diff --git a/java/arcs/core/data/testutil/BUILD b/java/arcs/core/data/testutil/BUILD
index 22bc8689bef..2ef19812723 100644
--- a/java/arcs/core/data/testutil/BUILD
+++ b/java/arcs/core/data/testutil/BUILD
@@ -7,12 +7,26 @@ licenses(["notice"])
 
 package(default_visibility = ["//java/arcs:allowed-packages"])
 
+GENERATOR_SRCS = ["Generators.kt"]
+
 arcs_kt_library(
-    name = "generators",
-    testonly = 1,
-    srcs = [
-        "Generators.kt",
+    name = "testutil",
+    testonly = True,
+    srcs = glob(
+        ["*.kt"],
+        exclude = GENERATOR_SRCS,
+    ),
+    deps = [
+        "//java/arcs/core/data:rawentity",
+        "//java/arcs/core/data/util:data-util",
+        "//third_party/java/truth:truth-android",
     ],
+)
+
+arcs_kt_library(
+    name = "generators",
+    testonly = True,
+    srcs = GENERATOR_SRCS,
     deps = [
         "//java/arcs/core/common",
         "//java/arcs/core/data",
diff --git a/java/arcs/core/data/testutil/RawEntitySubject.kt b/java/arcs/core/data/testutil/RawEntitySubject.kt
new file mode 100644
index 00000000000..e2671e34786
--- /dev/null
+++ b/java/arcs/core/data/testutil/RawEntitySubject.kt
@@ -0,0 +1,198 @@
+package arcs.core.data.testutil
+
+import arcs.core.data.RawEntity
+import arcs.core.data.util.ReferencableList
+import com.google.common.truth.Fact
+import com.google.common.truth.Fact.fact
+import com.google.common.truth.Fact.simpleFact
+import com.google.common.truth.FailureMetadata
+import com.google.common.truth.Subject
+import com.google.common.truth.Truth.assertAbout
+import java.lang.Integer.max
+
+/**
+ * Custom Truth [Subject] for asserting on [RawEntity] instances.
+ *
+ * Displays a much nicer diff between unequal entities than the default diff, e.g.:
+ *
+ *     RawEntity instances are different
+ *     for field: rawEntity.collections[foo][1]
+ *     expected : Primitive(0.695700200506916)
+ *     but was  : Primitive(0.46839655204527963)
+ *
+ * See [RawEntitySubject.Companion.assertThat] for instructions on how to use this.
+ */
+class RawEntitySubject private constructor(
+  failureMetadata: FailureMetadata,
+  subject: RawEntity?
+) : Subject(failureMetadata, subject) {
+  private val actual: RawEntity? = subject
+
+  override fun isEqualTo(expected: Any?) {
+    if (actual == expected) {
+      return
+    }
+    if (expected !is RawEntity || actual == null) {
+      // Standard failure scenarios, defer to Truth's standard error message.
+      super.isEqualTo(expected)
+      return // Redundant return, above line should always throw.
+    }
+
+    var diffs = RawEntityDiffer().diff(expected, actual)
+
+    if (diffs.isEmpty()) {
+      diffs = listOf(
+        simpleFact("<RawEntity instances are not equal, but no differences could be found>"),
+        fact("expected", expected.toString()),
+        fact("but was", actual.toString())
+      )
+    }
+
+    failWithoutActual(
+      simpleFact("RawEntity instances are different"),
+      *diffs.toTypedArray()
+    )
+  }
+
+  class RawEntityDiffer {
+    private val diffs = mutableListOf<Fact>()
+
+    fun diff(expected: RawEntity, actual: RawEntity): List<Fact> {
+      diffRawEntity(prefix = "rawEntity", expected, actual)
+      return diffs
+    }
+
+    private fun diffRawEntity(prefix: String, expected: RawEntity, actual: RawEntity) {
+      if (expected == actual) {
+        return
+      }
+      if (actual.id != expected.id) {
+        addDiff("$prefix.id", expected.id, actual.id)
+      }
+      if (actual.creationTimestamp != expected.creationTimestamp) {
+        addDiff(
+          "$prefix.creationTimestamp",
+          expected.creationTimestamp.toString(),
+          actual.creationTimestamp.toString()
+        )
+      }
+      if (actual.expirationTimestamp != expected.expirationTimestamp) {
+        addDiff(
+          "$prefix.expirationTimestamp",
+          expected.expirationTimestamp.toString(),
+          actual.expirationTimestamp.toString()
+        )
+      }
+      diffMap("$prefix.singletons", expected.singletons, actual.singletons)
+      diffMap("$prefix.collections", expected.collections, actual.collections)
+    }
+
+    private fun diffMap(prefix: String, expected: Map<String, Any?>, actual: Map<String, Any?>) {
+      if (expected == actual) {
+        return
+      }
+      val expectedKeys = expected.keys
+      val actualKeys = actual.keys
+      for (key in expectedKeys) {
+        if (key in actualKeys) {
+          // Check values.
+          diffValue("$prefix[$key]", expected[key], actual[key])
+        } else {
+          // Expected key is missing.
+          addDiff("$prefix[$key]", expected[key].toString(), "<absent>")
+        }
+      }
+      for (key in actualKeys) {
+        if (key !in expectedKeys) {
+          // Unexpected key.
+          addDiff("$prefix[$key]", "<absent>", actual[key].toString())
+        }
+      }
+    }
+
+    private fun diffValue(prefix: String, expected: Any?, actual: Any?) {
+      when {
+        expected == actual -> Unit
+        expected is RawEntity && actual is RawEntity -> {
+          diffRawEntity(prefix, expected, actual)
+        }
+        expected is List<*> && actual is List<*> -> {
+          diffList(prefix, expected, actual)
+        }
+        expected is Set<*> && actual is Set<*> -> {
+          diffSet(prefix, expected, actual)
+        }
+        expected is ReferencableList<*> && actual is ReferencableList<*> -> {
+          diffValue("$prefix.itemType", expected.itemType, actual.itemType)
+          diffList(prefix, expected.value, actual.value)
+        }
+        else -> {
+          addDiff(prefix, expected.toString(), actual.toString())
+        }
+      }
+    }
+
+    private fun diffList(prefix: String, expected: List<*>, actual: List<*>) {
+      for (i in 0 until max(expected.size, actual.size)) {
+        diffValue(
+          "$prefix[$i]",
+          expected.getOrElse(i) { "<absent>" },
+          actual.getOrElse(i) { "<absent>" }
+        )
+      }
+    }
+
+    private fun diffSet(prefix: String, expected: Set<*>, actual: Set<*>) {
+      // Convert each set to a stable sorted list, then compare lists.
+      diffList(
+        prefix,
+        expected.sortedBy { it.toString() }.toList(),
+        actual.sortedBy { it.toString() }.toList()
+      )
+    }
+
+    private fun addDiff(field: String, expected: String, actual: String) {
+      diffs.add(fact("for field", field))
+      diffs.add(fact("expected", expected))
+      diffs.add(fact("but was", actual))
+    }
+  }
+
+  private object Factory : Subject.Factory<RawEntitySubject, RawEntity> {
+    override fun createSubject(
+      failureMetadata: FailureMetadata,
+      actual: RawEntity?
+    ): RawEntitySubject {
+      return RawEntitySubject(failureMetadata, actual)
+    }
+  }
+
+  companion object {
+    /**
+     * Returns a Truth [Subject] that can be used to assert on a [RawEntity].
+     *
+     * To use this, simply import this method, and then use [assertThat] as usual on any [RawEntity]
+     * instance:
+     *
+     * ```
+     * import arcs.core.data.testutil.RawEntitySubject.Companion.assertThat
+     *
+     * val myEntity = RawEntity(...)
+     * assertThat(myEntity).isEqualTo(...)
+     * ```
+     */
+    fun assertThat(actual: RawEntity): RawEntitySubject {
+      return assertAbout(Factory).that(actual)
+    }
+
+    /**
+     * Factory method for creating a [Factory] instance, to be used with methods like
+     * `assertWithMessage`.
+     *
+     * ```kotlin
+     * assertWithMessage("blah").about(rawEntities()).that(...)
+     * ```
+     */
+    fun rawEntities(): Subject.Factory<RawEntitySubject, RawEntity> = Factory
+  }
+}
diff --git a/javatests/arcs/android/crdt/BUILD b/javatests/arcs/android/crdt/BUILD
index 615fc63573a..95c08573b7b 100644
--- a/javatests/arcs/android/crdt/BUILD
+++ b/javatests/arcs/android/crdt/BUILD
@@ -25,6 +25,7 @@ arcs_kt_android_test_suite(
         "//java/arcs/core/crdt/testutil:generators",
         "//java/arcs/core/data:rawentity",
         "//java/arcs/core/data:schema_fields",
+        "//java/arcs/core/data/testutil",
         "//java/arcs/core/data/testutil:generators",
         "//java/arcs/core/data/util:data-util",
         "//java/arcs/core/entity/testutil",
diff --git a/javatests/arcs/android/crdt/CrdtSetProtoTest.kt b/javatests/arcs/android/crdt/CrdtSetProtoTest.kt
index aa22ad4d91f..478cec38652 100644
--- a/javatests/arcs/android/crdt/CrdtSetProtoTest.kt
+++ b/javatests/arcs/android/crdt/CrdtSetProtoTest.kt
@@ -18,6 +18,7 @@ import arcs.core.common.Referencable
 import arcs.core.crdt.CrdtSet
 import arcs.core.crdt.VersionMap
 import arcs.core.data.RawEntity
+import arcs.core.data.testutil.RawEntitySubject.Companion.assertThat
 import com.google.common.truth.Truth.assertThat
 import org.junit.Test
 import org.junit.runner.RunWith
@@ -48,7 +49,8 @@ class CrdtSetProtoTest {
   @Test
   fun data_parcelableRoundTrip_works() {
     val data = CrdtSet.DataImpl(
-      versionMap, mutableMapOf(
+      versionMap,
+      mutableMapOf(
         entity1.id to CrdtSet.DataValue(VersionMap("alice" to 1), entity1),
         entity2.id to CrdtSet.DataValue(VersionMap("alice" to 1), entity2)
       )
diff --git a/javatests/arcs/android/crdt/RawEntityProtoTest.kt b/javatests/arcs/android/crdt/RawEntityProtoTest.kt
index 53d395c3aa7..3fea7eade20 100644
--- a/javatests/arcs/android/crdt/RawEntityProtoTest.kt
+++ b/javatests/arcs/android/crdt/RawEntityProtoTest.kt
@@ -15,6 +15,7 @@ import android.os.Parcel
 import androidx.test.ext.junit.runners.AndroidJUnit4
 import arcs.android.util.writeProto
 import arcs.core.data.RawEntity
+import arcs.core.data.testutil.RawEntitySubject.Companion.assertThat
 import arcs.core.entity.testutil.FixtureEntities
 import com.google.common.truth.Truth.assertThat
 import org.junit.Test
diff --git a/javatests/arcs/android/crdt/ReferencableProtoTest.kt b/javatests/arcs/android/crdt/ReferencableProtoTest.kt
index 24ac5682c8c..226ce733e25 100644
--- a/javatests/arcs/android/crdt/ReferencableProtoTest.kt
+++ b/javatests/arcs/android/crdt/ReferencableProtoTest.kt
@@ -19,6 +19,7 @@ import arcs.core.crdt.CrdtEntity
 import arcs.core.crdt.VersionMap
 import arcs.core.data.FieldType
 import arcs.core.data.RawEntity
+import arcs.core.data.testutil.RawEntitySubject.Companion.assertThat
 import arcs.core.data.util.ReferencableList
 import arcs.core.data.util.ReferencablePrimitive
 import arcs.core.data.util.toReferencable
diff --git a/javatests/arcs/android/crdt/ReferenceProtoTest.kt b/javatests/arcs/android/crdt/ReferenceProtoTest.kt
index eb5fc83908e..d9d86c5ebe3 100644
--- a/javatests/arcs/android/crdt/ReferenceProtoTest.kt
+++ b/javatests/arcs/android/crdt/ReferenceProtoTest.kt
@@ -16,6 +16,7 @@ import androidx.test.ext.junit.runners.AndroidJUnit4
 import arcs.android.util.writeProto
 import arcs.core.crdt.VersionMap
 import arcs.core.data.RawEntity
+import arcs.core.data.testutil.RawEntitySubject.Companion.assertThat
 import arcs.core.storage.RawReference
 import arcs.core.storage.StorageKeyManager
 import arcs.core.storage.keys.RamDiskStorageKey
diff --git a/javatests/arcs/android/storage/BUILD b/javatests/arcs/android/storage/BUILD
index 8ecd6f16cab..e03c4494b06 100644
--- a/javatests/arcs/android/storage/BUILD
+++ b/javatests/arcs/android/storage/BUILD
@@ -25,6 +25,7 @@ arcs_kt_android_test_suite(
         "//java/arcs/android/util/testutil",
         "//java/arcs/core/crdt",
         "//java/arcs/core/data",
+        "//java/arcs/core/data/testutil",
         "//java/arcs/core/entity/testutil",
         "//java/arcs/core/storage",
         "//java/arcs/core/storage/database",
diff --git a/javatests/arcs/core/crdt/BUILD b/javatests/arcs/core/crdt/BUILD
index cf9bf6e8756..b574b541f59 100644
--- a/javatests/arcs/core/crdt/BUILD
+++ b/javatests/arcs/core/crdt/BUILD
@@ -19,6 +19,7 @@ arcs_kt_jvm_test_suite(
         "//java/arcs/core/crdt/testutil:generators",
         "//java/arcs/core/data",
         "//java/arcs/core/data:schema_fields",
+        "//java/arcs/core/data/testutil",
         "//java/arcs/core/data/util:data-util",
         "//java/arcs/core/testutil",
         "//java/arcs/core/util",
diff --git a/javatests/arcs/core/crdt/CrdtEntityTest.kt b/javatests/arcs/core/crdt/CrdtEntityTest.kt
index 0607f1398b7..aea14325228 100644
--- a/javatests/arcs/core/crdt/CrdtEntityTest.kt
+++ b/javatests/arcs/core/crdt/CrdtEntityTest.kt
@@ -20,6 +20,7 @@ import arcs.core.crdt.CrdtEntity.Operation.SetSingleton
 import arcs.core.crdt.CrdtEntity.Reference.Companion.defaultReferenceBuilder
 import arcs.core.data.FieldType
 import arcs.core.data.RawEntity
+import arcs.core.data.testutil.RawEntitySubject.Companion.assertThat
 import arcs.core.data.util.ReferencableList
 import arcs.core.data.util.ReferencablePrimitive
 import arcs.core.data.util.toReferencable
diff --git a/javatests/arcs/core/data/testutil/BUILD b/javatests/arcs/core/data/testutil/BUILD
new file mode 100644
index 00000000000..86812441c02
--- /dev/null
+++ b/javatests/arcs/core/data/testutil/BUILD
@@ -0,0 +1,19 @@
+load(
+    "//third_party/java/arcs/build_defs:build_defs.bzl",
+    "arcs_kt_jvm_test_suite",
+)
+
+licenses(["notice"])
+
+arcs_kt_jvm_test_suite(
+    name = "testutil",
+    package = "arcs.core.data.testutil",
+    deps = [
+        "//java/arcs/core/data:rawentity",
+        "//java/arcs/core/data:schema_fields",
+        "//java/arcs/core/data/testutil",
+        "//java/arcs/core/data/util:data-util",
+        "//third_party/java/junit:junit-android",
+        "//third_party/java/truth:truth-android",
+    ],
+)
diff --git a/javatests/arcs/core/data/testutil/RawEntitySubjectTest.kt b/javatests/arcs/core/data/testutil/RawEntitySubjectTest.kt
new file mode 100644
index 00000000000..f9e969ca3dc
--- /dev/null
+++ b/javatests/arcs/core/data/testutil/RawEntitySubjectTest.kt
@@ -0,0 +1,267 @@
+package arcs.core.data.testutil
+
+import arcs.core.data.FieldType
+import arcs.core.data.RawEntity
+import arcs.core.data.testutil.RawEntitySubject.Companion.assertThat
+import arcs.core.data.testutil.RawEntitySubject.Companion.rawEntities
+import arcs.core.data.util.ReferencableList
+import arcs.core.data.util.toReferencable
+import com.google.common.truth.ExpectFailure.assertThat
+import com.google.common.truth.ExpectFailure.expectFailureAbout
+import com.google.common.truth.SimpleSubjectBuilder
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.junit.runners.JUnit4
+
+@RunWith(JUnit4::class)
+class RawEntitySubjectTest {
+  @Test
+  fun isEqualTo_null_fails() {
+    expectFailure {
+      that(ENTITY).isEqualTo(null)
+    }
+  }
+
+  @Test
+  fun isEqualTo_self_passes() {
+    assertThat(ENTITY).isEqualTo(ENTITY)
+    assertThat(ENTITY.copy()).isEqualTo(ENTITY)
+  }
+
+  @Test
+  fun isEqualTo_differentId_fails() {
+    val expected = ENTITY.copy(id = "id1")
+    val actual = ENTITY.copy(id = "id2")
+
+    val failure = expectFailure { that(actual).isEqualTo(expected) }
+
+    failure.expectDiff("rawEntity.id", expected = "id1", actual = "id2")
+  }
+
+  @Test
+  fun isEqualTo_differentCreationTimestamp_fails() {
+    val expected = ENTITY.copy(creationTimestamp = 111)
+    val actual = ENTITY.copy(creationTimestamp = 222)
+
+    val failure = expectFailure { that(actual).isEqualTo(expected) }
+
+    failure.expectDiff("rawEntity.creationTimestamp", expected = "111", actual = "222")
+  }
+
+  @Test
+  fun isEqualTo_differentExpirationTimestamp_fails() {
+    val expected = ENTITY.copy(expirationTimestamp = 111)
+    val actual = ENTITY.copy(expirationTimestamp = 222)
+
+    val failure = expectFailure { that(actual).isEqualTo(expected) }
+
+    failure.expectDiff("rawEntity.expirationTimestamp", expected = "111", actual = "222")
+  }
+
+  @Test
+  fun isEqualTo_sameSingletonFields_passes() {
+    val expected = ENTITY.copy(singletons = mapOf("a" to VALUE1, "b" to VALUE2))
+    val actual = expected.copy()
+
+    assertThat(actual).isEqualTo(expected)
+  }
+
+  @Test
+  fun isEqualTo_differentSingletonFields_fails() {
+    val expected = ENTITY.copy(singletons = mapOf("a" to VALUE1))
+    val actual = expected.copy(singletons = mapOf("a" to VALUE2))
+
+    val failure = expectFailure { that(actual).isEqualTo(expected) }
+
+    failure.expectDiff(
+      "rawEntity.singletons[a]",
+      expected = VALUE1.toString(),
+      actual = VALUE2.toString()
+    )
+  }
+
+  @Test
+  fun isEqualTo_extraSingletonField_fails() {
+    val expected = ENTITY.copy(singletons = emptyMap())
+    val actual = ENTITY.copy(singletons = mapOf("a" to VALUE1))
+
+    val failure = expectFailure { that(actual).isEqualTo(expected) }
+
+    failure.expectDiff(
+      "rawEntity.singletons[a]",
+      expected = "<absent>",
+      actual = VALUE1.toString()
+    )
+  }
+
+  @Test
+  fun isEqualTo_missingSingletonField_fails() {
+    val expected = ENTITY.copy(singletons = mapOf("a" to VALUE1))
+    val actual = ENTITY.copy(singletons = emptyMap())
+
+    val failure = expectFailure { that(actual).isEqualTo(expected) }
+
+    failure.expectDiff(
+      "rawEntity.singletons[a]",
+      expected = VALUE1.toString(),
+      actual = "<absent>"
+    )
+  }
+
+  @Test
+  fun isEqualTo_sameCollectionFields_passes() {
+    val expected = ENTITY.copy(collections = mapOf("a" to setOf(VALUE1), "b" to setOf(VALUE2)))
+    val actual = expected.copy()
+
+    assertThat(actual).isEqualTo(expected)
+  }
+
+  @Test
+  fun isEqualTo_differentCollectionFields_differentValue_fails() {
+    val expected = ENTITY.copy(collections = mapOf("a" to setOf(VALUE1)))
+    val actual = expected.copy(collections = mapOf("a" to setOf(VALUE2)))
+
+    val failure = expectFailure { that(actual).isEqualTo(expected) }
+
+    failure.expectDiff(
+      "rawEntity.collections[a][0]",
+      expected = VALUE1.toString(),
+      actual = VALUE2.toString()
+    )
+  }
+
+  @Test
+  fun isEqualTo_differentCollectionFields_additionalValue_fails() {
+    val expected = ENTITY.copy(collections = mapOf("a" to setOf(VALUE1)))
+    val actual = expected.copy(collections = mapOf("a" to setOf(VALUE1, VALUE2)))
+
+    val failure = expectFailure { that(actual).isEqualTo(expected) }
+
+    failure.expectDiff(
+      "rawEntity.collections[a][1]",
+      expected = "<absent>",
+      actual = VALUE2.toString()
+    )
+  }
+
+  @Test
+  fun isEqualTo_extraCollectionField_fails() {
+    val expected = ENTITY.copy(collections = emptyMap())
+    val actual = ENTITY.copy(collections = mapOf("a" to emptySet()))
+
+    val failure = expectFailure { that(actual).isEqualTo(expected) }
+
+    failure.expectDiff(
+      "rawEntity.collections[a]",
+      expected = "<absent>",
+      actual = "[]"
+    )
+  }
+
+  @Test
+  fun isEqualTo_missingCollectionField_fails() {
+    val expected = ENTITY.copy(collections = mapOf("a" to emptySet()))
+    val actual = ENTITY.copy(collections = emptyMap())
+
+    val failure = expectFailure { that(actual).isEqualTo(expected) }
+
+    failure.expectDiff(
+      "rawEntity.collections[a]",
+      expected = "[]",
+      actual = "<absent>"
+    )
+  }
+
+  @Test
+  fun isEqualTo_nestedRawEntityInSingletonField() {
+    fun createNestedEntity(id: String): RawEntity {
+      val deepInnerEntity = ENTITY.copy(id = id)
+      val innerEntity = ENTITY.copy(singletons = mapOf("deep" to deepInnerEntity))
+      return ENTITY.copy(singletons = mapOf("inner" to innerEntity))
+    }
+    val expected = createNestedEntity("id1")
+    val actual = createNestedEntity("id2")
+
+    val failure = expectFailure { that(actual).isEqualTo(expected) }
+
+    failure.expectDiff(
+      "rawEntity.singletons[inner].singletons[deep].id",
+      expected = "id1",
+      actual = "id2"
+    )
+  }
+
+  @Test
+  fun isEqualTo_nestedRawEntityInCollectionField() {
+    fun createNestedEntity(id: String): RawEntity {
+      val deepInnerEntity = ENTITY.copy(id = id)
+      val innerEntity = ENTITY.copy(collections = mapOf("deep" to setOf(VALUE1, deepInnerEntity)))
+      return ENTITY.copy(collections = mapOf("inner" to setOf(innerEntity)))
+    }
+    val expected = createNestedEntity("id1")
+    val actual = createNestedEntity("id2")
+
+    val failure = expectFailure { that(actual).isEqualTo(expected) }
+
+    failure.expectDiff(
+      "rawEntity.collections[inner][0].collections[deep][1].id",
+      expected = "id1",
+      actual = "id2"
+    )
+  }
+
+  @Test
+  fun isEqualTo_referencableListWithDifferentItemType_fails() {
+    val expected = ENTITY.copy(
+      singletons = mapOf("a" to ReferencableList(listOf(VALUE1, VALUE2), FieldType.BigInt))
+    )
+    val actual = ENTITY.copy(
+      singletons = mapOf("a" to ReferencableList(listOf(VALUE1, VALUE2), FieldType.Duration))
+    )
+
+    val failure = expectFailure { that(actual).isEqualTo(expected) }
+
+    failure.expectDiff(
+      "rawEntity.singletons[a].itemType",
+      expected = "BigInt",
+      actual = "Duration"
+    )
+  }
+
+  @Test
+  fun isEqualTo_referencableListWithDifferentItems_fails() {
+    val expected = ENTITY.copy(
+      singletons = mapOf("a" to ReferencableList(listOf(VALUE1, VALUE1), FieldType.BigInt))
+    )
+    val actual = ENTITY.copy(
+      singletons = mapOf("a" to ReferencableList(listOf(VALUE1, VALUE2), FieldType.BigInt))
+    )
+
+    val failure = expectFailure { that(actual).isEqualTo(expected) }
+
+    failure.expectDiff(
+      "rawEntity.singletons[a][1]",
+      expected = VALUE1.toString(),
+      actual = VALUE2.toString()
+    )
+  }
+
+  private companion object {
+    private fun expectFailure(
+      callback: (SimpleSubjectBuilder<RawEntitySubject, RawEntity>).() -> Unit
+    ): AssertionError {
+      return expectFailureAbout(rawEntities(), callback)
+    }
+
+    private val ENTITY = RawEntity(id = "id", creationTimestamp = 987, expirationTimestamp = 654)
+
+    private val VALUE1 = "value1".toReferencable()
+    private val VALUE2 = "value2".toReferencable()
+
+    private fun AssertionError.expectDiff(field: String, expected: String, actual: String) {
+      assertThat(this).factValue("for field").isEqualTo(field)
+      assertThat(this).factValue("expected").isEqualTo(expected)
+      assertThat(this).factValue("but was").isEqualTo(actual)
+    }
+  }
+}
diff --git a/javatests/arcs/core/entity/BUILD b/javatests/arcs/core/entity/BUILD
index 27e3ac72057..ef16a765a78 100644
--- a/javatests/arcs/core/entity/BUILD
+++ b/javatests/arcs/core/entity/BUILD
@@ -15,6 +15,7 @@ arcs_kt_jvm_test_suite(
         "//java/arcs/core/data",
         "//java/arcs/core/data:schema_fields",
         "//java/arcs/core/data/expression",
+        "//java/arcs/core/data/testutil",
         "//java/arcs/core/data/util:data-util",
         "//java/arcs/core/entity",
         "//java/arcs/core/entity/testutil",
diff --git a/javatests/arcs/core/entity/EntityBaseTest.kt b/javatests/arcs/core/entity/EntityBaseTest.kt
index 07991168a8e..76da267a066 100644
--- a/javatests/arcs/core/entity/EntityBaseTest.kt
+++ b/javatests/arcs/core/entity/EntityBaseTest.kt
@@ -19,6 +19,7 @@ import arcs.core.data.RawEntity
 import arcs.core.data.Schema
 import arcs.core.data.SchemaFields
 import arcs.core.data.SchemaRegistry
+import arcs.core.data.testutil.RawEntitySubject.Companion.assertThat
 import arcs.core.data.util.toReferencable
 import arcs.core.entity.testutil.DummyEntity
 import arcs.core.entity.testutil.DummyVariableEntity
diff --git a/javatests/arcs/core/entity/EntityDereferencerFactoryTest.kt b/javatests/arcs/core/entity/EntityDereferencerFactoryTest.kt
index b76de3b7fc3..ca368042ae3 100644
--- a/javatests/arcs/core/entity/EntityDereferencerFactoryTest.kt
+++ b/javatests/arcs/core/entity/EntityDereferencerFactoryTest.kt
@@ -6,6 +6,7 @@ import arcs.core.data.RawEntity
 import arcs.core.data.Schema
 import arcs.core.data.SchemaFields
 import arcs.core.data.SchemaRegistry
+import arcs.core.data.testutil.RawEntitySubject.Companion.assertThat
 import arcs.core.data.util.ReferencableList
 import arcs.core.data.util.toReferencable
 import arcs.core.storage.RawEntityDereferencer
diff --git a/javatests/arcs/core/entity/StorageAdapterTest.kt b/javatests/arcs/core/entity/StorageAdapterTest.kt
index f483ad4718a..9d662d29179 100644
--- a/javatests/arcs/core/entity/StorageAdapterTest.kt
+++ b/javatests/arcs/core/entity/StorageAdapterTest.kt
@@ -9,6 +9,7 @@ import arcs.core.data.SchemaRegistry
 import arcs.core.data.expression.asExpr
 import arcs.core.data.expression.neq
 import arcs.core.data.expression.text
+import arcs.core.data.testutil.RawEntitySubject.Companion.assertThat
 import arcs.core.data.util.toReferencable
 import arcs.core.entity.testutil.DummyEntity
 import arcs.core.entity.testutil.InlineDummyEntity
diff --git a/javatests/arcs/core/entity/VariableEntityBaseTest.kt b/javatests/arcs/core/entity/VariableEntityBaseTest.kt
index 378a95faeab..bba9027bcfc 100644
--- a/javatests/arcs/core/entity/VariableEntityBaseTest.kt
+++ b/javatests/arcs/core/entity/VariableEntityBaseTest.kt
@@ -2,6 +2,7 @@ package arcs.core.entity
 
 import arcs.core.crdt.VersionMap
 import arcs.core.data.SchemaRegistry
+import arcs.core.data.testutil.RawEntitySubject.Companion.assertThat
 import arcs.core.entity.testutil.DummyEntity
 import arcs.core.entity.testutil.DummyVariableEntity
 import arcs.core.entity.testutil.InlineDummyEntity
diff --git a/javatests/arcs/core/storage/BUILD b/javatests/arcs/core/storage/BUILD
index 5d3a1c4b1f3..eee3b81e45f 100644
--- a/javatests/arcs/core/storage/BUILD
+++ b/javatests/arcs/core/storage/BUILD
@@ -23,6 +23,7 @@ arcs_kt_android_test_suite(
         "//java/arcs/core/crdt/testing",
         "//java/arcs/core/data",
         "//java/arcs/core/data:schema_fields",
+        "//java/arcs/core/data/testutil",
         "//java/arcs/core/data/util:data-util",
         "//java/arcs/core/storage",
         "//java/arcs/core/storage/api",
diff --git a/javatests/arcs/core/storage/ReferenceModeStoreDatabaseIntegrationTest.kt b/javatests/arcs/core/storage/ReferenceModeStoreDatabaseIntegrationTest.kt
index cfee9b7710a..14c66094b14 100644
--- a/javatests/arcs/core/storage/ReferenceModeStoreDatabaseIntegrationTest.kt
+++ b/javatests/arcs/core/storage/ReferenceModeStoreDatabaseIntegrationTest.kt
@@ -16,6 +16,7 @@ import arcs.core.crdt.CrdtSet
 import arcs.core.crdt.VersionMap
 import arcs.core.data.RawEntity
 import arcs.core.data.SchemaRegistry
+import arcs.core.data.testutil.RawEntitySubject.Companion.assertThat
 import arcs.core.storage.driver.DatabaseDriver
 import arcs.core.storage.driver.DatabaseDriverProvider
 import arcs.core.storage.keys.DatabaseStorageKey
@@ -75,14 +76,16 @@ class ReferenceModeStoreDatabaseIntegrationTest(
 
     // Read data (using a new store ensures we read from the db instead of using cached values).
     val activeStore2 = collectionReferenceModeStore(scope = this)
-    val e1RefVersionMap = if (BuildFlags.REFERENCE_MODE_STORE_FIXES)
+    val e1RefVersionMap = if (BuildFlags.REFERENCE_MODE_STORE_FIXES) {
       VersionMap(activeStore.crdtKey to 1)
-    else
+    } else {
       VersionMap("me" to 1)
-    val e2RefVersionMap = if (BuildFlags.REFERENCE_MODE_STORE_FIXES)
+    }
+    val e2RefVersionMap = if (BuildFlags.REFERENCE_MODE_STORE_FIXES) {
       VersionMap(activeStore.crdtKey to 1)
-    else
+    } else {
       VersionMap("me" to 2)
+    }
 
     val e1Ref = CrdtSet.DataValue(
       VersionMap("me" to 1),
diff --git a/javatests/arcs/core/storage/ReferenceModeStoreTest.kt b/javatests/arcs/core/storage/ReferenceModeStoreTest.kt
index 49ba5059645..499dbedf6c1 100644
--- a/javatests/arcs/core/storage/ReferenceModeStoreTest.kt
+++ b/javatests/arcs/core/storage/ReferenceModeStoreTest.kt
@@ -15,6 +15,7 @@ import arcs.core.crdt.CrdtData
 import arcs.core.crdt.CrdtSet
 import arcs.core.crdt.VersionMap
 import arcs.core.data.RawEntity
+import arcs.core.data.testutil.RawEntitySubject.Companion.assertThat
 import arcs.core.storage.referencemode.RefModeStoreData
 import arcs.core.storage.referencemode.ReferenceModeStorageKey
 import arcs.core.storage.testutil.FakeDriver
diff --git a/javatests/arcs/core/storage/driver/BUILD b/javatests/arcs/core/storage/driver/BUILD
index f174cf4133d..ae440fd6067 100644
--- a/javatests/arcs/core/storage/driver/BUILD
+++ b/javatests/arcs/core/storage/driver/BUILD
@@ -15,6 +15,7 @@ arcs_kt_jvm_test_suite(
         "//java/arcs/core/crdt",
         "//java/arcs/core/data",
         "//java/arcs/core/data:schema_fields",
+        "//java/arcs/core/data/testutil",
         "//java/arcs/core/data/util:data-util",
         "//java/arcs/core/storage",
         "//java/arcs/core/storage/database",
diff --git a/javatests/arcs/core/storage/driver/DatabaseDriverTest.kt b/javatests/arcs/core/storage/driver/DatabaseDriverTest.kt
index 0b9cf051a08..71d23cc28d7 100644
--- a/javatests/arcs/core/storage/driver/DatabaseDriverTest.kt
+++ b/javatests/arcs/core/storage/driver/DatabaseDriverTest.kt
@@ -22,6 +22,7 @@ import arcs.core.data.RawEntity
 import arcs.core.data.Schema
 import arcs.core.data.SchemaFields
 import arcs.core.data.SchemaName
+import arcs.core.data.testutil.RawEntitySubject.Companion.assertThat
 import arcs.core.data.util.ReferencablePrimitive
 import arcs.core.data.util.toReferencable
 import arcs.core.storage.RawReference
diff --git a/javatests/arcs/sdk/BUILD b/javatests/arcs/sdk/BUILD
index fa302b42e97..c038e9c654c 100644
--- a/javatests/arcs/sdk/BUILD
+++ b/javatests/arcs/sdk/BUILD
@@ -20,6 +20,7 @@ arcs_kt_jvm_test_suite(
         "//java/arcs/core/common",
         "//java/arcs/core/data",
         "//java/arcs/core/data:schema_fields",
+        "//java/arcs/core/data/testutil",
         "//java/arcs/core/data/util:data-util",
         "//java/arcs/core/entity",
         "//java/arcs/core/entity/testutil",
diff --git a/javatests/arcs/sdk/GeneratedEntityTest.kt b/javatests/arcs/sdk/GeneratedEntityTest.kt
index e63af18181e..2fb3d462122 100755
--- a/javatests/arcs/sdk/GeneratedEntityTest.kt
+++ b/javatests/arcs/sdk/GeneratedEntityTest.kt
@@ -6,6 +6,7 @@ import arcs.core.data.FieldType
 import arcs.core.data.RawEntity
 import arcs.core.data.RawEntity.Companion.NO_REFERENCE_ID
 import arcs.core.data.SchemaRegistry
+import arcs.core.data.testutil.RawEntitySubject.Companion.assertThat
 import arcs.core.data.util.ReferencableList
 import arcs.core.data.util.toReferencable
 import arcs.core.entity.testutil.FixtureEntities