diff --git a/Firestore/Example/Tests/Local/FSTLevelDBMigrationsTests.mm b/Firestore/Example/Tests/Local/FSTLevelDBMigrationsTests.mm
index 5f25935ea41..818948b7fbf 100644
--- a/Firestore/Example/Tests/Local/FSTLevelDBMigrationsTests.mm
+++ b/Firestore/Example/Tests/Local/FSTLevelDBMigrationsTests.mm
@@ -20,6 +20,7 @@
 #include <string>
 #include <vector>
 
+#import "Firestore/Protos/objc/firestore/local/Mutation.pbobjc.h"
 #import "Firestore/Protos/objc/firestore/local/Target.pbobjc.h"
 #import "Firestore/Source/Local/FSTLevelDB.h"
 #import "Firestore/Source/Local/FSTLevelDBMutationQueue.h"
@@ -27,6 +28,7 @@
 
 #include "Firestore/core/src/firebase/firestore/local/leveldb_key.h"
 #include "Firestore/core/src/firebase/firestore/local/leveldb_migrations.h"
+#include "Firestore/core/src/firebase/firestore/model/document_key.h"
 #include "Firestore/core/src/firebase/firestore/util/ordered_code.h"
 #include "Firestore/core/src/firebase/firestore/util/status.h"
 #include "Firestore/core/test/firebase/firestore/testutil/testutil.h"
@@ -38,6 +40,7 @@
 NS_ASSUME_NONNULL_BEGIN
 
 using firebase::firestore::FirestoreErrorCode;
+using firebase::firestore::local::LevelDbDocumentMutationKey;
 using firebase::firestore::local::LevelDbDocumentTargetKey;
 using firebase::firestore::local::LevelDbMigrations;
 using firebase::firestore::local::LevelDbMutationKey;
@@ -47,6 +50,7 @@
 using firebase::firestore::local::LevelDbTargetKey;
 using firebase::firestore::local::LevelDbTransaction;
 using firebase::firestore::model::BatchId;
+using firebase::firestore::model::DocumentKey;
 using firebase::firestore::model::TargetId;
 using firebase::firestore::testutil::Key;
 using firebase::firestore::util::OrderedCode;
@@ -199,6 +203,111 @@ - (void)testDropsTheQueryCacheWithThousandsOfEntries {
   }
 }
 
+- (void)testRemovesMutationBatches {
+  std::string emptyBuffer;
+  DocumentKey testWriteFoo = DocumentKey::FromPathString("docs/foo");
+  DocumentKey testWriteBar = DocumentKey::FromPathString("docs/bar");
+  DocumentKey testWriteBaz = DocumentKey::FromPathString("docs/baz");
+  DocumentKey testWritePending = DocumentKey::FromPathString("docs/pending");
+  // Do everything up until the mutation batch migration.
+  LevelDbMigrations::RunMigrations(_db.get(), 3);
+  // Set up data
+  {
+    LevelDbTransaction transaction(_db.get(), "Setup Foo");
+    // User 'foo' has two acknowledged mutations and one that is pending.
+    FSTPBMutationQueue *fooQueue = [[FSTPBMutationQueue alloc] init];
+    fooQueue.lastAcknowledgedBatchId = 2;
+    std::string fooKey = LevelDbMutationQueueKey::Key("foo");
+    transaction.Put(fooKey, fooQueue);
+
+    FSTPBWriteBatch *fooBatch1 = [[FSTPBWriteBatch alloc] init];
+    fooBatch1.batchId = 1;
+    std::string fooBatchKey1 = LevelDbMutationKey::Key("foo", 1);
+    transaction.Put(fooBatchKey1, fooBatch1);
+    transaction.Put(LevelDbDocumentMutationKey::Key("foo", testWriteFoo, 1), emptyBuffer);
+
+    FSTPBWriteBatch *fooBatch2 = [[FSTPBWriteBatch alloc] init];
+    fooBatch2.batchId = 2;
+    std::string fooBatchKey2 = LevelDbMutationKey::Key("foo", 2);
+    transaction.Put(fooBatchKey2, fooBatch2);
+    transaction.Put(LevelDbDocumentMutationKey::Key("foo", testWriteFoo, 2), emptyBuffer);
+
+    FSTPBWriteBatch *fooBatch3 = [[FSTPBWriteBatch alloc] init];
+    fooBatch3.batchId = 5;
+    std::string fooBatchKey3 = LevelDbMutationKey::Key("foo", 5);
+    transaction.Put(fooBatchKey3, fooBatch3);
+    transaction.Put(LevelDbDocumentMutationKey::Key("foo", testWritePending, 5), emptyBuffer);
+
+    transaction.Commit();
+  }
+
+  {
+    LevelDbTransaction transaction(_db.get(), "Setup Bar");
+    // User 'bar' has one acknowledged mutation and one that is pending
+    FSTPBMutationQueue *barQueue = [[FSTPBMutationQueue alloc] init];
+    barQueue.lastAcknowledgedBatchId = 3;
+    std::string barKey = LevelDbMutationQueueKey::Key("bar");
+    transaction.Put(barKey, barQueue);
+
+    FSTPBWriteBatch *barBatch1 = [[FSTPBWriteBatch alloc] init];
+    barBatch1.batchId = 3;
+    std::string barBatchKey1 = LevelDbMutationKey::Key("bar", 3);
+    transaction.Put(barBatchKey1, barBatch1);
+    transaction.Put(LevelDbDocumentMutationKey::Key("bar", testWriteBar, 3), emptyBuffer);
+    transaction.Put(LevelDbDocumentMutationKey::Key("bar", testWriteBaz, 3), emptyBuffer);
+
+    FSTPBWriteBatch *barBatch2 = [[FSTPBWriteBatch alloc] init];
+    barBatch2.batchId = 4;
+    std::string barBatchKey2 = LevelDbMutationKey::Key("bar", 4);
+    transaction.Put(barBatchKey2, barBatch2);
+    transaction.Put(LevelDbDocumentMutationKey::Key("bar", testWritePending, 4), emptyBuffer);
+
+    transaction.Commit();
+  }
+
+  {
+    LevelDbTransaction transaction(_db.get(), "Setup Empty");
+    // User 'empty' has no mutations
+    FSTPBMutationQueue *emptyQueue = [[FSTPBMutationQueue alloc] init];
+    emptyQueue.lastAcknowledgedBatchId = -1;
+    std::string emptyKey = LevelDbMutationQueueKey::Key("empty");
+    transaction.Put(emptyKey, emptyQueue);
+
+    transaction.Commit();
+  }
+
+  LevelDbMigrations::RunMigrations(_db.get(), 4);
+
+  {
+    // Verify
+    std::string buffer;
+    LevelDbTransaction transaction(_db.get(), "Verify");
+    auto it = transaction.NewIterator();
+    // verify that we deleted the correct batches
+    XCTAssertTrue(transaction.Get(LevelDbMutationKey::Key("foo", 1), &buffer).IsNotFound());
+    XCTAssertTrue(transaction.Get(LevelDbMutationKey::Key("foo", 2), &buffer).IsNotFound());
+    XCTAssertTrue(transaction.Get(LevelDbMutationKey::Key("foo", 5), &buffer).ok());
+
+    XCTAssertTrue(transaction.Get(LevelDbMutationKey::Key("bar", 3), &buffer).IsNotFound());
+    XCTAssertTrue(transaction.Get(LevelDbMutationKey::Key("bar", 4), &buffer).ok());
+
+    // verify document associations have been removed
+    XCTAssertTrue(transaction.Get(LevelDbDocumentMutationKey::Key("foo", testWriteFoo, 1), &buffer)
+                      .IsNotFound());
+    XCTAssertTrue(transaction.Get(LevelDbDocumentMutationKey::Key("foo", testWriteFoo, 2), &buffer)
+                      .IsNotFound());
+    XCTAssertTrue(
+        transaction.Get(LevelDbDocumentMutationKey::Key("foo", testWritePending, 5), &buffer).ok());
+
+    XCTAssertTrue(transaction.Get(LevelDbDocumentMutationKey::Key("bar", testWriteBar, 3), &buffer)
+                      .IsNotFound());
+    XCTAssertTrue(transaction.Get(LevelDbDocumentMutationKey::Key("bar", testWriteBaz, 3), &buffer)
+                      .IsNotFound());
+    XCTAssertTrue(
+        transaction.Get(LevelDbDocumentMutationKey::Key("bar", testWritePending, 4), &buffer).ok());
+  }
+}
+
 /**
  * Creates the name of a dummy entry to make sure the iteration is correctly bounded.
  */
diff --git a/Firestore/Source/Local/FSTLevelDBQueryCache.mm b/Firestore/Source/Local/FSTLevelDBQueryCache.mm
index 978aa0d79eb..62b562553f0 100644
--- a/Firestore/Source/Local/FSTLevelDBQueryCache.mm
+++ b/Firestore/Source/Local/FSTLevelDBQueryCache.mm
@@ -193,7 +193,7 @@ - (void)enumerateOrphanedDocumentsUsingBlock:
   LevelDbDocumentTargetKey key;
   BOOL stop = NO;
   for (; !stop && it->Valid() && absl::StartsWith(it->key(), documentTargetPrefix); it->Next()) {
-    key.Decode(it->key());
+    HARD_ASSERT(key.Decode(it->key()), "Failed to decode DocumentTarget key");
     if (key.IsSentinel()) {
       // if nextToReport is non-zero, report it, this is a new key so the last one
       // must be not be a member of any targets.
diff --git a/Firestore/core/src/firebase/firestore/local/leveldb_key.cc b/Firestore/core/src/firebase/firestore/local/leveldb_key.cc
index c82a53f1dd8..a6d6bbe7bdf 100644
--- a/Firestore/core/src/firebase/firestore/local/leveldb_key.cc
+++ b/Firestore/core/src/firebase/firestore/local/leveldb_key.cc
@@ -656,7 +656,7 @@ std::string LevelDbMutationQueueKey::Key(absl::string_view user_id) {
   return writer.result();
 }
 
-bool LevelDbMutationQueueKey::Decode(leveldb::Slice key) {
+bool LevelDbMutationQueueKey::Decode(absl::string_view key) {
   Reader reader{key};
   reader.ReadTableNameMatching(kMutationQueuesTable);
   user_id_ = reader.ReadUserId();
diff --git a/Firestore/core/src/firebase/firestore/local/leveldb_key.h b/Firestore/core/src/firebase/firestore/local/leveldb_key.h
index c31abefd3c0..c57769f95a2 100644
--- a/Firestore/core/src/firebase/firestore/local/leveldb_key.h
+++ b/Firestore/core/src/firebase/firestore/local/leveldb_key.h
@@ -121,6 +121,7 @@ class LevelDbMutationKey {
    * returned, this instance is in an undefined state until the next call to
    * `Decode()`.
    */
+  ABSL_MUST_USE_RESULT
   bool Decode(absl::string_view key);
 
   /** The user that owns the mutation batches. */
@@ -184,6 +185,7 @@ class LevelDbDocumentMutationKey {
    * returned, this instance is in an undefined state until the next call to
    * `Decode()`.
    */
+  ABSL_MUST_USE_RESULT
   bool Decode(absl::string_view key);
 
   /** The user that owns the mutation batches. */
@@ -235,7 +237,8 @@ class LevelDbMutationQueueKey {
    * returned, this instance is in an undefined state until the next call to
    * `Decode()`.
    */
-  bool Decode(leveldb::Slice key);
+  ABSL_MUST_USE_RESULT
+  bool Decode(absl::string_view key);
 
   const std::string& user_id() const {
     return user_id_;
@@ -259,6 +262,7 @@ class LevelDbTargetGlobalKey {
    * Decodes the contents of a target global key, essentially just verifying
    * that the key has the correct table name.
    */
+  ABSL_MUST_USE_RESULT
   bool Decode(leveldb::Slice key);
 };
 
@@ -281,6 +285,7 @@ class LevelDbTargetKey {
    * returned, this instance is in an undefined state until the next call to
    * `Decode()`.
    */
+  ABSL_MUST_USE_RESULT
   bool Decode(leveldb::Slice key);
 
   model::TargetId target_id() {
@@ -322,6 +327,7 @@ class LevelDbQueryTargetKey {
    * returned, this instance is in an undefined state until the next call to
    * `Decode()`.
    */
+  ABSL_MUST_USE_RESULT
   bool Decode(absl::string_view key);
 
   /** The canonical_id derived from the query. */
@@ -370,6 +376,7 @@ class LevelDbTargetDocumentKey {
    * returned, this instance is in an undefined state until the next call to
    * `Decode()`.
    */
+  ABSL_MUST_USE_RESULT
   bool Decode(absl::string_view key);
 
   /** The target_id identifying a target. */
@@ -424,6 +431,7 @@ class LevelDbDocumentTargetKey {
    * returned, this instance is in an undefined state until the next call to
    * `Decode()`.
    */
+  ABSL_MUST_USE_RESULT
   bool Decode(absl::string_view key);
 
   /** The target_id identifying a target. */
@@ -487,6 +495,7 @@ class LevelDbRemoteDocumentKey {
    * returned, this instance is in an undefined state until the next call to
    * `Decode()`.
    */
+  ABSL_MUST_USE_RESULT
   bool Decode(absl::string_view key);
 
   /** The path to the document, as encoded in the key. */
diff --git a/Firestore/core/src/firebase/firestore/local/leveldb_migrations.cc b/Firestore/core/src/firebase/firestore/local/leveldb_migrations.cc
index dc4d1594e56..6e8b97035e7 100644
--- a/Firestore/core/src/firebase/firestore/local/leveldb_migrations.cc
+++ b/Firestore/core/src/firebase/firestore/local/leveldb_migrations.cc
@@ -19,8 +19,10 @@
 #include <string>
 #include <utility>
 
+#include "Firestore/Protos/nanopb/firestore/local/mutation.nanopb.h"
 #include "Firestore/Protos/nanopb/firestore/local/target.nanopb.h"
 #include "Firestore/core/src/firebase/firestore/local/leveldb_key.h"
+#include "Firestore/core/src/firebase/firestore/nanopb/reader.h"
 #include "Firestore/core/src/firebase/firestore/nanopb/writer.h"
 #include "absl/strings/match.h"
 
@@ -32,6 +34,7 @@ using leveldb::Iterator;
 using leveldb::Slice;
 using leveldb::Status;
 using leveldb::WriteOptions;
+using nanopb::Reader;
 using nanopb::Writer;
 
 namespace {
@@ -110,6 +113,72 @@ void ClearQueryCache(leveldb::DB* db) {
   transaction.Commit();
 }
 
+/**
+ * Removes document associations for the given user's mutation queue for
+ * any mutation with a `batch_id` less than or equal to
+ * `last_acknowledged_batch_id`.
+ */
+void RemoveMutationDocuments(LevelDbTransaction* transaction,
+                             absl::string_view user_id,
+                             int32_t last_acknowledged_batch_id) {
+  LevelDbDocumentMutationKey doc_key;
+  std::string prefix = LevelDbDocumentMutationKey::KeyPrefix(user_id);
+
+  auto it = transaction->NewIterator();
+  it->Seek(prefix);
+  for (; it->Valid() && absl::StartsWith(it->key(), prefix); it->Next()) {
+    HARD_ASSERT(doc_key.Decode(it->key()),
+                "Failed to decode document mutation key");
+    if (doc_key.batch_id() <= last_acknowledged_batch_id) {
+      transaction->Delete(it->key());
+    }
+  }
+}
+
+/**
+ * Removes mutation batches for the given user with a `batch_id` less than
+ * or equal to `last_acknowledged_batch_id`
+ */
+void RemoveMutationBatches(LevelDbTransaction* transaction,
+                           absl::string_view user_id,
+                           int32_t last_acknowledged_batch_id) {
+  std::string mutations_key = LevelDbMutationKey::KeyPrefix(user_id);
+  std::string last_key =
+      LevelDbMutationKey::Key(user_id, last_acknowledged_batch_id);
+  auto it = transaction->NewIterator();
+  it->Seek(mutations_key);
+  for (; it->Valid() && it->key() <= last_key; it->Next()) {
+    transaction->Delete(it->key());
+  }
+}
+
+/** Migration 4. */
+void RemoveAcknowledgedMutations(leveldb::DB* db) {
+  LevelDbTransaction transaction(db, "remove acknowledged mutations");
+  std::string mutation_queue_start = LevelDbMutationQueueKey::KeyPrefix();
+
+  LevelDbMutationQueueKey key;
+
+  auto it = transaction.NewIterator();
+  it->Seek(mutation_queue_start);
+  for (; it->Valid() && absl::StartsWith(it->key(), mutation_queue_start);
+       it->Next()) {
+    HARD_ASSERT(key.Decode(it->key()), "Failed to decode mutation queue key");
+    firestore_client_MutationQueue mutation_queue{};
+    Reader reader = Reader::Wrap(it->value());
+    reader.ReadNanopbMessage(firestore_client_MutationQueue_fields,
+                             &mutation_queue);
+    HARD_ASSERT(reader.status().ok(), "Failed to deserialize MutationQueue");
+    RemoveMutationBatches(&transaction, key.user_id(),
+                          mutation_queue.last_acknowledged_batch_id);
+    RemoveMutationDocuments(&transaction, key.user_id(),
+                            mutation_queue.last_acknowledged_batch_id);
+  }
+
+  SaveVersion(4, &transaction);
+  transaction.Commit();
+}
+
 }  // namespace
 
 LevelDbMigrations::SchemaVersion LevelDbMigrations::ReadSchemaVersion(
@@ -139,6 +208,9 @@ void LevelDbMigrations::RunMigrations(leveldb::DB* db,
   if (from_version < 3 && to_version >= 3) {
     ClearQueryCache(db);
   }
+  if (from_version < 4 && to_version >= 4) {
+    RemoveAcknowledgedMutations(db);
+  }
 }
 
 }  // namespace local
diff --git a/Firestore/core/src/firebase/firestore/nanopb/reader.cc b/Firestore/core/src/firebase/firestore/nanopb/reader.cc
index ae8f4cdef96..fdc49330d92 100644
--- a/Firestore/core/src/firebase/firestore/nanopb/reader.cc
+++ b/Firestore/core/src/firebase/firestore/nanopb/reader.cc
@@ -30,6 +30,12 @@ Reader Reader::Wrap(const uint8_t* bytes, size_t length) {
   return Reader{pb_istream_from_buffer(bytes, length)};
 }
 
+Reader Reader::Wrap(absl::string_view string_view) {
+  return Reader{pb_istream_from_buffer(
+      reinterpret_cast<const uint8_t*>(string_view.data()),
+      string_view.size())};
+}
+
 uint32_t Reader::ReadTag() {
   Tag tag;
   if (!status_.ok()) return 0;
diff --git a/Firestore/core/src/firebase/firestore/nanopb/reader.h b/Firestore/core/src/firebase/firestore/nanopb/reader.h
index af5ecd4cd3c..30b3b04e6f2 100644
--- a/Firestore/core/src/firebase/firestore/nanopb/reader.h
+++ b/Firestore/core/src/firebase/firestore/nanopb/reader.h
@@ -59,6 +59,15 @@ class Reader {
    */
   static Reader Wrap(const uint8_t* bytes, size_t length);
 
+  /**
+   * Creates an input stream from bytes backing the string_view. Note that
+   * the backing buffer must remain valid for the lifetime of this Reader.
+   *
+   * (This is roughly equivalent to the nanopb function
+   * pb_istream_from_buffer())
+   */
+  static Reader Wrap(absl::string_view);
+
   /**
    * Reads a message type from the input stream.
    *