diff --git a/CMakeLists.txt b/CMakeLists.txt
index f70def7..1990481 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -45,6 +45,8 @@ set(SOURCES
   compression/compress.cc
   compression/compress.h
   compression/compress-inl.h
+  compression/fields.cc
+  compression/fields.h
   compression/io_win.cc
   compression/io.cc
   compression/io.h
@@ -150,6 +152,7 @@ set(GEMMA_TEST_FILES
   compression/blob_store_test.cc
   compression/compress_test.cc
   compression/distortion_test.cc
+  compression/fields_test.cc
   compression/nuq_test.cc
   compression/sfp_test.cc
   evals/gemma_test.cc
diff --git a/compression/BUILD.bazel b/compression/BUILD.bazel
index b6508cd..832f635 100644
--- a/compression/BUILD.bazel
+++ b/compression/BUILD.bazel
@@ -37,6 +37,26 @@ cc_library(
     ] + FILE_DEPS,
 )
 
+cc_library(
+    name = "fields",
+    srcs = ["fields.cc"],
+    hdrs = ["fields.h"],
+    deps = [
+        "@highway//:hwy",
+    ],
+)
+
+cc_test(
+    name = "fields_test",
+    srcs = ["fields_test.cc"],
+    deps = [
+        ":fields",
+        "@googletest//:gtest_main",  # buildcleaner: keep
+        "@highway//:hwy",
+        "@highway//:hwy_test_util",
+    ],
+)
+
 cc_library(
     name = "blob_store",
     srcs = ["blob_store.cc"],
diff --git a/compression/fields.cc b/compression/fields.cc
new file mode 100644
index 0000000..de90ec8
--- /dev/null
+++ b/compression/fields.cc
@@ -0,0 +1,320 @@
+// Copyright 2024 Google LLC
+// SPDX-License-Identifier: Apache-2.0
+//
+// 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 "compression/fields.h"
+
+#include <stdarg.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <string.h>
+
+#include <string>
+#include <vector>
+
+#include "hwy/aligned_allocator.h"
+#include "hwy/base.h"
+
+namespace gcpp {
+
+IFieldsVisitor::~IFieldsVisitor() = default;
+
+void IFieldsVisitor::NotifyInvalid(const char* fmt, ...) {
+  va_list args;
+  va_start(args, fmt);
+  vfprintf(stderr, fmt, args);
+  va_end(args);
+
+  any_invalid_ = true;
+}
+
+class VisitorBase : public IFieldsVisitor {
+ public:
+  VisitorBase() = default;
+  ~VisitorBase() override = default;
+
+  // This is the only call site of IFields::VisitFields.
+  void operator()(IFields& fields) override { fields.VisitFields(*this); }
+
+ protected:  // Functions shared between ReadVisitor and WriteVisitor:
+  void CheckF32(float value) {
+    if (HWY_UNLIKELY(hwy::ScalarIsInf(value) || hwy::ScalarIsNaN(value))) {
+      NotifyInvalid("Invalid float %g\n", value);
+    }
+  }
+
+  // Return bool to avoid having to check AnyInvalid() after calling.
+  bool CheckStringLength(uint32_t num_u32) {
+    // Disallow long strings for safety, and to prevent them being used for
+    // arbitrary data (we also require them to be ASCII).
+    if (HWY_UNLIKELY(num_u32 > 64)) {
+      NotifyInvalid("String num_u32=%u too large\n", num_u32);
+      return false;
+    }
+    return true;
+  }
+
+  bool CheckStringU32(uint32_t u32, uint32_t i, uint32_t num_u32) {
+    // Although strings are zero-padded to u32, an entire u32 should not be
+    // zero, and upper bits should not be set (ASCII-only).
+    if (HWY_UNLIKELY(u32 == 0 || (u32 & 0x80808080))) {
+      NotifyInvalid("Invalid characters %x at %u of %u\n", u32, i, num_u32);
+      return false;
+    }
+    return true;
+  }
+};
+
+class PrintVisitor : public VisitorBase {
+ public:
+  void operator()(uint32_t& value) override {
+    fprintf(stderr, "%sU32 %u\n", indent_.c_str(), value);
+  }
+
+  void operator()(float& value) override {
+    fprintf(stderr, "%sF32 %f\n", indent_.c_str(), value);
+  }
+
+  void operator()(std::string& value) override {
+    fprintf(stderr, "%sStr %s\n", indent_.c_str(), value.c_str());
+  }
+
+  void operator()(IFields& fields) override {
+    fprintf(stderr, "%s%s\n", indent_.c_str(), fields.Name());
+    indent_ += "  ";
+
+    VisitorBase::operator()(fields);
+
+    HWY_ASSERT(!indent_.empty());
+    indent_.resize(indent_.size() - 2);
+  }
+
+ private:
+  std::string indent_;
+};
+
+class ReadVisitor : public VisitorBase {
+ public:
+  ReadVisitor(const hwy::Span<const uint32_t>& span, size_t pos)
+      : span_(span), result_(pos) {}
+  ~ReadVisitor() {
+    HWY_ASSERT(end_.empty());  // Bug if push/pop are not balanced.
+  }
+
+  // All data is read through this overload.
+  void operator()(uint32_t& value) override {
+    if (HWY_UNLIKELY(SkipField())) return;
+
+    value = span_[result_.pos++];
+  }
+
+  void operator()(float& value) override {
+    if (HWY_UNLIKELY(SkipField())) return;
+
+    uint32_t u32 = hwy::BitCastScalar<uint32_t>(value);
+    operator()(u32);
+    value = hwy::BitCastScalar<float>(u32);
+    CheckF32(value);
+  }
+
+  void operator()(std::string& value) override {
+    if (HWY_UNLIKELY(SkipField())) return;
+
+    uint32_t num_u32;     // not including itself because this..
+    operator()(num_u32);  // increments result_.pos for the num_u32 field
+    if (HWY_UNLIKELY(!CheckStringLength(num_u32))) return;
+
+    // Ensure we have that much data.
+    if (HWY_UNLIKELY(result_.pos + num_u32 > end_.back())) {
+      NotifyInvalid("Invalid string: pos %zu + num_u32 %u > end %zu\n",
+                    result_.pos, num_u32, span_.size());
+      return;
+    }
+
+    constexpr size_t k4 = sizeof(uint32_t);
+    value.resize(num_u32 * k4);
+    for (uint32_t i = 0; i < num_u32; ++i) {
+      uint32_t u32;
+      operator()(u32);
+      (void)CheckStringU32(u32, i, num_u32);
+      hwy::CopyBytes(&u32, value.data() + i * k4, k4);
+    }
+
+    // Trim 0..3 trailing nulls.
+    const size_t pos = value.find_last_not_of('\0');
+    if (pos != std::string::npos) {
+      value.resize(pos + 1);
+    }
+  }
+
+  void operator()(IFields& fields) override {
+    // Our SkipField requires end_ to be set before reading num_u32, which
+    // determines the actual end, so use an upper bound which is tight if this
+    // IFields is last one in span_.
+    end_.push_back(span_.size());
+
+    if (HWY_UNLIKELY(SkipField())) {
+      end_.pop_back();  // undo `push_back` to keep the stack balanced
+      return;
+    }
+
+    uint32_t num_u32;     // not including itself because this..
+    operator()(num_u32);  // increments result_.pos for the num_u32 field
+
+    // Ensure we have that much data and set end_.
+    if (HWY_UNLIKELY(result_.pos + num_u32 > span_.size())) {
+      NotifyInvalid("Invalid IFields: pos %zu + num_u32 %u > size %zu\n",
+                    result_.pos, num_u32, span_.size());
+      return;
+    }
+    end_.back() = result_.pos + num_u32;
+
+    VisitorBase::operator()(fields);
+
+    HWY_ASSERT(!end_.empty() && result_.pos <= end_.back());
+    // Count extra, which indicates old code and new data.
+    result_.extra_u32 += end_.back() - result_.pos;
+    end_.pop_back();
+  }
+
+  // Override because ReadVisitor also does bounds checking.
+  bool SkipField() override {
+    // If invalid, all bets are off and we don't count missing fields.
+    if (HWY_UNLIKELY(AnyInvalid())) return true;
+
+    // Reaching the end of the stored size, or the span, is not invalid -
+    // it happens when we read old data with new code.
+    if (HWY_UNLIKELY(result_.pos >= end_.back())) {
+      result_.missing_fields++;
+      return true;
+    }
+
+    return false;
+  }
+
+  // Override so that operator()(std::vector<T>&) resizes the vector.
+  bool IsReading() const override { return true; }
+
+  IFields::ReadResult Result() {
+    if (HWY_UNLIKELY(AnyInvalid())) result_.pos = 0;
+    return result_;
+  }
+
+ private:
+  const hwy::Span<const uint32_t> span_;
+  IFields::ReadResult result_;
+  // Stack of end positions of nested IFields. Updated in operator()(IFields&),
+  // but read in SkipField.
+  std::vector<uint32_t> end_;
+};
+
+class WriteVisitor : public VisitorBase {
+ public:
+  WriteVisitor(std::vector<uint32_t>& storage) : storage_(storage) {}
+
+  // Note: while writing, only string lengths/characters can trigger AnyInvalid,
+  // so we don't have to check SkipField.
+
+  void operator()(uint32_t& value) override { storage_.push_back(value); }
+
+  void operator()(float& value) override {
+    storage_.push_back(hwy::BitCastScalar<uint32_t>(value));
+    CheckF32(value);
+  }
+
+  void operator()(std::string& value) override {
+    constexpr size_t k4 = sizeof(uint32_t);
+
+    // Write length.
+    uint32_t num_u32 = hwy::DivCeil(value.size(), k4);
+    if (HWY_UNLIKELY(!CheckStringLength(num_u32))) return;
+    operator()(num_u32);  // always valid
+
+    // Copy whole uint32_t.
+    const size_t num_whole_u32 = value.size() / k4;
+    for (uint32_t i = 0; i < num_whole_u32; ++i) {
+      uint32_t u32 = 0;
+      hwy::CopyBytes(value.data() + i * k4, &u32, k4);
+      if (HWY_UNLIKELY(!CheckStringU32(u32, i, num_u32))) return;
+      storage_.push_back(u32);
+    }
+
+    // Read remaining bytes into least-significant bits of u32.
+    const size_t remainder = value.size() - num_whole_u32 * k4;
+    if (remainder != 0) {
+      HWY_DASSERT(remainder < k4);
+      uint32_t u32 = 0;
+      for (size_t i = 0; i < remainder; ++i) {
+        const char c = value[num_whole_u32 * k4 + i];
+        const uint32_t next = static_cast<uint32_t>(static_cast<uint8_t>(c));
+        u32 += next << (i * 8);
+      }
+      if (HWY_UNLIKELY(!CheckStringU32(u32, num_whole_u32, num_u32))) return;
+      storage_.push_back(u32);
+    }
+  }
+
+  void operator()(IFields& fields) override {
+    const size_t pos_before_size = storage_.size();
+    storage_.push_back(0);  // placeholder, updated below
+
+    VisitorBase::operator()(fields);
+
+    HWY_ASSERT(storage_[pos_before_size] == 0);
+    // Number of u32 written, including the one storing that number.
+    const uint32_t num_u32 = storage_.size() - pos_before_size;
+    HWY_ASSERT(num_u32 != 0);  // at least one due to push_back above
+    // Store the payload size, not including the num_u32 field itself, because
+    // that is more convenient for ReadVisitor.
+    storage_[pos_before_size] = num_u32 - 1;
+  }
+
+ private:
+  std::vector<uint32_t>& storage_;
+};
+
+IFields::~IFields() = default;
+
+void IFields::Print() const {
+  PrintVisitor visitor;
+  // VisitFields is non-const. It is safe to cast because PrintVisitor does not
+  // modify the fields.
+  visitor(*const_cast<IFields*>(this));
+}
+
+IFields::ReadResult IFields::Read(const hwy::Span<const uint32_t>& span,
+                                  size_t pos) {
+  ReadVisitor visitor(span, pos);
+  visitor(*this);
+  return visitor.Result();
+}
+
+bool IFields::AppendTo(std::vector<uint32_t>& storage) const {
+  // VisitFields is non-const. It is safe to cast because WriteVisitor does not
+  // modify the fields.
+  IFields& fields = *const_cast<IFields*>(this);
+
+  // Reduce allocations, but not in debug builds so we notice any iterator
+  // invalidation bugs.
+  if constexpr (!HWY_IS_DEBUG_BUILD) {
+    storage.reserve(storage.size() + 256);
+  }
+
+  WriteVisitor visitor(storage);
+  visitor(fields);
+  return !visitor.AnyInvalid();
+}
+
+}  // namespace gcpp
diff --git a/compression/fields.h b/compression/fields.h
new file mode 100644
index 0000000..2ee409a
--- /dev/null
+++ b/compression/fields.h
@@ -0,0 +1,200 @@
+// Copyright 2024 Google LLC
+// SPDX-License-Identifier: Apache-2.0
+//
+// 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_GEMMA_CPP_COMPRESSION_FIELDS_H_
+#define THIRD_PARTY_GEMMA_CPP_COMPRESSION_FIELDS_H_
+
+// Simple serialization/deserialization for user-defined classes, inspired by
+// BSD-licensed code Copyright (c) the JPEG XL Project Authors:
+// https://github.com/libjxl/libjxl, lib/jxl/fields.h.
+
+// IWYU pragma: begin_exports
+#include <stddef.h>
+#include <stdint.h>
+
+#include <string>
+#include <vector>
+
+#include "hwy/aligned_allocator.h"
+#include "hwy/base.h"
+// IWYU pragma: end_exports
+
+namespace gcpp {
+
+// Design goals:
+// - self-contained to simplify installing/building, no separate compiler (rules
+//   out Protocol Buffers, FlatBuffers, Cap'n Proto, Apache Thrift).
+// - simplicity: small codebase without JIT (rules out Apache Fury and bitsery).
+// - old code can read new data, and new code can read old data (rules out yas
+//   and msgpack). This avoids rewriting weights when we add a new field.
+// - no user-specified versions (rules out cereal) nor field names (rules out
+//   JSON and GGUF). These are error-prone; users should just be able to append
+//   new fields.
+//
+// Non-goals:
+// - anything better than reasonable encoding size and decode speed: we only
+//   anticipate ~KiB of data, alongside ~GiB of separately compressed weights.
+// - features such as maps, interfaces, random access, and optional/deleted
+//   fields: not required for the intended use case of `ModelConfig`.
+// - support any other languages than C++ and Python (for the exporter).
+
+class IFields;  // breaks circular dependency
+
+// Visitors are internal-only, but their base class is visible to user code
+// because their `IFields::VisitFields` calls `visitor.operator()`.
+//
+// Supported field types `T`: `uint32_t`, `float`, `std::string`, classes
+// derived from `IFields`, `bool`, `enum`, `std::vector<T>`.
+class IFieldsVisitor {
+ public:
+  virtual ~IFieldsVisitor();
+
+  // Indicates whether NotifyInvalid was called for any field. Once set, this is
+  // sticky for all IFields visited by this visitor.
+  bool AnyInvalid() const { return any_invalid_; }
+
+  // None of these fail directly, but they call NotifyInvalid() if any value
+  // is out of range. A single generic/overloaded function is required to
+  // support `std::vector<T>`.
+  virtual void operator()(uint32_t& value) = 0;
+  virtual void operator()(float& value) = 0;
+  virtual void operator()(std::string& value) = 0;
+  virtual void operator()(IFields& fields) = 0;  // recurse into nested fields
+
+  // bool and enum fields are actually stored as uint32_t.
+  void operator()(bool& value) {
+    if (HWY_UNLIKELY(SkipField())) return;
+
+    uint32_t u32 = value ? 1 : 0;
+    operator()(u32);
+    if (HWY_UNLIKELY(u32 > 1)) {
+      return NotifyInvalid("Invalid bool %u\n", u32);
+    }
+    value = (u32 == 1);
+  }
+
+  template <typename EnumT, hwy::EnableIf<std::is_enum_v<EnumT>>* = nullptr>
+  void operator()(EnumT& value) {
+    if (HWY_UNLIKELY(SkipField())) return;
+
+    uint32_t u32 = static_cast<uint32_t>(value);
+    operator()(u32);
+    if (HWY_UNLIKELY(!EnumValid(static_cast<EnumT>(u32)))) {
+      return NotifyInvalid("Invalid enum %u\n");
+    }
+    value = static_cast<EnumT>(u32);
+  }
+
+  template <typename T>
+  void operator()(std::vector<T>& value) {
+    if (HWY_UNLIKELY(SkipField())) return;
+
+    uint32_t num = static_cast<uint32_t>(value.size());
+    operator()(num);
+    if (HWY_UNLIKELY(num > 64 * 1024)) {
+      return NotifyInvalid("Vector too long %u\n", num);
+    }
+
+    if (IsReading()) {
+      value.resize(num);
+    }
+    for (size_t i = 0; i < value.size(); ++i) {
+      operator()(value[i]);
+    }
+  }
+
+ protected:
+  // Prints a message and causes subsequent AnyInvalid() to return true.
+  void NotifyInvalid(const char* fmt, ...);
+
+  // Must check this before modifying any field, and if it returns true,
+  // avoid doing so. This is important for strings and vectors in the
+  // "new code, old data" test: resizing them may destroy their contents.
+  virtual bool SkipField() { return AnyInvalid(); }
+  // For operator()(std::vector&).
+  virtual bool IsReading() const { return false; }
+
+ private:
+  bool any_invalid_ = false;
+};
+
+// Abstract base class for user-defined serializable classes, which are
+// forward- and backward compatible collection of fields (members). This means
+// old code can safely read new data, and new code can still handle old data.
+//
+// Fields are written in the unchanging order established by the user-defined
+// `VisitFields`; any new fields must be visited after all existing fields in
+// the same `IFields`. We encode each into `uint32_t` storage for simplicity.
+//
+// HOWTO:
+// - basic usage: define a struct with member variables ("fields") and their
+//   initializers, e.g. `uint32_t field = 0;`. Then define a
+//   `void VisitFields(IFieldsVisitor& v)` member function that calls
+//   `v(field);` etc. for each field, and a `const char* Name()` function used
+//   as a caption when printing.
+//
+// - enum fields: define `enum class EnumT` and `bool EnumValid(EnumT)`, then
+//   call `v(field);` as usual. Note that `EnumT` is not extendable insofar as
+//   `EnumValid` returns false for values beyond the initially known ones. You
+//   can add placeholders, which requires user code to know how to handle them,
+//   or later add new fields including enums to override the first enum.
+struct IFields {
+  virtual ~IFields();
+
+  // User-defined caption used during Print().
+  virtual const char* Name() const = 0;
+
+  // User-defined, called by IFieldsVisitor::operator()(IFields&).
+  virtual void VisitFields(IFieldsVisitor& visitor) = 0;
+
+  // Prints name and fields to stderr.
+  void Print() const;
+
+  struct ReadResult {
+    ReadResult(size_t pos) : pos(pos), missing_fields(0), extra_u32(0) {}
+
+    // Where to resume reading in the next Read() call, or 0 if there was an
+    // unrecoverable error: any field has an invalid value, or the span is
+    // shorter than the data says it should be. If so, do not use the fields nor
+    // continue reading.
+    size_t pos;
+    // From the perspective of VisitFields, how many more fields would have
+    // been read beyond the stored size. If non-zero, the data is older than
+    // the code, but valid, and extra_u32 should be zero.
+    uint32_t missing_fields;
+    // How many extra u32 are in the stored size, vs. what we actually read as
+    // requested by VisitFields. If non-zero,, the data is newer than the code,
+    // but valid, and missing_fields should be zero.
+    uint32_t extra_u32;
+  };
+
+  // Reads fields starting at `span[pos]`.
+  ReadResult Read(const hwy::Span<const uint32_t>& span, size_t pos);
+
+  // Returns false if there was an unrecoverable error, typically because a
+  // field has an invalid value. If so, `storage` is undefined.
+  bool AppendTo(std::vector<uint32_t>& storage) const;
+
+  // Convenience wrapper for AppendTo when we only write once.
+  std::vector<uint32_t> Write() const {
+    std::vector<uint32_t> storage;
+    if (!AppendTo(storage)) storage.clear();
+    return storage;
+  }
+};
+
+}  // namespace gcpp
+
+#endif  // THIRD_PARTY_GEMMA_CPP_COMPRESSION_FIELDS_H_
diff --git a/compression/fields_test.cc b/compression/fields_test.cc
new file mode 100644
index 0000000..d6c1c51
--- /dev/null
+++ b/compression/fields_test.cc
@@ -0,0 +1,354 @@
+// Copyright 2024 Google LLC
+// SPDX-License-Identifier: Apache-2.0
+//
+// 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
+//
+//      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.
+
+#include "compression/fields.h"
+
+#include <stddef.h>
+#include <stdint.h>
+#include <stdio.h>
+
+#include <limits>
+#include <type_traits>
+
+#include "hwy/tests/hwy_gtest.h"
+
+namespace gcpp {
+namespace {
+
+#if !HWY_TEST_STANDALONE
+class FieldsTest : public testing::Test {};
+#endif
+
+void MaybePrint(const IFields& fields) {
+  if (HWY_IS_DEBUG_BUILD) {
+    fields.Print();
+  }
+}
+
+template <typename T>
+void CheckVectorEqual(const std::vector<T>& a, const std::vector<T>& b) {
+  EXPECT_EQ(a.size(), b.size());
+  for (size_t i = 0; i < a.size(); ++i) {
+    if constexpr (std::is_base_of_v<IFields, T>) {
+      a[i].CheckEqual(b[i]);
+    } else {
+      EXPECT_EQ(a[i], b[i]);
+    }
+  }
+}
+
+enum class Enum : uint32_t {
+  k1 = 1,
+  k3 = 3,
+  k8 = 8,
+};
+HWY_MAYBE_UNUSED bool EnumValid(Enum e) {
+  return e == Enum::k1 || e == Enum::k3 || e == Enum::k8;
+}
+
+// Contains all supported types except IFields and std::vector<IFields>.
+struct Nested : public IFields {
+  Nested() : nested_u32(0) {}  // for std::vector
+  explicit Nested(uint32_t u32) : nested_u32(u32) {}
+
+  const char* Name() const override { return "Nested"; }
+  void VisitFields(IFieldsVisitor& visitor) override {
+    visitor(nested_u32);
+    visitor(nested_bool);
+    visitor(nested_vector);
+    visitor(nested_enum);
+    visitor(nested_str);
+    visitor(nested_f);
+  }
+
+  void CheckEqual(const Nested& n) const {
+    EXPECT_EQ(nested_u32, n.nested_u32);
+    EXPECT_EQ(nested_bool, n.nested_bool);
+    CheckVectorEqual(nested_vector, n.nested_vector);
+    EXPECT_EQ(nested_enum, n.nested_enum);
+    EXPECT_EQ(nested_str, n.nested_str);
+    EXPECT_EQ(nested_f, n.nested_f);
+  }
+
+  uint32_t nested_u32;  // set in ctor
+  bool nested_bool = true;
+  std::vector<uint32_t> nested_vector = {1, 2, 3};
+  Enum nested_enum = Enum::k1;
+  std::string nested_str = "nested";
+  float nested_f = 1.125f;
+};
+
+// Contains all supported types.
+struct OldFields : public IFields {
+  const char* Name() const override { return "OldFields"; }
+  void VisitFields(IFieldsVisitor& visitor) override {
+    visitor(old_str);
+    visitor(old_nested);
+    visitor(old1);
+    visitor(old_vec_str);
+    visitor(old_vec_nested);
+    visitor(old_f);
+    visitor(old_enum);
+    visitor(old_bool);
+  }
+
+  // Template allows comparing with NewFields.
+  template <typename Other>
+  void CheckEqual(const Other& n) const {
+    EXPECT_EQ(old_str, n.old_str);
+    old_nested.CheckEqual(n.old_nested);
+    EXPECT_EQ(old1, n.old1);
+    CheckVectorEqual(old_vec_str, n.old_vec_str);
+    CheckVectorEqual(old_vec_nested, n.old_vec_nested);
+    EXPECT_EQ(old_f, n.old_f);
+    EXPECT_EQ(old_enum, n.old_enum);
+    EXPECT_EQ(old_bool, n.old_bool);
+  }
+
+  std::string old_str = "old";
+  Nested old_nested = Nested(0);
+  uint32_t old1 = 1;
+  std::vector<std::string> old_vec_str = {"abc", "1234"};
+  std::vector<Nested> old_vec_nested = {Nested(1), Nested(4)};
+  float old_f = 1.125f;
+  Enum old_enum = Enum::k1;
+  bool old_bool = true;
+};  // OldFields
+
+// Simulates adding new fields of all types to an existing struct.
+struct NewFields : public IFields {
+  const char* Name() const override { return "NewFields"; }
+  void VisitFields(IFieldsVisitor& visitor) override {
+    visitor(old_str);
+    visitor(old_nested);
+    visitor(old1);
+    visitor(old_vec_str);
+    visitor(old_vec_nested);
+    visitor(old_f);
+    visitor(old_enum);
+    visitor(old_bool);
+
+    // Change order of field types relative to OldFields to ensure that works.
+    visitor(new_nested);
+    visitor(new_bool);
+    visitor(new_vec_nested);
+    visitor(new_f);
+    visitor(new_vec_str);
+    visitor(new_enum);
+    visitor(new2);
+    visitor(new_str);
+  }
+
+  void CheckEqual(const NewFields& n) const {
+    EXPECT_EQ(old_str, n.old_str);
+    old_nested.CheckEqual(n.old_nested);
+    EXPECT_EQ(old1, n.old1);
+    CheckVectorEqual(old_vec_str, n.old_vec_str);
+    CheckVectorEqual(old_vec_nested, n.old_vec_nested);
+    EXPECT_EQ(old_f, n.old_f);
+    EXPECT_EQ(old_enum, n.old_enum);
+    EXPECT_EQ(old_bool, n.old_bool);
+
+    new_nested.CheckEqual(n.new_nested);
+    EXPECT_EQ(new_bool, n.new_bool);
+    CheckVectorEqual(new_vec_nested, n.new_vec_nested);
+    EXPECT_EQ(new_f, n.new_f);
+    CheckVectorEqual(new_vec_str, n.new_vec_str);
+    EXPECT_EQ(new_enum, n.new_enum);
+    EXPECT_EQ(new2, n.new2);
+    EXPECT_EQ(new_str, n.new_str);
+  }
+
+  // Copied from OldFields to match the use case of adding new fields. If we
+  // write an OldFields member, that would change the layout due to its size.
+  std::string old_str = "old";
+  Nested old_nested = Nested(0);
+  uint32_t old1 = 1;
+  std::vector<std::string> old_vec_str = {"abc", "1234"};
+  std::vector<Nested> old_vec_nested = {Nested(1), Nested(4)};
+  float old_f = 1.125f;
+  Enum old_enum = Enum::k1;
+  bool old_bool = true;
+
+  Nested new_nested = Nested(999);
+  bool new_bool = false;
+  std::vector<Nested> new_vec_nested = {Nested(2), Nested(3)};
+  float new_f = -2.0f;
+  std::vector<std::string> new_vec_str = {"AB", std::string(), "56789"};
+  Enum new_enum = Enum::k3;
+  uint32_t new2 = 2;
+  std::string new_str = std::string();  // empty is allowed
+};  // NewFields
+
+// Changes all fields to non-default values.
+NewFields ModifiedNewFields() {
+  NewFields n;
+  n.old_str = "old2";
+  n.old_nested = Nested(5);
+  n.old1 = 11;
+  n.old_vec_str = {"abc2", "431", "ZZ"};
+  n.old_vec_nested = {Nested(9)};
+  n.old_f = -2.5f;
+  n.old_enum = Enum::k3;
+  n.old_bool = false;
+
+  n.new_nested = Nested(55);
+  n.new_bool = true;
+  n.new_vec_nested = {Nested(3), Nested(33), Nested(333)};
+  n.new_f = 4.f;
+  n.new_vec_str = {"4321", "321", "21", "1"};
+  n.new_enum = Enum::k8;
+  n.new2 = 22;
+  n.new_str = "new and even longer";
+
+  return n;
+}
+
+using Span = hwy::Span<const uint32_t>;
+
+using ReadResult = IFields::ReadResult;
+void CheckConsumedAll(const ReadResult& result, size_t storage_size) {
+  EXPECT_NE(0, storage_size);  // Ensure we notice failure (pos == 0).
+  EXPECT_EQ(storage_size, result.pos);
+  EXPECT_EQ(0, result.missing_fields);
+  EXPECT_EQ(0, result.extra_u32);
+}
+
+// If we do not change any fields, Write+Read returns the defaults.
+TEST(FieldsTest, TestNewMatchesDefaults) {
+  NewFields new_fields;
+  const std::vector<uint32_t> storage = new_fields.Write();
+
+  const ReadResult result = new_fields.Read(Span(storage), 0);
+  CheckConsumedAll(result, storage.size());
+
+  NewFields().CheckEqual(new_fields);
+}
+
+// Change fields from default and check that Write+Read returns them again.
+TEST(FieldsTest, TestRoundTrip) {
+  NewFields new_fields = ModifiedNewFields();
+  const std::vector<uint32_t> storage = new_fields.Write();
+
+  NewFields copy;
+  const ReadResult result = copy.Read(Span(storage), 0);
+  CheckConsumedAll(result, storage.size());
+
+  new_fields.CheckEqual(copy);
+}
+
+// Refuse to write invalid floats.
+TEST(FieldsTest, TestInvalidFloat) {
+  NewFields new_fields;
+  new_fields.new_f = std::numeric_limits<float>::infinity();
+  EXPECT_TRUE(new_fields.Write().empty());
+
+  new_fields.new_f = std::numeric_limits<float>::quiet_NaN();
+  EXPECT_TRUE(new_fields.Write().empty());
+}
+
+// Refuse to write invalid strings.
+TEST(FieldsTest, TestInvalidString) {
+  NewFields new_fields;
+  // Four zero bytes
+  new_fields.new_str.assign(4, '\0');
+  EXPECT_TRUE(new_fields.Write().empty());
+
+  // Too long
+  new_fields.new_str.assign(257, 'a');
+  EXPECT_TRUE(new_fields.Write().empty());
+
+  // First byte not ASCII
+  new_fields.new_str.assign("123");
+  new_fields.new_str[0] = 128;
+  EXPECT_TRUE(new_fields.Write().empty());
+
+  // Upper byte in later u32 not ASCII
+  new_fields.new_str.assign("ABCDEFGH");
+  new_fields.new_str[7] = 255;
+  EXPECT_TRUE(new_fields.Write().empty());
+}
+
+// Write two structs to the same storage.
+TEST(FieldsTest, TestMultipleWrite) {
+  const NewFields modified = ModifiedNewFields();
+  std::vector<uint32_t> storage = modified.Write();
+  const size_t modified_size = storage.size();
+  const NewFields defaults;
+  defaults.AppendTo(storage);
+
+  // Start with defaults to ensure Read retrieves the modified values.
+  NewFields modified_copy;
+  const ReadResult result1 = modified_copy.Read(Span(storage), 0);
+  CheckConsumedAll(result1, modified_size);
+  modified.CheckEqual(modified_copy);
+
+  // Start with modified values to ensure Read retrieves the defaults.
+  NewFields defaults_copy = modified;
+  const ReadResult result2 = defaults_copy.Read(Span(storage), result1.pos);
+  CheckConsumedAll(result2, storage.size());
+  defaults.CheckEqual(defaults_copy);
+}
+
+// Write old defaults, read old using new code.
+TEST(FieldsTest, TestNewCodeOldData) {
+  OldFields old_fields;
+  const std::vector<uint32_t> storage = old_fields.Write();
+
+  // Start with modified old values to ensure old defaults overwrite them.
+  NewFields new_fields = ModifiedNewFields();
+  const ReadResult result = new_fields.Read(Span(storage), 0);
+  MaybePrint(new_fields);
+  EXPECT_NE(0, result.pos);  // did not fail
+  EXPECT_NE(0, result.missing_fields);
+  EXPECT_EQ(0, result.extra_u32);
+  old_fields.CheckEqual(new_fields);  // old fields are the same in both
+}
+
+// Write old defaults, ensure new defaults remain unchanged.
+TEST(FieldsTest, TestNewCodeOldDataNewUnchanged) {
+  OldFields old_fields;
+  const std::vector<uint32_t> storage = old_fields.Write();
+
+  NewFields new_fields;
+  const ReadResult result = new_fields.Read(Span(storage), 0);
+  MaybePrint(new_fields);
+  EXPECT_NE(0, result.pos);  // did not fail
+  EXPECT_NE(0, result.missing_fields);
+  EXPECT_EQ(0, result.extra_u32);
+  NewFields().CheckEqual(new_fields);  // new fields match their defaults
+}
+
+// Write new defaults, read using old code.
+TEST(FieldsTest, TestOldCodeNewData) {
+  NewFields new_fields;
+  const std::vector<uint32_t> storage = new_fields.Write();
+
+  OldFields old_fields;
+  const ReadResult result = old_fields.Read(Span(storage), 0);
+  MaybePrint(old_fields);
+  EXPECT_NE(0, result.pos);  // did not fail
+  EXPECT_EQ(0, result.missing_fields);
+  EXPECT_NE(0, result.extra_u32);
+  EXPECT_EQ(storage.size(), result.pos + result.extra_u32);
+
+  old_fields.CheckEqual(new_fields);  // old fields are the same in both
+  // (Can't check new fields because we only read OldFields)
+}
+
+}  // namespace
+}  // namespace gcpp
+
+HWY_TEST_MAIN();