diff --git a/README.md b/README.md
index 225c9e1..0f5c89f 100644
--- a/README.md
+++ b/README.md
@@ -8,6 +8,12 @@
 
 Swift Object Graph
 
+## Requirements
+
+Swift 6.0 or later, macOS 10.15, iOS 13, tvOS 13, and watchOS 6.
+
+`InMemoryStorage`, `InMemoryModelStorage`, and `InMemoryViewContext` are the exception: they serialize access to their shared backing with `Synchronization.Mutex`, which is not back-deployed, so on Apple platforms they require macOS 15, iOS 18, tvOS 18, watchOS 11, or visionOS 2. Everything else in `CoreModel` — the schema types, predicates, and the `ModelStorage`/`ViewContext` protocols — is available at the package's own deployment targets, as are third-party backends such as SQLite and MongoDB. Linux, Windows, Android, and WebAssembly have no such restriction.
+
 ## Backends
 
 - CoreData
diff --git a/Sources/CoreModel/InMemoryStorage.swift b/Sources/CoreModel/InMemoryStorage.swift
index 6b1be41..2153dc1 100644
--- a/Sources/CoreModel/InMemoryStorage.swift
+++ b/Sources/CoreModel/InMemoryStorage.swift
@@ -7,7 +7,7 @@
 //
 
 #if !hasFeature(Embedded)
-import Foundation
+import Synchronization
 #endif
 
 /// Shared in-memory backing store.
@@ -17,51 +17,64 @@ import Foundation
 /// reference type, a store and a view context that share the same instance observe
 /// the exact same data.
 ///
-/// The type is thread-safe: all access is serialized so the actor-isolated
-/// ``InMemoryModelStorage`` and the main-actor ``InMemoryViewContext`` can operate
-/// on the same instance concurrently. Under Embedded Swift the lock is elided:
-/// with a concurrency runtime the backing is only ever touched from within its
+/// The type is thread-safe: the mutable state lives inside a `Mutex`, so the
+/// actor-isolated ``InMemoryModelStorage`` and the main-actor ``InMemoryViewContext``
+/// can operate on the same instance concurrently. Under Embedded Swift the mutex is
+/// elided: with a concurrency runtime the backing is only ever touched from within its
 /// owning actor, and without one (e.g. bare-metal ARM, where ``ModelStorage``
 /// itself is unavailable) this synchronous store is the storage API, used
 /// directly from the single-threaded main loop.
-public final class InMemoryStorage {
+///
+/// - Note: On Apple platforms this type is gated on the availability of
+///   `Synchronization.Mutex`, which is not back-deployed. The rest of `CoreModel`
+///   keeps the package's lower deployment targets.
+@available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
+internal final class InMemoryStorage {
 
     /// The schema entities are validated against.
     public let model: Model
 
-    private var objects = [EntityName: [ObjectID: ModelData]]()
+    /// The mutable state, only reachable through ``withState(_:)``.
+    private struct State {
+
+        var objects = [EntityName: [ObjectID: ModelData]]()
 
-    private var functions = [String: DatabaseFunction]()
+        var functions = [String: DatabaseFunction]()
+    }
 
-    #if !hasFeature(Embedded)
-    private let lock = NSLock()
+    #if hasFeature(Embedded)
+    private var state = State()
+    #else
+    private let state = Mutex(State())
     #endif
 
     public init(model: Model) {
         self.model = model
     }
 
-    private func withLock<T, E>(_ body: () throws(E) -> T) throws(E) -> T where E: Error {
-        #if !hasFeature(Embedded)
-        lock.lock()
-        defer { lock.unlock() }
+    private func withState<T, E>(_ body: (inout State) throws(E) -> T) throws(E) -> T where E: Error {
+        #if hasFeature(Embedded)
+        return try body(&state)
+        #else
+        return try state.withLock { (state) throws(E) in
+            try body(&state)
+        }
         #endif
-        return try body()
     }
 
     public func fetch(_ entity: EntityName, for id: ObjectID) throws(CoreModelError) -> ModelData? {
-        try withLock { () throws(CoreModelError) in
+        try withState { (state) throws(CoreModelError) in
             try validate(entity)
-            return objects[entity]?[id].map { normalized(entity: entity, $0) }
+            return state.objects[entity]?[id].map { normalized(entity: entity, $0, objects: state.objects) }
         }
     }
 
     public func fetch(_ fetchRequest: FetchRequest) throws(CoreModelError) -> [ModelData] {
-        try withLock { () throws(CoreModelError) in
+        try withState { (state) throws(CoreModelError) in
             try validate(fetchRequest.entity)
-            let values = (objects[fetchRequest.entity].map { Array($0.values) } ?? [])
-                .map { normalized(entity: fetchRequest.entity, $0) }
-            return fetchRequest.evaluate(values, functions: functions)
+            let values = (state.objects[fetchRequest.entity].map { Array($0.values) } ?? [])
+                .map { normalized(entity: fetchRequest.entity, $0, objects: state.objects) }
+            return fetchRequest.evaluate(values, functions: state.functions)
         }
     }
 
@@ -85,7 +98,11 @@ public final class InMemoryStorage {
     /// assigned — previously decoded as `keyNotFound` instead of the collection it actually
     /// has. To-one relationships are left alone: an absent required reference is a real data
     /// problem, not a default.
-    private func normalized(entity: EntityName, _ value: ModelData) -> ModelData {
+    private func normalized(
+        entity: EntityName,
+        _ value: ModelData,
+        objects: [EntityName: [ObjectID: ModelData]]
+    ) -> ModelData {
         guard let description = model[entity] else { return value }
         var value = value
         for attribute in description.attributes where value.attributes[attribute.id] == nil {
@@ -139,7 +156,7 @@ public final class InMemoryStorage {
     }
 
     public func insert(_ value: ModelData) throws(CoreModelError) {
-        try withLock { () throws(CoreModelError) in
+        try withState { (state) throws(CoreModelError) in
             try validate(value.entity)
             // A key present in `value` overrides; a key the existing row already had that
             // `value` doesn't mention is preserved — the same "only touch the columns you
@@ -148,7 +165,7 @@ public final class InMemoryStorage {
             // touch every relationship (e.g. a site catalog refresh that never re-states
             // `parkingReservations`, which is written by an entirely separate sync) would
             // silently wipe those links instead of leaving them alone.
-            if var existing = objects[value.entity]?[value.id] {
+            if var existing = state.objects[value.entity]?[value.id] {
                 // - Note: Explicit loops rather than `Dictionary.merge(_:uniquingKeysWith:)` —
                 //   the closure-based overload does dynamic casting internally, which is
                 //   disallowed under Embedded Swift.
@@ -158,9 +175,9 @@ public final class InMemoryStorage {
                 for (key, relationship) in value.relationships {
                     existing.relationships[key] = relationship
                 }
-                objects[value.entity, default: [:]][value.id] = existing
+                state.objects[value.entity, default: [:]][value.id] = existing
             } else {
-                objects[value.entity, default: [:]][value.id] = value
+                state.objects[value.entity, default: [:]][value.id] = value
             }
         }
     }
@@ -172,24 +189,24 @@ public final class InMemoryStorage {
     }
 
     public func delete(_ entity: EntityName, for id: ObjectID) throws(CoreModelError) {
-        try withLock { () throws(CoreModelError) in
+        try withState { (state) throws(CoreModelError) in
             try validate(entity)
-            objects[entity]?[id] = nil
+            state.objects[entity]?[id] = nil
         }
     }
 
     public func delete(_ entity: EntityName, for ids: [ObjectID]) throws(CoreModelError) {
-        try withLock { () throws(CoreModelError) in
+        try withState { (state) throws(CoreModelError) in
             try validate(entity)
             for id in ids {
-                objects[entity]?[id] = nil
+                state.objects[entity]?[id] = nil
             }
         }
     }
 
     public func register(function: DatabaseFunction) {
-        withLock {
-            functions[function.name] = function
+        withState { state in
+            state.functions[function.name] = function
         }
     }
 
@@ -200,6 +217,14 @@ public final class InMemoryStorage {
     }
 }
 
-// - Note: Safe because every access is serialized through `withLock` (non-Embedded)
-//   or confined to the owning actor (Embedded).
+#if hasFeature(Embedded)
+// - Note: Safe because the backing is confined to the owning actor, or to the
+//   single-threaded main loop on targets without a concurrency runtime.
+@available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
 extension InMemoryStorage: @unchecked Sendable {}
+#else
+// - Note: Checked: `model` is an immutable `Sendable` value and every piece of
+//   mutable state lives inside the `Mutex`.
+@available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
+extension InMemoryStorage: Sendable {}
+#endif
diff --git a/Sources/CoreModel/InMemoryStore.swift b/Sources/CoreModel/InMemoryStore.swift
index bda223e..d1fa836 100644
--- a/Sources/CoreModel/InMemoryStore.swift
+++ b/Sources/CoreModel/InMemoryStore.swift
@@ -22,6 +22,10 @@
 ///
 /// On platforms that support it, ``viewContext`` returns a synchronous,
 /// main-actor ``InMemoryViewContext`` backed by the same data.
+///
+/// - Note: Inherits ``InMemoryStorage``'s Apple-platform availability, which is
+///   gated on `Synchronization.Mutex`.
+@available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
 public actor InMemoryModelStorage {
 
     internal let backing: InMemoryStorage
@@ -90,6 +94,7 @@ public actor InMemoryModelStorage {
 // MARK: - ViewContext
 
 #if !hasFeature(Embedded)
+@available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
 public extension InMemoryModelStorage {
 
     /// A synchronous, main-actor view context backed by the same data as this store.
@@ -116,6 +121,7 @@ public extension InMemoryModelStorage {
 //   `CoreModelError` to `any Error` is disallowed (`#EmbeddedRestrictions`).
 //   Embedded consumers call the store's methods directly; they provide the
 //   same API with typed throws.
+@available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
 extension InMemoryModelStorage: ModelStorage {}
 #endif
 
diff --git a/Sources/CoreModel/InMemoryViewContext.swift b/Sources/CoreModel/InMemoryViewContext.swift
index d7933be..65fefa3 100644
--- a/Sources/CoreModel/InMemoryViewContext.swift
+++ b/Sources/CoreModel/InMemoryViewContext.swift
@@ -22,6 +22,10 @@
 ///
 /// Useful for SwiftUI previews, unit tests, and lightweight main-thread caches
 /// where the `async` ``InMemoryModelStorage`` would be inconvenient.
+///
+/// - Note: Inherits ``InMemoryStorage``'s Apple-platform availability, which is
+///   gated on `Synchronization.Mutex`.
+@available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
 @MainActor
 public final class InMemoryViewContext {
 
@@ -90,6 +94,7 @@ public final class InMemoryViewContext {
 
 // MARK: - ViewContext
 
+@available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
 extension InMemoryViewContext: ViewContext {}
 
 #endif
diff --git a/Tests/CoreModelTests/InMemoryStoreTests.swift b/Tests/CoreModelTests/InMemoryStoreTests.swift
index d5ed7a0..941545f 100644
--- a/Tests/CoreModelTests/InMemoryStoreTests.swift
+++ b/Tests/CoreModelTests/InMemoryStoreTests.swift
@@ -16,6 +16,7 @@ import Testing
         EntityDescription(entity: Event.self)
     ])
 
+    @available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
     @Test func insertAndFetch() async throws {
         let store = InMemoryModelStorage(model: Self.model)
         let person = Person(name: "Alice", age: 30)
@@ -27,6 +28,7 @@ import Testing
         #expect(missing == nil)
     }
 
+    @available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
     @Test func update() async throws {
         let store = InMemoryModelStorage(model: Self.model)
         var person = Person(name: "Alice", age: 30)
@@ -39,6 +41,7 @@ import Testing
         #expect(count == 1)
     }
 
+    @available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
     @Test func batchInsert() async throws {
         let store = InMemoryModelStorage(model: Self.model)
         let people = (1...10).map { Person(name: "Person \($0)", age: UInt(20 + $0)) }
@@ -47,6 +50,7 @@ import Testing
         #expect(count == 10)
     }
 
+    @available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
     @Test func fetchRequest() async throws {
         let store = InMemoryModelStorage(model: Self.model)
         let people = (1...5).map { Person(name: "Person \($0)", age: UInt(20 + $0)) }
@@ -77,6 +81,7 @@ import Testing
         #expect(count == 2)
     }
 
+    @available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
     @Test func fetchID() async throws {
         let store = InMemoryModelStorage(model: Self.model)
         let person = Person(name: "Alice", age: 30)
@@ -85,6 +90,7 @@ import Testing
         #expect(ids == [ObjectID(person.id)])
     }
 
+    @available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
     @Test func delete() async throws {
         let store = InMemoryModelStorage(model: Self.model)
         let people = (1...3).map { Person(name: "Person \($0)", age: UInt(20 + $0)) }
@@ -98,6 +104,7 @@ import Testing
         #expect(count == 0)
     }
 
+    @available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
     @Test func relationshipPredicate() async throws {
         let store = InMemoryModelStorage(model: Self.model)
         let event = Event(name: "WWDC", date: Date())
@@ -111,6 +118,7 @@ import Testing
         #expect(attendees == [attendee])
     }
 
+    @available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
     @Test func customFunction() async throws {
         let store = InMemoryModelStorage(model: Self.model)
         let stringLength = DatabaseFunction(name: "LENGTH", argumentCount: 1) { arguments in
@@ -136,6 +144,7 @@ import Testing
         #expect(longNames.map { $0.name } == ["Alexandra"])
     }
 
+    @available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
     @Test func modelValidation() async throws {
         let store = InMemoryModelStorage(model: Self.model)
         let person = Person(name: "Alice", age: 30)
diff --git a/Tests/CoreModelTests/InMemoryViewContextTests.swift b/Tests/CoreModelTests/InMemoryViewContextTests.swift
index 5740177..44a2630 100644
--- a/Tests/CoreModelTests/InMemoryViewContextTests.swift
+++ b/Tests/CoreModelTests/InMemoryViewContextTests.swift
@@ -17,6 +17,7 @@ import Testing
         EntityDescription(entity: Event.self)
     ])
 
+    @available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
     @Test func insertAndFetch() throws {
         let context = InMemoryViewContext(model: Self.model)
         let person = Person(name: "Alice", age: 30)
@@ -28,6 +29,7 @@ import Testing
         #expect(missing == nil)
     }
 
+    @available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
     @Test func update() throws {
         let context = InMemoryViewContext(model: Self.model)
         var person = Person(name: "Alice", age: 30)
@@ -40,6 +42,7 @@ import Testing
         #expect(count == 1)
     }
 
+    @available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
     @Test func batchInsert() throws {
         let context = InMemoryViewContext(model: Self.model)
         let people = (1...10).map { Person(name: "Person \($0)", age: UInt(20 + $0)) }
@@ -48,6 +51,7 @@ import Testing
         #expect(count == 10)
     }
 
+    @available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
     @Test func fetchRequest() throws {
         let context = InMemoryViewContext(model: Self.model)
         let people = (1...5).map { Person(name: "Person \($0)", age: UInt(20 + $0)) }
@@ -78,6 +82,7 @@ import Testing
         #expect(count == 2)
     }
 
+    @available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
     @Test func fetchID() throws {
         let context = InMemoryViewContext(model: Self.model)
         let person = Person(name: "Alice", age: 30)
@@ -86,6 +91,7 @@ import Testing
         #expect(ids == [ObjectID(person.id)])
     }
 
+    @available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
     @Test func delete() throws {
         let context = InMemoryViewContext(model: Self.model)
         let people = (1...3).map { Person(name: "Person \($0)", age: UInt(20 + $0)) }
@@ -99,6 +105,7 @@ import Testing
         #expect(count == 0)
     }
 
+    @available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
     @Test func relationshipPredicate() throws {
         let context = InMemoryViewContext(model: Self.model)
         let event = Event(name: "WWDC", date: Date())
@@ -112,6 +119,7 @@ import Testing
         #expect(attendees == [attendee])
     }
 
+    @available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
     @Test func customFunction() throws {
         let context = InMemoryViewContext(model: Self.model)
         let stringLength = DatabaseFunction(name: "LENGTH", argumentCount: 1) { arguments in
@@ -137,6 +145,7 @@ import Testing
         #expect(longNames.map { $0.name } == ["Alexandra"])
     }
 
+    @available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
     @Test func sharedDataWithStore() async throws {
         let store = InMemoryModelStorage(model: Self.model)
         let context = store.viewContext
@@ -158,6 +167,30 @@ import Testing
         #expect(try context.count(FetchRequest(entity: Person.entityName)) == 1)
     }
 
+    @available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
+    @Test func concurrentAccessWithStore() async throws {
+        let store = InMemoryModelStorage(model: Self.model)
+        let context = store.viewContext
+        let people = (0 ..< 100).map { Person(name: "Person \($0)", age: UInt($0)) }
+        // writers run on the store's actor while this main-actor task reads the same
+        // backing, so the two isolation domains genuinely overlap
+        async let inserts: Void = withThrowingTaskGroup(of: Void.self) { group in
+            for person in people {
+                group.addTask {
+                    try await store.insert(person)
+                }
+            }
+            try await group.waitForAll()
+        }
+        for _ in 0 ..< 100 {
+            _ = try context.count(FetchRequest(entity: Person.entityName))
+        }
+        try await inserts
+        let count = try context.count(FetchRequest(entity: Person.entityName))
+        #expect(count == 100)
+    }
+
+    @available(macOS 15, iOS 18, tvOS 18, watchOS 11, visionOS 2, *)
     @Test func modelValidation() throws {
         let context = InMemoryViewContext(model: Self.model)
         let person = Person(name: "Alice", age: 30)