diff --git a/lib/internal/quic/quic.js b/lib/internal/quic/quic.js
index 6ca59469faf2de..a5ce3d8c14fb23 100644
--- a/lib/internal/quic/quic.js
+++ b/lib/internal/quic/quic.js
@@ -1279,19 +1279,14 @@ function waitForDrain(stream) {
 
 // Writes a batch to the handle, awaiting drain if backpressured.
 // Returns true if the stream was destroyed during the wait.
-// Checks writeDesiredSize before writing to enforce backpressure
-// against the outbound DataQueue's uncommitted bytes.
+// Only waits when writeDesiredSize is 0 (no capacity at all).
+// When there is any capacity, the write proceeds even if the batch
+// is larger -- the C++ side buffers the data and writeDesiredSize
+// drops toward 0, letting the normal drain mechanism take over.
 async function writeBatchWithDrain(handle, stream, batch) {
   const state = getQuicStreamState(stream);
 
-  // Calculate total batch size for the capacity check.
-  let len = 0;
-  for (const chunk of batch) len += TypedArrayPrototypeGetByteLength(chunk);
-
-  // If insufficient capacity, wait for the C++ drain signal which
-  // fires when writeDesiredSize transitions from 0 to > 0 (i.e.,
-  // ngtcp2 has consumed data from the outbound DataQueue).
-  if (len > state.writeDesiredSize) {
+  if (state.writeDesiredSize === 0) {
     await waitForDrain(stream);
     if (stream.destroyed) return true;
   }
@@ -2029,9 +2024,15 @@ class QuicStream {
       chunk = toUint8Array(chunk);
       const len = TypedArrayPrototypeGetByteLength(chunk);
       if (len === 0) return true;
-      // Refuse the write if the chunk doesn't fit in the available
-      // buffer capacity. The caller should wait for drain and retry.
-      if (len > stream.#state.writeDesiredSize) return false;
+      // Refuse the write only when there is no available capacity at
+      // all. When writeDesiredSize > 0 we allow the write even if the
+      // chunk is larger than the remaining capacity -- the C++ side
+      // will accept the data into the DataQueue and
+      // UpdateWriteDesiredSize() will drop writeDesiredSize toward 0,
+      // at which point the standard drain mechanism takes over.
+      // This follows the Web Streams model where writes beyond the HWM
+      // succeed and backpressure applies to *subsequent* writes.
+      if (stream.#state.writeDesiredSize === 0) return false;
       const result = handle.write([chunk]);
       if (result === undefined) return false;
       totalBytesWritten += len;
@@ -2070,7 +2071,7 @@ class QuicStream {
       let len = 0;
       for (const c of chunks) len += TypedArrayPrototypeGetByteLength(c);
       if (len === 0) return true;
-      if (len > stream.#state.writeDesiredSize) return false;
+      if (stream.#state.writeDesiredSize === 0) return false;
       const result = handle.write(chunks);
       if (result === undefined) return false;
       totalBytesWritten += len;
diff --git a/src/quic/streams.cc b/src/quic/streams.cc
index dd7f7ecbb3880e..81e619e28d3720 100644
--- a/src/quic/streams.cc
+++ b/src/quic/streams.cc
@@ -1592,8 +1592,10 @@ void Stream::UpdateWriteDesiredSize() {
   uint32_t old_size = state_->write_desired_size;
   state_->write_desired_size = clamped;
 
-  // Fire drain when transitioning from 0 to non-zero
-  if (old_size == 0 && desired > 0) {
+  // Fire drain when transitioning from 0 to non-zero.
+  // writeDesiredSize == 0 means the buffer is full or flow control is
+  // exhausted, so the JS side may be waiting for capacity.
+  if (old_size == 0 && clamped > 0) {
     EmitDrain();
   }
 }
diff --git a/test/parallel/test-quic-stream-bidi-varchunklen.mjs b/test/parallel/test-quic-stream-bidi-varchunklen.mjs
new file mode 100644
index 00000000000000..a8928c7ced34dc
--- /dev/null
+++ b/test/parallel/test-quic-stream-bidi-varchunklen.mjs
@@ -0,0 +1,91 @@
+// Flags: --experimental-quic --experimental-stream-iter --no-warnings
+
+// Test: bidirectional data transfer with varying chunk sizes.
+// This is a regression test for a stall caused by a mismatch between
+// writeSync (which rejects when chunk > writeDesiredSize) and
+// drainableProtocol (which returned null when writeDesiredSize > 0).
+// When chunks don't evenly fill the high water mark, writeDesiredSize
+// can be positive but smaller than the next chunk, causing the
+// while(!writeSync) { dp(); await } loop to spin without yielding.
+// See: https://github.com/nodejs/node/issues/63216
+
+import { hasQuic, skip, mustCall } from '../common/index.mjs';
+import assert from 'node:assert';
+
+const { strictEqual } = assert;
+
+if (!hasQuic) {
+  skip('QUIC is not enabled');
+}
+
+const { listen, connect } = await import('../common/quic.mjs');
+const { bytes, drainableProtocol: dp } = await import('stream/iter');
+
+// Varying chunk sizes — the pattern of alternating large and small
+// chunks is effective at triggering the writeDesiredSize gap.
+const chunkSizes = [60000, 12, 50000, 1600, 20000, 30000, 0, 100];
+const numChunks = chunkSizes.length;
+const byteLength = chunkSizes.reduce((a, b) => a + b, 0);
+
+// Build a deterministic payload so we can verify integrity.
+function buildChunk(index) {
+  const chunk = new Uint8Array(chunkSizes[index]);
+  const val = index & 0xff;
+  for (let i = 0; i < chunkSizes[index]; i++) {
+    chunk[i] = (val + i) & 0xff;
+  }
+  return chunk;
+}
+
+function checksum(data) {
+  let sum = 0;
+  for (let i = 0; i < data.byteLength; i++) {
+    sum = (sum + data[i]) | 0;
+  }
+  return sum;
+}
+
+// Compute expected checksum.
+let expectedChecksum = 0;
+for (let i = 0; i < numChunks; i++) {
+  const chunk = buildChunk(i);
+  expectedChecksum = (expectedChecksum + checksum(chunk)) | 0;
+}
+
+const done = Promise.withResolvers();
+
+const serverEndpoint = await listen(mustCall((serverSession) => {
+  serverSession.onstream = mustCall(async (stream) => {
+    const received = await bytes(stream);
+    strictEqual(received.byteLength, byteLength);
+    strictEqual(checksum(received), expectedChecksum);
+
+    stream.writer.endSync();
+    await stream.closed;
+    serverSession.close();
+    done.resolve();
+  });
+}));
+
+const clientSession = await connect(serverEndpoint.address);
+await clientSession.opened;
+
+const stream = await clientSession.createBidirectionalStream();
+const w = stream.writer;
+
+// Write chunks, respecting backpressure via drainableProtocol.
+for (let i = 0; i < numChunks; i++) {
+  const chunk = buildChunk(i);
+  while (!w.writeSync(chunk)) {
+    // Flow controlled — wait for drain before retrying.
+    const drainable = w[dp]();
+    if (drainable) await drainable;
+  }
+}
+
+const totalWritten = w.endSync();
+strictEqual(totalWritten, byteLength);
+
+await Promise.all([stream.closed, done.promise]);
+await clientSession.close();
+await serverEndpoint.close();