diff --git a/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/impl/RealMutableStore.kt b/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/impl/RealMutableStore.kt
index 39c2ac83..6e74462c 100644
--- a/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/impl/RealMutableStore.kt
+++ b/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/impl/RealMutableStore.kt
@@ -24,6 +24,7 @@ import org.mobilenativefoundation.store.store5.impl.extensions.now
 import org.mobilenativefoundation.store.store5.internal.concurrent.ThreadSafety
 import org.mobilenativefoundation.store.store5.internal.definition.WriteRequestQueue
 import org.mobilenativefoundation.store.store5.internal.result.EagerConflictResolutionResult
+import org.mobilenativefoundation.store.store5.internal.result.StoreDelegateWriteResult
 
 @OptIn(ExperimentalStoreApi::class)
 internal class RealMutableStore<Key : Any, Network : Any, Output : Any, Local : Any>(
@@ -85,25 +86,30 @@ internal class RealMutableStore<Key : Any, Network : Any, Output : Any, Local :
                     val storeWriteResponse =
                         try {
                             // Always write to local first.
-                            delegate.write(writeRequest.key, writeRequest.value)
-
-                            // Try to sync to network.
-                            val updaterResult = tryUpdateServer(writeRequest)
-
-                            // Convert UpdaterResult -> StoreWriteResponse.
-                            when (updaterResult) {
-                                is UpdaterResult.Error.Exception -> StoreWriteResponse.Error.Exception(updaterResult.error)
-                                is UpdaterResult.Error.Message -> StoreWriteResponse.Error.Message(updaterResult.message)
-                                is UpdaterResult.Success.Typed<*> -> {
-                                    val typedValue = updaterResult.value as? Response
-                                    if (typedValue == null) {
-                                        StoreWriteResponse.Success.Untyped(updaterResult.value)
-                                    } else {
-                                        StoreWriteResponse.Success.Typed(updaterResult.value)
+                            // Only proceed to network if local write succeeded.
+                            when (val delegateWriteResult = delegate.write(writeRequest.key, writeRequest.value)) {
+                                is StoreDelegateWriteResult.Error.Exception -> {
+                                    StoreWriteResponse.Error.Exception(delegateWriteResult.error)
+                                }
+                                is StoreDelegateWriteResult.Error.Message -> {
+                                    StoreWriteResponse.Error.Message(delegateWriteResult.error)
+                                }
+                                is StoreDelegateWriteResult.Success -> {
+                                    // Try to sync to network.
+                                    when (val updaterResult = tryUpdateServer(writeRequest)) {
+                                        is UpdaterResult.Error.Exception -> StoreWriteResponse.Error.Exception(updaterResult.error)
+                                        is UpdaterResult.Error.Message -> StoreWriteResponse.Error.Message(updaterResult.message)
+                                        is UpdaterResult.Success.Typed<*> -> {
+                                            val typedValue = updaterResult.value as? Response
+                                            if (typedValue == null) {
+                                                StoreWriteResponse.Success.Untyped(updaterResult.value)
+                                            } else {
+                                                StoreWriteResponse.Success.Typed(updaterResult.value)
+                                            }
+                                        }
+                                        is UpdaterResult.Success.Untyped -> StoreWriteResponse.Success.Untyped(updaterResult.value)
                                     }
                                 }
-
-                                is UpdaterResult.Success.Untyped -> StoreWriteResponse.Success.Untyped(updaterResult.value)
                             }
                         } catch (throwable: Throwable) {
                             StoreWriteResponse.Error.Exception(throwable)
diff --git a/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/impl/RealStore.kt b/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/impl/RealStore.kt
index d0198297..20f7fe8b 100644
--- a/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/impl/RealStore.kt
+++ b/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/impl/RealStore.kt
@@ -332,9 +332,13 @@ internal class RealStore<Key : Any, Network : Any, Output : Any, Local : Any>(
         value: Output,
     ): StoreDelegateWriteResult =
         try {
-            memCache?.put(key, value)
-            sourceOfTruth?.write(key, converter.fromOutputToLocal(value))
-            StoreDelegateWriteResult.Success
+            val writeException = sourceOfTruth?.write(key, converter.fromOutputToLocal(value))
+            if (writeException != null) {
+                StoreDelegateWriteResult.Error.Exception(writeException)
+            } else {
+                memCache?.put(key, value)
+                StoreDelegateWriteResult.Success
+            }
         } catch (error: Throwable) {
             StoreDelegateWriteResult.Error.Exception(error)
         }
diff --git a/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/impl/SourceOfTruthWithBarrier.kt b/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/impl/SourceOfTruthWithBarrier.kt
index a0a0953b..d791757a 100644
--- a/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/impl/SourceOfTruthWithBarrier.kt
+++ b/store/src/commonMain/kotlin/org/mobilenativefoundation/store/store5/impl/SourceOfTruthWithBarrier.kt
@@ -144,11 +144,18 @@ internal class SourceOfTruthWithBarrier<Key : Any, Network : Any, Output : Any,
         }
     }
 
+    /**
+     * Writes a value to the underlying [SourceOfTruth] and returns any error that occurred.
+     *
+     * @return The [SourceOfTruth.WriteException] if the write failed, or null if successful.
+     *         Callers like [RealStore.write] can check this to determine if the write succeeded.
+     *         The barrier mechanism also notifies readers of the error via [BarrierMsg.Open.writeError].
+     */
     @Suppress("UNCHECKED_CAST")
     suspend fun write(
         key: Key,
         value: Local,
-    ) {
+    ): SourceOfTruth.WriteException? {
         val barrier = barriers.acquire(key)
         try {
             barrier.emit(BarrierMsg.Blocked(versionCounter.incrementAndGet()))
@@ -164,24 +171,27 @@ internal class SourceOfTruthWithBarrier<Key : Any, Network : Any, Output : Any,
                     }
                 }
 
+            // Avoid double-wrapping if the error is already a WriteException.
+            val writeException =
+                writeError?.let {
+                    writeError as? SourceOfTruth.WriteException
+                        ?: SourceOfTruth.WriteException(
+                            key = key,
+                            value = value,
+                            cause = writeError,
+                        )
+                }
+
             barrier.emit(
                 BarrierMsg.Open(
                     version = versionCounter.incrementAndGet(),
-                    writeError =
-                        writeError?.let {
-                            SourceOfTruth.WriteException(
-                                key = key,
-                                value = value,
-                                cause = writeError,
-                            )
-                        },
+                    writeError = writeException,
                 ),
             )
-            if (writeError is CancellationException) {
-                // only throw if it failed because of cancelation.
-                // otherwise, we take care of letting downstream know that there was a write error
-                throw writeError
-            }
+
+            // Return the error so callers know the operation failed.
+            // The barrier message above notifies readers of the error.
+            return writeException
         } finally {
             barriers.release(key, barrier)
         }
diff --git a/store/src/commonTest/kotlin/org/mobilenativefoundation/store/store5/mutablestore/RealMutableStoreTest.kt b/store/src/commonTest/kotlin/org/mobilenativefoundation/store/store5/mutablestore/RealMutableStoreTest.kt
index 18ea7c72..734193b7 100644
--- a/store/src/commonTest/kotlin/org/mobilenativefoundation/store/store5/mutablestore/RealMutableStoreTest.kt
+++ b/store/src/commonTest/kotlin/org/mobilenativefoundation/store/store5/mutablestore/RealMutableStoreTest.kt
@@ -11,6 +11,7 @@ import kotlinx.coroutines.flow.toList
 import kotlinx.coroutines.test.runTest
 import org.mobilenativefoundation.store.core5.ExperimentalStoreApi
 import org.mobilenativefoundation.store.store5.FetcherResult
+import org.mobilenativefoundation.store.store5.SourceOfTruth
 import org.mobilenativefoundation.store.store5.StoreReadRequest
 import org.mobilenativefoundation.store.store5.StoreReadResponse
 import org.mobilenativefoundation.store.store5.StoreWriteRequest
@@ -297,6 +298,115 @@ class RealMutableStoreTest {
             assertNotNull(testBookkeeper.getLastFailedSync("exceptionKey"))
         }
 
+    @Test
+    fun write_givenSourceOfTruthFailure_whenCalled_thenSurfacesWriteError() =
+        runTest {
+            // Given
+            val key = "key"
+            val errorMessage = "write error"
+            testSourceOfTruth.throwOnWrite(key) {
+                IllegalStateException(errorMessage)
+            }
+            val request =
+                StoreWriteRequest.of<String, Note, Unit>(
+                    key = key,
+                    value = Note(key, "content"),
+                    created = 3333L,
+                    onCompletions = null,
+                )
+
+            // When
+            val response = mutableStore.write(request)
+
+            // Then
+            val errorResponse = assertIs<StoreWriteResponse.Error.Exception>(response)
+            val writeException = assertIs<SourceOfTruth.WriteException>(errorResponse.error)
+            val cause = assertIs<IllegalStateException>(writeException.cause)
+            assertEquals(errorMessage, cause.message)
+        }
+
+    @Test
+    fun write_givenSourceOfTruthFailure_whenCalled_thenNetworkSyncNotAttempted() =
+        runTest {
+            // Given
+            val key = "key"
+            testUpdater.postCallCount = 0
+            testSourceOfTruth.throwOnWrite(key) { IllegalStateException("SOT failure") }
+
+            val request =
+                StoreWriteRequest.of<String, Note, Unit>(
+                    key = key,
+                    value = Note(key, "content"),
+                    created = 4444L,
+                    onCompletions = null,
+                )
+
+            // When
+            val response = mutableStore.write(request)
+
+            // Then
+            assertIs<StoreWriteResponse.Error.Exception>(response)
+            assertEquals(0, testUpdater.postCallCount, "Network updater should not be called when SOT write fails")
+        }
+
+    @Test
+    fun write_givenSourceOfTruthFailure_whenCalled_thenMemCacheNotUpdated() =
+        runTest {
+            // Given
+            val key = "key"
+            testSourceOfTruth.throwOnWrite(key) { IllegalStateException("SOT failure") }
+
+            val request =
+                StoreWriteRequest.of<String, Note, Unit>(
+                    key = key,
+                    value = Note(key, "content"),
+                    created = 6666L,
+                    onCompletions = null,
+                )
+
+            // When
+            val response = mutableStore.write(request)
+
+            // Then
+            assertIs<StoreWriteResponse.Error.Exception>(response)
+            assertNull(delegateStore.latestOrNull(key), "Value should not be in cache after SOT write failure")
+        }
+
+    @Test
+    fun write_givenNoSourceOfTruth_whenCalled_thenSucceeds() =
+        runTest {
+            // Given
+            val storeWithoutSot =
+                testStore(
+                    fetcher = testFetcher,
+                    sourceOfTruth = null,
+                    converter = testConverter,
+                    validator = testValidator,
+                    memoryCache = testCache,
+                )
+            val mutableStoreWithoutSot =
+                RealMutableStore(
+                    delegate = storeWithoutSot,
+                    updater = testUpdater,
+                    bookkeeper = testBookkeeper,
+                    logger = testLogger,
+                )
+
+            val request =
+                StoreWriteRequest.of<String, Note, Unit>(
+                    key = "noSotKey",
+                    value = Note("id", "content"),
+                    created = 5555L,
+                    onCompletions = null,
+                )
+
+            // When
+            val response = mutableStoreWithoutSot.write(request)
+
+            // Then
+            assertIs<StoreWriteResponse.Success>(response)
+        }
+
     @Test
     fun clearAll_givenSomeKeys_whenCalled_thenDelegateIsCleared() =
         runTest {
diff --git a/store/src/commonTest/kotlin/org/mobilenativefoundation/store/store5/mutablestore/util/TestStore.kt b/store/src/commonTest/kotlin/org/mobilenativefoundation/store/store5/mutablestore/util/TestStore.kt
index 1a2fb1f5..62504297 100644
--- a/store/src/commonTest/kotlin/org/mobilenativefoundation/store/store5/mutablestore/util/TestStore.kt
+++ b/store/src/commonTest/kotlin/org/mobilenativefoundation/store/store5/mutablestore/util/TestStore.kt
@@ -14,7 +14,7 @@ internal fun <Key : Any, Network : Any, Output : Any, Local : Any> testStore(
     dispatcher: CoroutineDispatcher = Dispatchers.Default,
     scope: CoroutineScope = CoroutineScope(dispatcher),
     fetcher: Fetcher<Key, Network> = TestFetcher(),
-    sourceOfTruth: SourceOfTruth<Key, Local, Output> = TestSourceOfTruth(),
+    sourceOfTruth: SourceOfTruth<Key, Local, Output>? = TestSourceOfTruth(),
     converter: Converter<Network, Local, Output> = TestConverter(),
     validator: Validator<Output> = TestValidator(),
     memoryCache: Cache<Key, Output> = TestCache(),
diff --git a/store/src/commonTest/kotlin/org/mobilenativefoundation/store/store5/mutablestore/util/TestUpdater.kt b/store/src/commonTest/kotlin/org/mobilenativefoundation/store/store5/mutablestore/util/TestUpdater.kt
index 802faa34..f83dc893 100644
--- a/store/src/commonTest/kotlin/org/mobilenativefoundation/store/store5/mutablestore/util/TestUpdater.kt
+++ b/store/src/commonTest/kotlin/org/mobilenativefoundation/store/store5/mutablestore/util/TestUpdater.kt
@@ -8,11 +8,13 @@ class TestUpdater<Key : Any, Output : Any, Response : Any> : Updater<Key, Output
     var exception: Throwable? = null
     var errorMessage: String? = null
     var successValue: Response? = null
+    var postCallCount: Int = 0
 
     override suspend fun post(
         key: Key,
         value: Output,
     ): UpdaterResult {
+        postCallCount++
         exception?.let { return UpdaterResult.Error.Exception(it) }
         errorMessage?.let { return UpdaterResult.Error.Message(it) }
         successValue?.let { return UpdaterResult.Success.Typed(it) }