers per channel; each successive
+    // DataFrame on the same channel gets the next number. Pin so a
+    // regression that resets seq on flush is visible.
+    val (buf, cap) = newBuffer(batchSize = 1) // each addTuple flushes
+    (0 until 4).foreach(i => buf.addTuple(tuple(i)))
+    val seqs = cap.messages.map(_.sequenceNumber).toList
+    assert(seqs == List(0L, 1L, 2L, 3L), s"unexpected sequence: $seqs")
+  }
+
+  // --- sendState ----------------------------------------------------------
+
+  "NetworkOutputBuffer.sendState" should "flush pending tuples FIRST, then send the StateFrame" in {
+    val (buf, cap) = newBuffer(batchSize = 100)
+    buf.addTuple(tuple(0))
+    buf.addTuple(tuple(1))
+    val state = State(Map("checkpoint" -> 99))
+    buf.sendState(state)
+    // Expected order: DataFrame (the buffered tuples) → StateFrame.
+    assert(cap.messages.size == 2)
+    val first = cap.messages.head.payload
+    val second = cap.messages(1).payload
+    assert(first.isInstanceOf[DataFrame], s"first frame should be DataFrame, got $first")
+    assert(first.asInstanceOf[DataFrame].frame.toList == List(tuple(0), tuple(1)))
+    assert(second == StateFrame(state))
+  }
+
+  it should "send only the StateFrame when no tuples are pending (empty pre-flush is a no-op)" in {
+    val (buf, cap) = newBuffer()
+    val state = State(Map("k" -> "v"))
+    buf.sendState(state)
+    assert(cap.messages.size == 1)
+    assert(cap.messages.head.payload == StateFrame(state))
+  }
+
+  it should "leave the tuple buffer empty after sendState (trailing flush no-op)" in {
+    // sendState calls flush() AFTER sending the state too. Pin that the
+    // trailing flush doesn't double-send and that subsequent addTuple
+    // starts from a clean buffer.
+    val (buf, cap) = newBuffer(batchSize = 100)
+    buf.addTuple(tuple(0))
+    buf.sendState(State(Map.empty))
+    val countBefore = cap.messages.size // DataFrame + StateFrame = 2
+    assert(countBefore == 2)
+    // Add another tuple and explicit flush — must produce one fresh frame.
+    buf.addTuple(tuple(99))
+    buf.flush()
+    assert(cap.messages.size == 3)
+    val third = cap.messages(2).payload.asInstanceOf[DataFrame]
+    assert(third.frame.toList == List(tuple(99)), "post-state buffer must start empty")
+  }
+
+  it should "share a single sequence-number stream across DataFrames and the StateFrame on the same channel" in {
+    // Pin: DataFrame and StateFrame go through the same `sendTo` path on
+    // the same channel, so they share the gateway's sequence-number
+    // counter. A regression that opens a side-channel for StateFrame
+    // would produce a non-monotonic stream and fail this.
+    val (buf, cap) = newBuffer(batchSize = 100)
+    buf.addTuple(tuple(0))
+    buf.addTuple(tuple(1))
+    buf.sendState(State(Map("x" -> 1)))
+    buf.addTuple(tuple(2))
+    buf.flush()
+    val seqs = cap.messages.map(_.sequenceNumber).toList
+    assert(seqs == List(0L, 1L, 2L), s"unexpected sequence: $seqs")
+  }
+
+  // --- batchSize edge cases -------------------------------------------------
+
+  "NetworkOutputBuffer with batchSize = 1" should "flush immediately after every addTuple" in {
+    val (buf, cap) = newBuffer(batchSize = 1)
+    buf.addTuple(tuple(0))
+    assert(cap.messages.size == 1)
+    buf.addTuple(tuple(1))
+    assert(cap.messages.size == 2)
+    val frames = cap.messages.toList.map(_.payload.asInstanceOf[DataFrame].frame.toList)
+    assert(frames == List(List(tuple(0)), List(tuple(1))))
+  }
+
+  // `batchSize <= 0` IS reachable from production today: the
+  // workflow-settings UI restricts the value to `>= 1`, but
+  // `SyncExecutionResource` accepts `request.workflowSettings` directly
+  // from the API and the backend forwards `workflowSettings
+  // .dataTransferBatchSize` into `NetworkOutputBuffer` without
+  // validating it. The reachable path is covered by a characterization
+  // test (current lenient `>=` behavior — flush every tuple) plus a
+  // pendingUntilFixed test pinning the desired hardening (rejection
+  // at construction). When the hardening lands the characterization
+  // test breaks on purpose AND pendingUntilFixed flips into a
+  // deliberate failure forcing both markers to be updated together.
+
+  "NetworkOutputBuffer with non-positive batchSize" should
+    "currently flush per-tuple under the `>=` guard (characterization, today's lenient behavior)" in {
+    // Pin the current observable behavior for the reachable-from-API
+    // `batchSize <= 0` path so a regression that breaks per-tuple
+    // flush (e.g. a partial change that disables flushing entirely
+    // for non-positive batch sizes) surfaces here. A future hardening
+    // that rejects `<= 0` at construction WILL break this test on
+    // purpose — and the pendingUntilFixed test below will flip into
+    // a deliberate failure at the same time, forcing both markers to
+    // be updated together.
+    val (buf0, cap0) = newBuffer(batchSize = 0)
+    buf0.addTuple(tuple(1))
+    buf0.addTuple(tuple(2))
+    val frames0 = cap0.messages.toList.map(_.payload.asInstanceOf[DataFrame].frame.toList)
+    assert(frames0 == List(List(tuple(1)), List(tuple(2))))
+
+    val (bufNeg, capNeg) = newBuffer(batchSize = -1)
+    bufNeg.addTuple(tuple(99))
+    val framesNeg = capNeg.messages.toList.map(_.payload.asInstanceOf[DataFrame].frame.toList)
+    assert(framesNeg == List(List(tuple(99))))
+  }
+
+  it should "eventually reject construction (pendingUntilFixed)" in pendingUntilFixed {
+    // Today the constructor accepts `batchSize <= 0` and the `>=`
+    // guard then fires after every append (the characterization
+    // above pins that behavior). The intended contract is that a
+    // non-positive batch size is invalid input and should be
+    // rejected at construction (e.g. `require(batchSize > 0, ...)`).
+    // Asserting `IllegalArgumentException` here flips this from
+    // pending to passing once the hardening lands.
+    val cap = new Capture
+    intercept[IllegalArgumentException] {
+      new NetworkOutputBuffer(receiver, cap.gateway, batchSize = 0)
+    }
+    intercept[IllegalArgumentException] {
+      new NetworkOutputBuffer(receiver, cap.gateway, batchSize = -1)
+    }
+  }
+}