diff --git a/Package.swift b/Package.swift
index 9712deb4..03fc2193 100644
--- a/Package.swift
+++ b/Package.swift
@@ -521,6 +521,7 @@ let package = Package(
         "internal/platform/input_stream_test.cc",
         "internal/platform/single_thread_executor_test.cc",
         "internal/platform/scheduled_executor_test.cc",
+        "internal/platform/stream_writer_test.cc",
         "internal/platform/count_down_latch_test.cc",
         "internal/platform/pipe_test.cc",
         "internal/platform/timer_impl_test.cc",
diff --git a/internal/platform/BUILD b/internal/platform/BUILD
index 1c53d60d..626649c4 100644
--- a/internal/platform/BUILD
+++ b/internal/platform/BUILD
@@ -84,10 +84,12 @@ cc_library(
     srcs = [
         "base_input_stream.cc",
         "byte_utils.cc",
+        "stream_writer.cc",
     ],
     hdrs = [
         "base_input_stream.h",
         "byte_utils.h",
+        "stream_writer.h",
     ],
     visibility = [
         "//connections:__subpackages__",
@@ -399,6 +401,7 @@ cc_test(
     srcs = [
         "base_input_stream_test.cc",
         "byte_utils_test.cc",
+        "stream_writer_test.cc",
     ],
     deps = [
         ":base",
diff --git a/internal/platform/base_input_stream.cc b/internal/platform/base_input_stream.cc
index 7392ce9b..3335ea1b 100644
--- a/internal/platform/base_input_stream.cc
+++ b/internal/platform/base_input_stream.cc
@@ -39,7 +39,7 @@ ExceptionOr<ByteArray> BaseInputStream::Read(std::int64_t size) {
 }
 
 std::optional<std::uint8_t> BaseInputStream::ReadBits(int bits) {
-  if (bits > 8) {
+  if (bits > 8 || bits <= 0) {
     return std::nullopt;
   }
   if (bits_unused_ == 0) {
diff --git a/internal/platform/stream_writer.cc b/internal/platform/stream_writer.cc
new file mode 100644
index 00000000..a6971fc8
--- /dev/null
+++ b/internal/platform/stream_writer.cc
@@ -0,0 +1,126 @@
+// Copyright 2025 Google LLC
+//
+// Licensed 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
+//
+//     https://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.
+
+#include "internal/platform/stream_writer.h"
+
+#include <cstdint>
+#include <string>
+
+#include "internal/platform/exception.h"
+
+namespace nearby {
+
+std::string StreamWriter::GetData() {
+  if (bits_used_ != 0) {
+    return buffer_ + std::string(1, bits_buffer_);
+  }
+  return buffer_;
+}
+
+Exception StreamWriter::WriteBits(uint8_t value, int bits) {
+  if (bits >= 8 || bits <= 0 || 8 - bits < bits_used_) {
+    return Exception{Exception::kInvalidProtocolBuffer};
+  }
+
+  bits_buffer_ |= (value & ((1 << bits) - 1)) << (8 - bits - bits_used_);
+  bits_used_ += bits;
+
+  if (bits_used_ == 8) {
+    buffer_.push_back(bits_buffer_);
+    bits_used_ = 0;
+    bits_buffer_ = 0;
+  }
+
+  return Exception{Exception::kSuccess};
+}
+
+Exception StreamWriter::WriteUint8(uint8_t value) {
+  if (bits_used_ != 0) {
+    return Exception{Exception::kInvalidProtocolBuffer};
+  }
+
+  buffer_.push_back(value);
+  return Exception{Exception::kSuccess};
+}
+
+Exception StreamWriter::WriteInt8(int8_t value) {
+  return WriteUint8(static_cast<uint8_t>(value));
+}
+
+Exception StreamWriter::WriteUint16(uint16_t value) {
+  if (bits_used_ != 0) {
+    return Exception{Exception::kInvalidProtocolBuffer};
+  }
+
+  char bytes[2];
+  bytes[0] = (value >> 8) & 0xff;
+  bytes[1] = value & 0xff;
+  buffer_.append(bytes, 2);
+  return Exception{Exception::kSuccess};
+}
+
+Exception StreamWriter::WriteInt16(int16_t value) {
+  return WriteUint16(static_cast<uint16_t>(value));
+}
+
+Exception StreamWriter::WriteUint32(uint32_t value) {
+  if (bits_used_ != 0) {
+    return Exception{Exception::kInvalidProtocolBuffer};
+  }
+
+  char bytes[4];
+  bytes[0] = (value >> 24) & 0xff;
+  bytes[1] = (value >> 16) & 0xff;
+  bytes[2] = (value >> 8) & 0xff;
+  bytes[3] = value & 0xff;
+  buffer_.append(bytes, 4);
+  return Exception{Exception::kSuccess};
+}
+
+Exception StreamWriter::WriteInt32(int32_t value) {
+  return WriteUint32(static_cast<uint32_t>(value));
+}
+
+Exception StreamWriter::WriteUint64(uint64_t value) {
+  if (bits_used_ != 0) {
+    return Exception{Exception::kInvalidProtocolBuffer};
+  }
+
+  char bytes[8];
+  bytes[0] = (value >> 56) & 0xff;
+  bytes[1] = (value >> 48) & 0xff;
+  bytes[2] = (value >> 40) & 0xff;
+  bytes[3] = (value >> 32) & 0xff;
+  bytes[4] = (value >> 24) & 0xff;
+  bytes[5] = (value >> 16) & 0xff;
+  bytes[6] = (value >> 8) & 0xff;
+  bytes[7] = value & 0xff;
+  buffer_.append(bytes, 8);
+  return Exception{Exception::kSuccess};
+}
+
+Exception StreamWriter::WriteInt64(int64_t value) {
+  return WriteUint64(static_cast<uint64_t>(value));
+}
+
+Exception StreamWriter::WriteBytes(const std::string& data) {
+  if (bits_used_ != 0) {
+    return Exception{Exception::kInvalidProtocolBuffer};
+  }
+
+  buffer_.append(data);
+  return Exception{Exception::kSuccess};
+}
+
+}  // namespace nearby
diff --git a/internal/platform/stream_writer.h b/internal/platform/stream_writer.h
new file mode 100644
index 00000000..1f94a405
--- /dev/null
+++ b/internal/platform/stream_writer.h
@@ -0,0 +1,55 @@
+// Copyright 2025 Google LLC
+//
+// Licensed 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
+//
+//     https://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.
+
+#ifndef THIRD_PARTY_NEARBY_INTERNAL_PLATFORM_STREAM_WRITER_H_
+#define THIRD_PARTY_NEARBY_INTERNAL_PLATFORM_STREAM_WRITER_H_
+
+#include <cstdint>
+#include <string>
+
+#include "internal/platform/exception.h"
+
+namespace nearby {
+
+// A base {@link StreamWriter } for writing the contents of a byte array.
+// All internal data is stored in network order.
+class StreamWriter {
+ public:
+  StreamWriter() = default;
+  ~StreamWriter() = default;
+
+  std::string GetData();
+
+  // Writes less than 8 bits to the stream. The written bits cannot across a
+  // byte boundary.
+  Exception WriteBits(uint8_t value, int bits);
+  Exception WriteUint8(uint8_t value);
+  Exception WriteInt8(int8_t value);
+  Exception WriteUint16(uint16_t value);
+  Exception WriteInt16(int16_t value);
+  Exception WriteUint32(uint32_t value);
+  Exception WriteInt32(int32_t value);
+  Exception WriteUint64(uint64_t value);
+  Exception WriteInt64(int64_t value);
+  Exception WriteBytes(const std::string& data);
+
+ private:
+  uint8_t bits_used_{0};
+  uint8_t bits_buffer_{0};
+  std::string buffer_;
+};
+
+}  // namespace nearby
+
+#endif  // THIRD_PARTY_NEARBY_INTERNAL_PLATFORM_STREAM_WRITER_H_
diff --git a/internal/platform/stream_writer_test.cc b/internal/platform/stream_writer_test.cc
new file mode 100644
index 00000000..d85e877e
--- /dev/null
+++ b/internal/platform/stream_writer_test.cc
@@ -0,0 +1,87 @@
+// Copyright 2025 Google LLC
+//
+// Licensed 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
+//
+//     https://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.
+
+#include "internal/platform/stream_writer.h"
+
+#include <string>
+
+#include "gtest/gtest.h"
+#include "internal/platform/exception.h"
+
+namespace nearby {
+namespace {
+
+TEST(StreamWriterTest, WriteBits) {
+  StreamWriter writer;
+  EXPECT_EQ(writer.WriteBits(0x01, 1), Exception{Exception::kSuccess});
+  EXPECT_EQ(writer.WriteBits(0x01, 1), Exception{Exception::kSuccess});
+  EXPECT_EQ(writer.WriteBits(0x01, 1), Exception{Exception::kSuccess});
+  EXPECT_EQ(writer.WriteBits(0x00, 1), Exception{Exception::kSuccess});
+  EXPECT_EQ(writer.GetData(), std::string("\xe0"));
+  EXPECT_EQ(writer.WriteBits(0x01, 1), Exception{Exception::kSuccess});
+  EXPECT_EQ(writer.WriteBits(0x01, 1), Exception{Exception::kSuccess});
+  EXPECT_EQ(writer.WriteBits(0x01, 1), Exception{Exception::kSuccess});
+  EXPECT_EQ(writer.WriteBits(0x01, 1), Exception{Exception::kSuccess});
+  EXPECT_EQ(writer.GetData(), std::string("\xef"));
+}
+
+TEST(StreamWriterTest, WriteInvalidBits) {
+  StreamWriter writer;
+  EXPECT_EQ(writer.WriteBits(0x01, 1), Exception{Exception::kSuccess});
+  EXPECT_EQ(writer.WriteBits(0x01, 9),
+            Exception{Exception::kInvalidProtocolBuffer});
+  EXPECT_EQ(writer.WriteBits(0x03, 4), Exception{Exception::kSuccess});
+  EXPECT_EQ(writer.WriteBits(0x01, 5),
+            Exception{Exception::kInvalidProtocolBuffer});
+}
+
+TEST(StreamWriterTest, WriteIntegerValues) {
+  StreamWriter writer;
+  EXPECT_EQ(writer.WriteUint8(0x01), Exception{Exception::kSuccess});
+  EXPECT_EQ(writer.WriteInt8(0xf1), Exception{Exception::kSuccess});
+  EXPECT_EQ(writer.WriteUint16(0x0102), Exception{Exception::kSuccess});
+  EXPECT_EQ(writer.WriteInt16(0xf102), Exception{Exception::kSuccess});
+  EXPECT_EQ(writer.GetData(), "\x01\xF1\x01\x02\xF1\x02");
+  EXPECT_EQ(writer.WriteUint32(0x01020304), Exception{Exception::kSuccess});
+  EXPECT_EQ(writer.WriteInt32(0xf1020304), Exception{Exception::kSuccess});
+  writer = {};
+  EXPECT_EQ(writer.WriteUint64(0x0102030405060708),
+            Exception{Exception::kSuccess});
+  EXPECT_EQ(writer.WriteInt64(0xf102030405060708),
+            Exception{Exception::kSuccess});
+  EXPECT_EQ(writer.GetData(),
+            "\x01\x02\x03\x04\x05\x06\x07\x08\xF1\x02\x03\x04\x05\x06\x07\x08");
+}
+
+TEST(StreamWriterTest, InvalidWriteIntegerValues) {
+  StreamWriter writer;
+  EXPECT_EQ(writer.WriteBits(0x01, 1), Exception{Exception::kSuccess});
+  EXPECT_EQ(writer.WriteUint8(0x01),
+            Exception{Exception::kInvalidProtocolBuffer});
+  EXPECT_EQ(writer.WriteUint16(0x0102),
+            Exception{Exception::kInvalidProtocolBuffer});
+  EXPECT_EQ(writer.WriteUint32(0x01020304),
+            Exception{Exception::kInvalidProtocolBuffer});
+  EXPECT_EQ(writer.WriteUint64(0x0102030405060708),
+            Exception{Exception::kInvalidProtocolBuffer});
+}
+
+TEST(StreamWriterTest, WriteBytes) {
+  StreamWriter writer;
+  EXPECT_EQ(writer.WriteBytes("test"), Exception{Exception::kSuccess});
+  EXPECT_EQ(writer.GetData(), std::string("test"));
+}
+
+}  // namespace
+}  // namespace nearby