diff --git a/mbo/container/BUILD.bazel b/mbo/container/BUILD.bazel
index 38c9d6488..e91c3ae6e 100644
--- a/mbo/container/BUILD.bazel
+++ b/mbo/container/BUILD.bazel
@@ -154,6 +154,30 @@ cc_library(
     ],
 )
 
+cc_library(
+    name = "hamt_replace_cc",
+    hdrs = ["internal/hamt_replace.h"],
+    deps = [
+        ":hamt_hash_path_cc",
+        ":hamt_lookup_cc",
+        ":hamt_shared_node_cc",
+        "//mbo/memory:block_source_cc",
+    ],
+)
+
+cc_test(
+    name = "hamt_replace_test",
+    size = "small",
+    srcs = ["internal/hamt_replace_test.cc"],
+    deps = [
+        ":hamt_insert_cc",
+        ":hamt_replace_cc",
+        "//mbo/memory:block_source_cc",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
 cc_test(
     name = "hamt_clone_test",
     size = "small",
diff --git a/mbo/container/HAMT_STORAGE.md b/mbo/container/HAMT_STORAGE.md
index ec13458e5..e9754c1fa 100644
--- a/mbo/container/HAMT_STORAGE.md
+++ b/mbo/container/HAMT_STORAGE.md
@@ -241,3 +241,16 @@ Child staging uses bounded stack storage rather than another allocator. Repeated
 references are copied independently; this primitive does not preserve graph aliasing or perform
 cross-domain structural sharing. Public `clone_to(source)` wrappers also preserve hash/equality
 state and container-level metadata; those responsibilities are not handled by this node primitive.
+
+## Persistent value replacement
+
+`TryReplaceHamtEntry` finds an existing key and copies only its affected path, replacing the
+stored entry without changing the original snapshot. The replacement must preserve the key
+and full hash; public map wrappers enforce immutable keys while replacing mapped values.
+Collision siblings and unaffected branches remain unchanged.
+
+Success returns an owned root and `replaced == true`. A missing key returns a retained original
+root and `replaced == false`, without allocating; a null root is a successful missing-key result.
+`std::nullopt` means allocation failure, with temporary nodes reclaimed and original ownership
+unchanged. Release successful roots through their original block source. Copies, destruction,
+hash/key extraction, and equality must be non-throwing; access remains externally synchronized.
diff --git a/mbo/container/internal/hamt_replace.h b/mbo/container/internal/hamt_replace.h
new file mode 100644
index 000000000..aa7dce8d3
--- /dev/null
+++ b/mbo/container/internal/hamt_replace.h
@@ -0,0 +1,102 @@
+// SPDX-FileCopyrightText: Copyright (c) M. Boerger, the MBO Works authors
+// SPDX-License-Identifier: Apache-2.0
+
+#ifndef MBO_CONTAINER_INTERNAL_HAMT_REPLACE_H_
+#define MBO_CONTAINER_INTERNAL_HAMT_REPLACE_H_
+
+#include <concepts>
+#include <cstddef>
+#include <iterator>
+#include <optional>
+#include <type_traits>
+
+#include "mbo/container/internal/hamt_hash_path.h"
+#include "mbo/container/internal/hamt_lookup.h"
+#include "mbo/container/internal/hamt_shared_node.h"
+#include "mbo/memory/block_source.h"
+
+namespace mbo::container::container_internal {
+
+template<typename Node>
+struct HamtReplaceResult final {
+  Node* root;
+  bool replaced;
+};
+
+namespace hamt_replace_internal {
+
+// The target has already been found in this root using this hash.
+template<std::size_t FragmentBits, typename Entry, std::unsigned_integral Hash, mbo::memory::BlockSource Source>
+[[nodiscard]] std::optional<HamtSharedNode<FragmentBits, Entry>*> TryReplaceAt(
+    Source& source,
+    const HamtSharedNode<FragmentBits, Entry>& original,
+    const HamtHashPath<Hash, FragmentBits>& path,
+    std::size_t level,
+    const Entry* target,
+    const Entry& replacement) noexcept
+requires std::is_nothrow_copy_constructible_v<Entry>
+{
+  using Node = HamtSharedNode<FragmentBits, Entry>;
+  if (original.is_collision()) {
+    const auto position = static_cast<std::size_t>(std::distance(original.entries().data(), target));
+    return Node::TryReplaceEntry(source, original, position, replacement);
+  }
+  const std::size_t fragment = path.Fragment(level);
+  if (original.index().Kind(fragment) == HamtSlotKind::kData) {
+    return Node::TryReplaceEntry(source, original, original.index().DataIndex(fragment), replacement);
+  }
+  const std::size_t position = original.index().NodeIndex(fragment);
+  const Node* const child = original.children().subspan(position).front();
+  auto* const replaced = TryReplaceAt(source, *child, path, level + 1, target, replacement).value_or(nullptr);
+  if (replaced == nullptr) {
+    return std::nullopt;
+  }
+  const auto result = Node::TryReplaceChild(source, original, position, replaced);
+  Node::Release(source, replaced);
+  return result;
+}
+
+}  // namespace hamt_replace_internal
+
+// Replacement must preserve the existing key and full hash. Borrows original;
+// success owns one root reference, including a retained root for missing keys.
+template<
+    std::unsigned_integral Hash,
+    std::size_t FragmentBits,
+    typename Entry,
+    typename Key,
+    mbo::memory::BlockSource Source,
+    typename HashOf,
+    typename KeyOf,
+    typename Equal>
+requires(
+    std::is_nothrow_copy_constructible_v<Entry> && std::is_nothrow_invocable_r_v<Hash, const HashOf&, const Entry&>
+    && std::is_nothrow_invocable_v<const KeyOf&, const Entry&>
+    && std::is_nothrow_invocable_r_v<bool, const Equal&, std::invoke_result_t<const KeyOf&, const Entry&>, const Key&>)
+[[nodiscard]] std::optional<HamtReplaceResult<HamtSharedNode<FragmentBits, Entry>>> TryReplaceHamtEntry(
+    Source& source,
+    HamtSharedNode<FragmentBits, Entry>* original,
+    Hash hash,
+    const Key& key,
+    const Entry& replacement,
+    const HashOf& hash_of,
+    const KeyOf& key_of,
+    const Equal& equal) noexcept {
+  using Node = HamtSharedNode<FragmentBits, Entry>;
+  const Entry* const target = FindHamtEntry(original, hash, key, hash_of, key_of, equal);
+  if (target == nullptr) {
+    Node::Retain(original);
+    return HamtReplaceResult<Node>{.root = original, .replaced = false};
+  }
+  const HamtHashPath<Hash, FragmentBits> path(hash);
+  auto* const replaced =
+      hamt_replace_internal::TryReplaceAt(source, *original, path, 0, target, replacement).value_or(nullptr);
+  if (replaced == nullptr) {
+    return std::nullopt;
+  }
+  return HamtReplaceResult<Node>{.root = replaced, .replaced = true};
+}
+
+}  // namespace mbo::container::container_internal
+
+#endif  // MBO_CONTAINER_INTERNAL_HAMT_REPLACE_H_
diff --git a/mbo/container/internal/hamt_replace_test.cc b/mbo/container/internal/hamt_replace_test.cc
new file mode 100644
index 000000000..26f12b1b4
--- /dev/null
+++ b/mbo/container/internal/hamt_replace_test.cc
@@ -0,0 +1,168 @@
+// SPDX-FileCopyrightText: Copyright (c) M. Boerger, the MBO Works authors
+// SPDX-License-Identifier: Apache-2.0
+
+#include "mbo/container/internal/hamt_replace.h"
+
+#include <cstdint>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "mbo/container/internal/hamt_insert.h"
+#include "mbo/memory/block_source.h"
+
+namespace mbo::container::container_internal {
+namespace {
+
+using ::testing::Eq;
+using ::testing::Field;
+using ::testing::IsNull;
+using ::testing::NotNull;
+using ::testing::Optional;
+
+struct Entry final {
+  std::uint64_t hash;
+  int key;
+  int value;
+};
+
+struct HashOf final {
+  constexpr std::uint64_t operator()(const Entry& entry) const noexcept { return entry.hash; }
+};
+
+struct KeyOf final {
+  constexpr int operator()(const Entry& entry) const noexcept { return entry.key; }
+};
+
+struct Equal final {
+  constexpr bool operator()(int lhs, int rhs) const noexcept { return lhs == rhs; }
+};
+
+using Node = HamtSharedNode<5, Entry>;
+
+struct HamtReplaceTest : ::testing::Test {
+ protected:
+  void TearDown() override { Node::Release(source, root); }
+
+ public:
+  void Insert(Entry entry) {
+    const auto inserted = TryInsertHamtEntry(source, root, entry.hash, entry.key, entry, HashOf{}, KeyOf{}, Equal{})
+                              .value_or(HamtInsertResult<Node>{.root = nullptr, .inserted = false});
+    ASSERT_THAT(inserted.root, NotNull());
+    Node::Release(source, root);
+    root = inserted.root;
+  }
+
+  static const Entry* Find(const Node* node, std::uint64_t hash, int key) {
+    return FindHamtEntry(node, hash, key, HashOf{}, KeyOf{}, Equal{});
+  }
+
+  mbo::memory::NewDeleteBlockSource source;
+  Node* root = nullptr;
+};
+
+TEST_F(HamtReplaceTest, NullRootAndMissingKeysSucceedWithoutAllocation) {
+  mbo::memory::InlineBlockSource<1> exhausted;
+  constexpr Entry kReplacement{.hash = 1, .key = 10, .value = 99};
+  const auto empty = TryReplaceHamtEntry(
+                         exhausted, root, kReplacement.hash, kReplacement.key, kReplacement, HashOf{}, KeyOf{}, Equal{})
+                         .value_or(HamtReplaceResult<Node>{.root = nullptr, .replaced = true});
+  EXPECT_THAT(empty.root, IsNull());
+  EXPECT_THAT(empty.replaced, Eq(false));
+  Insert(Entry{.hash = 2, .key = 20, .value = 20});
+  const auto missing =
+      TryReplaceHamtEntry(
+          exhausted, root, kReplacement.hash, kReplacement.key, kReplacement, HashOf{}, KeyOf{}, Equal{})
+          .value_or(HamtReplaceResult<Node>{.root = nullptr, .replaced = true});
+  ASSERT_THAT(missing.root, NotNull());
+  EXPECT_THAT(missing.root, Eq(root));
+  EXPECT_THAT(missing.replaced, Eq(false));
+  EXPECT_THAT(root->use_count(), Eq(2));
+  Node::Release(source, missing.root);
+}
+
+TEST_F(HamtReplaceTest, ReplacesDeepCollisionValueWithoutChangingTheOriginalSnapshot) {
+  constexpr std::uint64_t kHash = 1 + (std::uint64_t{3} << 15);
+  Insert(Entry{.hash = 1, .key = 10, .value = 10});
+  Insert(Entry{.hash = kHash, .key = 20, .value = 20});
+  Insert(Entry{.hash = kHash, .key = 30, .value = 30});
+  constexpr Entry kReplacement{.hash = kHash, .key = 20, .value = 99};
+  const auto changed = TryReplaceHamtEntry(source, root, kHash, 20, kReplacement, HashOf{}, KeyOf{}, Equal{})
+                           .value_or(HamtReplaceResult<Node>{.root = nullptr, .replaced = false});
+  ASSERT_THAT(changed.root, NotNull());
+  EXPECT_THAT(changed.replaced, Eq(true));
+  const Entry* original = Find(root, kHash, 20);
+  const Entry* updated = Find(changed.root, kHash, 20);
+  const Entry* other = Find(changed.root, kHash, 30);
+  ASSERT_THAT(original, NotNull());
+  ASSERT_THAT(updated, NotNull());
+  ASSERT_THAT(other, NotNull());
+  EXPECT_THAT(original->value, Eq(20));
+  EXPECT_THAT(updated->value, Eq(99));
+  EXPECT_THAT(other->value, Eq(30));
+  Node::Release(source, changed.root);
+}
+
+TEST_F(HamtReplaceTest, AllocationFailurePreservesDirectEntry) {
+  Insert(Entry{.hash = 1, .key = 10, .value = 10});
+  mbo::memory::InlineBlockSource<1> exhausted;
+  constexpr Entry kReplacement{.hash = 1, .key = 10, .value = 99};
+  EXPECT_THAT(
+      TryReplaceHamtEntry(exhausted, root, std::uint64_t{1}, 10, kReplacement, HashOf{}, KeyOf{}, Equal{}),
+      Eq(std::nullopt));
+  const Entry* unchanged = Find(root, 1, 10);
+  ASSERT_THAT(unchanged, NotNull());
+  EXPECT_THAT(unchanged->value, Eq(10));
+  EXPECT_THAT(root->use_count(), Eq(1));
+}
+
+TEST_F(HamtReplaceTest, DirectReplacementCanAliasTheExistingEntry) {
+  Insert(Entry{.hash = 1, .key = 10, .value = 10});
+  const Entry* const existing = Find(root, 1, 10);
+  ASSERT_THAT(existing, NotNull());
+  const auto changed = TryReplaceHamtEntry(source, root, std::uint64_t{1}, 10, *existing, HashOf{}, KeyOf{}, Equal{})
+                           .value_or(HamtReplaceResult<Node>{.root = nullptr, .replaced = false});
+  ASSERT_THAT(changed.root, NotNull());
+  EXPECT_THAT(changed.replaced, Eq(true));
+  const Entry* const copied = Find(changed.root, 1, 10);
+  ASSERT_THAT(copied, NotNull());
+  EXPECT_THAT(copied == existing, Eq(false));
+  EXPECT_THAT(copied->value, Eq(10));
+  EXPECT_THAT(existing->value, Eq(10));
+  Node::Release(source, changed.root);
+}
+
+TEST_F(HamtReplaceTest, FailedChildCopyPreservesTheEntireOriginalPath) {
+  Insert(Entry{.hash = 1, .key = 10, .value = 10});
+  Insert(Entry{.hash = 33, .key = 20, .value = 20});
+  mbo::memory::InlineBlockSource<1> exhausted;
+  constexpr Entry kReplacement{.hash = 33, .key = 20, .value = 99};
+  EXPECT_THAT(
+      TryReplaceHamtEntry(exhausted, root, std::uint64_t{33}, 20, kReplacement, HashOf{}, KeyOf{}, Equal{}),
+      Eq(std::nullopt));
+  const Entry* const unchanged = Find(root, 33, 20);
+  const Entry* const sibling = Find(root, 1, 10);
+  ASSERT_THAT(unchanged, NotNull());
+  ASSERT_THAT(sibling, NotNull());
+  EXPECT_THAT(unchanged->value, Eq(20));
+  EXPECT_THAT(sibling->value, Eq(10));
+  EXPECT_THAT(root->use_count(), Eq(1));
+  EXPECT_THAT(root->children().front()->use_count(), Eq(1));
+}
+
+TEST_F(HamtReplaceTest, FailedParentCopyReclaimsTheAlreadyCopiedChild) {
+  Insert(Entry{.hash = 1, .key = 10, .value = 10});
+  Insert(Entry{.hash = 33, .key = 20, .value = 20});
+  mbo::memory::InlineBlockSource<4'096> single_allocation;
+  constexpr Entry kReplacement{.hash = 33, .key = 20, .value = 99};
+  EXPECT_THAT(
+      TryReplaceHamtEntry(single_allocation, root, std::uint64_t{33}, 20, kReplacement, HashOf{}, KeyOf{}, Equal{}),
+      Eq(std::nullopt));
+  const Entry* const unchanged = Find(root, 33, 20);
+  ASSERT_THAT(unchanged, NotNull());
+  EXPECT_THAT(unchanged->value, Eq(20));
+  EXPECT_THAT(root->use_count(), Eq(1));
+  EXPECT_THAT(single_allocation.TryAcquire(1, 1), Optional(Field("data", &mbo::memory::MemoryBlock::data, NotNull())));
+}
+
+}  // namespace
+}  // namespace mbo::container::container_internal