diff --git a/src/workerd/api/sockets.c++ b/src/workerd/api/sockets.c++
index b335f10214d..2270f505a40 100644
--- a/src/workerd/api/sockets.c++
+++ b/src/workerd/api/sockets.c++
@@ -160,6 +160,7 @@ jsg::Ref<Socket> setupSocket(jsg::Lock& js,
   auto openedPrPair = js.newPromiseAndResolver<SocketInfo>();
   openedPrPair.promise.markAsHandled(js);
   auto writable = jsg::alloc<WritableStream>(ioContext, kj::mv(sysStreams.writable),
+      ioContext.getMetrics().tryCreateWritableByteStreamObserver(),
       getWritableHighWaterMark(options), openedPrPair.promise.whenResolved(js));
 
   auto result = jsg::alloc<Socket>(js, ioContext, kj::mv(refcountedConnection),
diff --git a/src/workerd/api/streams/common.h b/src/workerd/api/streams/common.h
index 3706734efdf..b0a7a9d87d5 100644
--- a/src/workerd/api/streams/common.h
+++ b/src/workerd/api/streams/common.h
@@ -726,7 +726,8 @@ class WritableStreamController {
 
 kj::Own<WritableStreamController> newWritableStreamJsController();
 kj::Own<WritableStreamController> newWritableStreamInternalController(IoContext& ioContext,
-    kj::Own<WritableStreamSink> source,
+    kj::Own<WritableStreamSink> sink,
+    kj::Maybe<kj::Own<ByteStreamObserver>> observer,
     kj::Maybe<uint64_t> maybeHighWaterMark = kj::none,
     kj::Maybe<jsg::Promise<void>> maybeClosureWaitable = kj::none);
 
diff --git a/src/workerd/api/streams/compression.c++ b/src/workerd/api/streams/compression.c++
index 6a0c76a2fa0..71106973d66 100644
--- a/src/workerd/api/streams/compression.c++
+++ b/src/workerd/api/streams/compression.c++
@@ -504,7 +504,8 @@ jsg::Ref<CompressionStream> CompressionStream::constructor(kj::String format) {
   auto& ioContext = IoContext::current();
 
   return jsg::alloc<CompressionStream>(jsg::alloc<ReadableStream>(ioContext, kj::mv(readableSide)),
-      jsg::alloc<WritableStream>(ioContext, kj::mv(writableSide)));
+      jsg::alloc<WritableStream>(ioContext, kj::mv(writableSide),
+          ioContext.getMetrics().tryCreateWritableByteStreamObserver()));
 }
 
 jsg::Ref<DecompressionStream> DecompressionStream::constructor(jsg::Lock& js, kj::String format) {
@@ -521,7 +522,8 @@ jsg::Ref<DecompressionStream> DecompressionStream::constructor(jsg::Lock& js, kj
 
   return jsg::alloc<DecompressionStream>(
       jsg::alloc<ReadableStream>(ioContext, kj::mv(readableSide)),
-      jsg::alloc<WritableStream>(ioContext, kj::mv(writableSide)));
+      jsg::alloc<WritableStream>(ioContext, kj::mv(writableSide),
+          ioContext.getMetrics().tryCreateWritableByteStreamObserver()));
 }
 
 }  // namespace workerd::api
diff --git a/src/workerd/api/streams/internal-test.c++ b/src/workerd/api/streams/internal-test.c++
index bd2c3df9701..ece64b9c3b8 100644
--- a/src/workerd/api/streams/internal-test.c++
+++ b/src/workerd/api/streams/internal-test.c++
@@ -230,7 +230,8 @@ KJ_TEST("WritableStreamInternalController queue size assertion") {
     // allowed to be queued.
 
     jsg::Ref<ReadableStream> source = ReadableStream::constructor(env.js, kj::none, kj::none);
-    jsg::Ref<WritableStream> sink = jsg::alloc<WritableStream>(env.context, kj::heap<MySink>());
+    jsg::Ref<WritableStream> sink =
+        jsg::alloc<WritableStream>(env.context, kj::heap<MySink>(), kj::none);
 
     auto pipeTo = source->pipeTo(env.js, sink.addRef(), PipeToOptions{.preventClose = true});
 
@@ -272,5 +273,84 @@ KJ_TEST("WritableStreamInternalController queue size assertion") {
   });
 }
 
+KJ_TEST("WritableStreamInternalController observability") {
+
+  capnp::MallocMessageBuilder message;
+  auto flags = message.initRoot<CompatibilityFlags>();
+  flags.setNodeJsCompat(true);
+  flags.setWorkerdExperimental(true);
+  flags.setStreamsJavaScriptControllers(true);
+
+  TestFixture::SetupParams params;
+  TestFixture fixture({.featureFlags = flags.asReader()});
+
+  class MySink final: public WritableStreamSink {
+  public:
+    kj::Promise<void> write(kj::ArrayPtr<const byte> buffer) override {
+      ++writeCount;
+      return kj::READY_NOW;
+    }
+    kj::Promise<void> write(kj::ArrayPtr<const kj::ArrayPtr<const byte>> pieces) override {
+      return kj::READY_NOW;
+    }
+    kj::Promise<void> end() override {
+      return kj::READY_NOW;
+    }
+    void abort(kj::Exception reason) override {}
+    uint getWriteCount() {
+      return writeCount;
+    }
+
+  private:
+    uint writeCount = 0;
+  };
+
+  class MyObserver final: public ByteStreamObserver {
+  public:
+    virtual void onChunkEnqueued(size_t bytes) {
+      ++queueSize;
+      queueSizeBytes += bytes;
+    };
+    virtual void onChunkDequeued(size_t bytes) {
+      queueSizeBytes -= bytes;
+      --queueSize;
+    };
+    uint64_t queueSize = 0;
+    uint64_t queueSizeBytes = 0;
+  };
+
+  auto myObserver = kj::heap<MyObserver>();
+  auto& observer = *myObserver;
+  kj::Maybe<jsg::Ref<WritableStream>> stream;
+  fixture.runInIoContext([&](const TestFixture::Environment& env) -> kj::Promise<void> {
+    stream = jsg::alloc<WritableStream>(env.context, kj::heap<MySink>(), kj::mv(myObserver));
+
+    auto write = [&](size_t size) {
+      auto buffersource = env.js.bytes(kj::heapArray<kj::byte>(size));
+      return env.context.awaitJs(env.js,
+          KJ_ASSERT_NONNULL(stream)->getController().write(env.js, buffersource.getHandle(env.js)));
+    };
+
+    KJ_ASSERT(observer.queueSize == 0);
+    KJ_ASSERT(observer.queueSizeBytes == 0);
+
+    auto builder = kj::heapArrayBuilder<kj::Promise<void>>(2);
+    builder.add(write(1));
+
+    KJ_ASSERT(observer.queueSize == 1);
+    KJ_ASSERT(observer.queueSizeBytes == 1);
+
+    builder.add(write(10));
+
+    KJ_ASSERT(observer.queueSize == 2);
+    KJ_ASSERT(observer.queueSizeBytes == 11);
+
+    return kj::joinPromises(builder.finish());
+  });
+
+  KJ_ASSERT(observer.queueSize == 0);
+  KJ_ASSERT(observer.queueSizeBytes == 0);
+}
+
 }  // namespace
 }  // namespace workerd::api
diff --git a/src/workerd/api/streams/internal.c++ b/src/workerd/api/streams/internal.c++
index c451d9d3b7a..5bae8a30624 100644
--- a/src/workerd/api/streams/internal.c++
+++ b/src/workerd/api/streams/internal.c++
@@ -933,6 +933,9 @@ jsg::Promise<void> WritableStreamInternalController::write(
 
       auto prp = js.newPromiseAndResolver<void>();
       increaseCurrentWriteBufferSize(js, byteLength);
+      KJ_IF_SOME(o, observer) {
+        o->onChunkEnqueued(byteLength);
+      }
       auto ptr =
           kj::ArrayPtr<kj::byte>(static_cast<kj::byte*>(store->Data()) + byteOffset, byteLength);
       queue.push_back(
@@ -1598,6 +1601,9 @@ jsg::Promise<void> WritableStreamInternalController::writeLoopAfterFrontOutputLo
         auto& request = check();
         maybeResolvePromise(js, request.promise);
         decreaseCurrentWriteBufferSize(js, amountToWrite);
+        KJ_IF_SOME(o, observer) {
+          o->onChunkDequeued(amountToWrite);
+        }
         queue.pop_front();
         maybeAbort(js, request);
         return writeLoop(js, IoContext::current());
@@ -1610,6 +1616,9 @@ jsg::Promise<void> WritableStreamInternalController::writeLoopAfterFrontOutputLo
         auto& request = check();
         auto& writable = state.get<IoOwn<Writable>>();
         decreaseCurrentWriteBufferSize(js, amountToWrite);
+        KJ_IF_SOME(o, observer) {
+          o->onChunkDequeued(amountToWrite);
+        }
         maybeRejectPromise<void>(js, request.promise, handle);
         queue.pop_front();
         if (!maybeAbort(js, request)) {
@@ -2468,10 +2477,11 @@ kj::Own<ReadableStreamController> newReadableStreamInternalController(
 
 kj::Own<WritableStreamController> newWritableStreamInternalController(IoContext& ioContext,
     kj::Own<WritableStreamSink> sink,
+    kj::Maybe<kj::Own<ByteStreamObserver>> observer,
     kj::Maybe<uint64_t> maybeHighWaterMark,
     kj::Maybe<jsg::Promise<void>> maybeClosureWaitable) {
   return kj::heap<WritableStreamInternalController>(
-      kj::mv(sink), maybeHighWaterMark, kj::mv(maybeClosureWaitable));
+      kj::mv(sink), kj::mv(observer), maybeHighWaterMark, kj::mv(maybeClosureWaitable));
 }
 
 kj::StringPtr WritableStreamInternalController::jsgGetMemoryName() const {
diff --git a/src/workerd/api/streams/internal.h b/src/workerd/api/streams/internal.h
index bddb9eef31f..3319251bb4d 100644
--- a/src/workerd/api/streams/internal.h
+++ b/src/workerd/api/streams/internal.h
@@ -8,6 +8,7 @@
 #include "writable.h"
 
 #include <workerd/io/io-context.h>
+#include <workerd/io/observer.h>
 
 #include <deque>
 
@@ -183,9 +184,11 @@ class WritableStreamInternalController: public WritableStreamController {
   explicit WritableStreamInternalController(StreamStates::Errored errored)
       : state(kj::mv(errored)) {}
   explicit WritableStreamInternalController(kj::Own<WritableStreamSink> writable,
+      kj::Maybe<kj::Own<ByteStreamObserver>> observer,
       kj::Maybe<uint64_t> maybeHighWaterMark = kj::none,
       kj::Maybe<jsg::Promise<void>> maybeClosureWaitable = kj::none)
       : state(IoContext::current().addObject(kj::heap<Writable>(kj::mv(writable)))),
+        observer(kj::mv(observer)),
         maybeHighWaterMark(maybeHighWaterMark),
         maybeClosureWaitable(kj::mv(maybeClosureWaitable)) {}
 
@@ -279,6 +282,8 @@ class WritableStreamInternalController: public WritableStreamController {
   kj::OneOf<StreamStates::Closed, StreamStates::Errored, IoOwn<Writable>> state;
   kj::OneOf<Unlocked, Locked, PipeLocked, WriterLocked> writeState = Unlocked();
 
+  kj::Maybe<kj::Own<ByteStreamObserver>> observer;
+
   kj::Maybe<PendingAbort> maybePendingAbort;
 
   uint64_t currentWriteBufferSize = 0;
diff --git a/src/workerd/api/streams/transform.c++ b/src/workerd/api/streams/transform.c++
index ae40d9d58ed..fd8cd656cf9 100644
--- a/src/workerd/api/streams/transform.c++
+++ b/src/workerd/api/streams/transform.c++
@@ -64,7 +64,7 @@ jsg::Ref<TransformStream> TransformStream::constructor(jsg::Lock& js,
           .pull = maybeAddFunctor<UnderlyingSource::PullAlgorithm>(
               JSG_VISITABLE_LAMBDA((controller = controller.addRef()), (controller),
                   (jsg::Lock & js, auto c) mutable { return controller->pull(js); })),
-          .cancel = maybeAddFunctor<UnderlyingSource::CancelAlgorithm>(JSG_VISITABLE_LAMBDA(
+          .cancel = maybeAddFunctor<UnderlyingSource::CancelAlgorithm>( JSG_VISITABLE_LAMBDA(
               (controller = controller.addRef()), (controller),
               (jsg::Lock & js, auto reason) mutable { return controller->cancel(js, reason); })),
           .expectedLength = transformer.expectedLength.map(
@@ -123,9 +123,9 @@ jsg::Ref<IdentityTransformStream> IdentityTransformStream::constructor(
   KJ_IF_SOME(queuingStrategy, maybeQueuingStrategy) {
     maybeHighWaterMark = queuingStrategy.highWaterMark;
   }
-
   return jsg::alloc<IdentityTransformStream>(jsg::alloc<ReadableStream>(ioContext, kj::mv(pipe.in)),
-      jsg::alloc<WritableStream>(ioContext, kj::mv(pipe.out), maybeHighWaterMark));
+      jsg::alloc<WritableStream>(ioContext, kj::mv(pipe.out),
+          ioContext.getMetrics().tryCreateWritableByteStreamObserver(), maybeHighWaterMark));
 }
 
 jsg::Ref<FixedLengthStream> FixedLengthStream::constructor(jsg::Lock& js,
@@ -147,7 +147,8 @@ jsg::Ref<FixedLengthStream> FixedLengthStream::constructor(jsg::Lock& js,
   }
 
   return jsg::alloc<FixedLengthStream>(jsg::alloc<ReadableStream>(ioContext, kj::mv(pipe.in)),
-      jsg::alloc<WritableStream>(ioContext, kj::mv(pipe.out), maybeHighWaterMark));
+      jsg::alloc<WritableStream>(ioContext, kj::mv(pipe.out),
+          ioContext.getMetrics().tryCreateWritableByteStreamObserver(), maybeHighWaterMark));
 }
 
 OneWayPipe newIdentityPipe(kj::Maybe<uint64_t> expectedLength) {
diff --git a/src/workerd/api/streams/writable.c++ b/src/workerd/api/streams/writable.c++
index 85478c0c014..8d81b536211 100644
--- a/src/workerd/api/streams/writable.c++
+++ b/src/workerd/api/streams/writable.c++
@@ -216,10 +216,14 @@ void WritableStreamDefaultWriter::visitForGc(jsg::GcVisitor& visitor) {
 
 WritableStream::WritableStream(IoContext& ioContext,
     kj::Own<WritableStreamSink> sink,
+    kj::Maybe<kj::Own<ByteStreamObserver>> maybeObserver,
     kj::Maybe<uint64_t> maybeHighWaterMark,
     kj::Maybe<jsg::Promise<void>> maybeClosureWaitable)
-    : WritableStream(newWritableStreamInternalController(
-          ioContext, kj::mv(sink), maybeHighWaterMark, kj::mv(maybeClosureWaitable))) {}
+    : WritableStream(newWritableStreamInternalController(ioContext,
+          kj::mv(sink),
+          kj::mv(maybeObserver),
+          maybeHighWaterMark,
+          kj::mv(maybeClosureWaitable))) {}
 
 WritableStream::WritableStream(kj::Own<WritableStreamController> controller)
     : ioContext(tryGetIoContext()),
@@ -611,7 +615,8 @@ jsg::Ref<WritableStream> WritableStream::deserialize(
   auto stream = ioctx.getByteStreamFactory().capnpToKjExplicitEnd(ws.getByteStream());
   auto sink = newSystemStream(kj::mv(stream), encoding, ioctx);
 
-  return jsg::alloc<WritableStream>(ioctx, kj::mv(sink));
+  return jsg::alloc<WritableStream>(
+      ioctx, kj::mv(sink), ioctx.getMetrics().tryCreateWritableByteStreamObserver());
 }
 
 void WritableStreamDefaultWriter::visitForMemoryInfo(jsg::MemoryTracker& tracker) const {
diff --git a/src/workerd/api/streams/writable.h b/src/workerd/api/streams/writable.h
index 10e636e4d6f..b46e312fe66 100644
--- a/src/workerd/api/streams/writable.h
+++ b/src/workerd/api/streams/writable.h
@@ -101,6 +101,7 @@ class WritableStream: public jsg::Object {
 public:
   explicit WritableStream(IoContext& ioContext,
       kj::Own<WritableStreamSink> sink,
+      kj::Maybe<kj::Own<ByteStreamObserver>> observer,
       kj::Maybe<uint64_t> maybeHighWaterMark = kj::none,
       kj::Maybe<jsg::Promise<void>> maybeClosureWaitable = kj::none);
 
diff --git a/src/workerd/io/io-context.h b/src/workerd/io/io-context.h
index e146af2c0fa..35ee29639f8 100644
--- a/src/workerd/io/io-context.h
+++ b/src/workerd/io/io-context.h
@@ -480,7 +480,7 @@ class IoContext final: public kj::Refcounted, private kj::TaskSet::ErrorHandler
   // immediately if any of these happen:
   // - The JS promise is GC'd without resolving.
   // - The JS promise is resolved from the wrong context.
-  // - The system detects that no further process will be made in this context (because there is no
+  // - The system detects that no further progress will be made in this context (because there is no
   //   more JavaScript to run, and there is no outstanding I/O scheduled with awaitIo()).
   //
   // If `T` is `IoOwn<U>`, it will be unwrapped to just `U` in the result. If `U` is in turn
diff --git a/src/workerd/io/observer.h b/src/workerd/io/observer.h
index e1eed46e03c..b4043921341 100644
--- a/src/workerd/io/observer.h
+++ b/src/workerd/io/observer.h
@@ -24,12 +24,30 @@ class TimerChannel;
 
 class WebSocketObserver: public kj::Refcounted {
 public:
+  virtual ~WebSocketObserver() noexcept(false) = default;
   // Called when a worker sends a message on this WebSocket (includes close messages).
   virtual void sentMessage(size_t bytes) {};
   // Called when a worker receives a message on this WebSocket (includes close messages).
   virtual void receivedMessage(size_t bytes) {};
 };
 
+// Observes a byte stream. Byte streams which use instances of this observer should call enqueue()
+// and dequeue() once for each chunk that passes through the stream. The order of enqueues should
+// match the order of dequeues.
+//
+// Byte observer implementations can then calculate the current number of chunks and the sum of the
+// size of the chunks in the internal queue by incrementing and decrementing each metric in
+// enqueue() and dequeue() respectively.
+class ByteStreamObserver {
+public:
+  virtual ~ByteStreamObserver() noexcept(false) = default;
+  // Called when a chunk of size `bytes` is enqueued on the stream.
+  virtual void onChunkEnqueued(size_t bytes) {};
+  // Called when a chunk of size `bytes` is dequeued from the stream (e.g. when a writable byte
+  // stream writes the chunk to its corresponding sink).
+  virtual void onChunkDequeued(size_t bytes) {};
+};
+
 // Observes a specific request to a specific worker. Also observes outgoing subrequests.
 //
 // Observing anything is optional. Default implementations of all methods observe nothing.
@@ -45,6 +63,12 @@ class RequestObserver: public kj::Refcounted {
     return kj::none;
   };
 
+  // This is called when a writable byte stream is created whilst processing this request. It will
+  // be destroyed when the corresponding byte stream is destroyed.
+  virtual kj::Maybe<kj::Own<ByteStreamObserver>> tryCreateWritableByteStreamObserver() {
+    return kj::none;
+  }
+
   // Invoked when the request is actually delivered.
   //
   // If, for some reason, this is not invoked before the object is destroyed, this indicate that