diff --git a/Sources/CMakeLists.txt b/Sources/CMakeLists.txt
index 09e5e9fd6..4fc0847b7 100644
--- a/Sources/CMakeLists.txt
+++ b/Sources/CMakeLists.txt
@@ -104,6 +104,7 @@ endif()
 include(AvailabilityDefinitions)
 include(CompilerSettings)
 add_subdirectory(_TestDiscovery)
+add_subdirectory(_TestingInterop)
 add_subdirectory(_TestingInternals)
 add_subdirectory(Overlays)
 add_subdirectory(Testing)
diff --git a/Sources/_TestingInterop/CMakeLists.txt b/Sources/_TestingInterop/CMakeLists.txt
new file mode 100644
index 000000000..adbf7037b
--- /dev/null
+++ b/Sources/_TestingInterop/CMakeLists.txt
@@ -0,0 +1,24 @@
+# This source file is part of the Swift.org open source project
+#
+# Copyright (c) 2025 Apple Inc. and the Swift project authors
+# Licensed under Apache License v2.0 with Runtime Library Exception
+#
+# See http://swift.org/LICENSE.txt for license information
+# See http://swift.org/CONTRIBUTORS.txt for Swift project authors
+
+add_library(_TestingInterop
+  FallbackEventHandler.swift)
+
+target_link_libraries(_TestingInterop PRIVATE
+  _TestingInternals)
+if(NOT BUILD_SHARED_LIBS)
+  # When building a static library, tell clients to autolink the internal
+  # libraries.
+  target_compile_options(_TestingInterop PRIVATE
+    "SHELL:-Xfrontend -public-autolink-library -Xfrontend _TestingInternals")
+endif()
+target_compile_options(_TestingInterop PRIVATE
+  -enable-library-evolution
+  -emit-module-interface -emit-module-interface-path $<TARGET_PROPERTY:_TestingInterop,Swift_MODULE_DIRECTORY>/_TestingInterop.swiftinterface)
+
+_swift_testing_install_target(_TestingInterop)
diff --git a/Sources/_TestingInterop/FallbackEventHandler.swift b/Sources/_TestingInterop/FallbackEventHandler.swift
new file mode 100644
index 000000000..2e33cd04d
--- /dev/null
+++ b/Sources/_TestingInterop/FallbackEventHandler.swift
@@ -0,0 +1,106 @@
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+//
+
+#if !SWT_NO_INTEROP
+#if SWT_TARGET_OS_APPLE && !SWT_NO_OS_UNFAIR_LOCK
+private import _TestingInternals
+#else
+private import Synchronization
+#endif
+
+#if SWT_TARGET_OS_APPLE && !SWT_NO_OS_UNFAIR_LOCK
+/// The installed event handler.
+private nonisolated(unsafe) let _fallbackEventHandler = {
+  let result = ManagedBuffer<FallbackEventHandler?, os_unfair_lock>.create(
+    minimumCapacity: 1,
+    makingHeaderWith: { _ in nil }
+  )
+  result.withUnsafeMutablePointerToHeader { $0.initialize(to: nil) }
+  return result
+}()
+#else
+/// The installed event handler.
+private nonisolated(unsafe) let _fallbackEventHandler = Atomic<UnsafeRawPointer?>(nil)
+#endif
+
+/// A type describing a fallback event handler that testing API can invoke as an
+/// alternate method of reporting test events to the current test runner.
+///
+/// For example, an `XCTAssert` failure in the body of a Swift Testing test
+/// cannot record issues directly with the Swift Testing runner. Instead, the
+/// framework packages the assertion failure as a JSON `Event` and invokes this
+/// handler to report the failure.
+///
+/// - Parameters:
+///   - recordJSONSchemaVersionNumber: The JSON schema version used to encode
+///     the event record.
+///   - recordJSONBaseAddress: A pointer to the first byte of the encoded event.
+///   - recordJSONByteCount: The size of the encoded event in bytes.
+///   - reserved: Reserved for future use.
+@usableFromInline
+package typealias FallbackEventHandler = @Sendable @convention(c) (
+  _ recordJSONSchemaVersionNumber: UnsafePointer<CChar>,
+  _ recordJSONBaseAddress: UnsafeRawPointer,
+  _ recordJSONByteCount: Int,
+  _ reserved: UnsafeRawPointer?
+) -> Void
+
+/// Get the current fallback event handler.
+///
+/// - Returns: The currently-set handler function, if any.
+@_cdecl("_swift_testing_getFallbackEventHandler")
+@usableFromInline
+package func _swift_testing_getFallbackEventHandler() -> FallbackEventHandler? {
+#if SWT_TARGET_OS_APPLE && !SWT_NO_OS_UNFAIR_LOCK
+  return _fallbackEventHandler.withUnsafeMutablePointers { fallbackEventHandler, lock in
+    os_unfair_lock_lock(lock)
+    defer {
+      os_unfair_lock_unlock(lock)
+    }
+    return fallbackEventHandler.pointee
+  }
+#else
+  return _fallbackEventHandler.load(ordering: .sequentiallyConsistent).flatMap { fallbackEventHandler in
+    unsafeBitCast(fallbackEventHandler, to: FallbackEventHandler?.self)
+  }
+#endif
+}
+
+/// Set the current fallback event handler if one has not already been set.
+///
+/// - Parameters:
+///   - handler: The handler function to set.
+///
+/// - Returns: Whether or not `handler` was installed.
+///
+/// The fallback event handler can only be installed once per process, typically
+/// by the first testing library to run. If this function has already been
+/// called and the handler set, it does not replace the previous handler.
+@_cdecl("_swift_testing_installFallbackEventHandler")
+@usableFromInline
+package func _swift_testing_installFallbackEventHandler(_ handler: FallbackEventHandler) -> CBool {
+#if SWT_TARGET_OS_APPLE && !SWT_NO_OS_UNFAIR_LOCK
+  return _fallbackEventHandler.withUnsafeMutablePointers { fallbackEventHandler, lock in
+    os_unfair_lock_lock(lock)
+    defer {
+      os_unfair_lock_unlock(lock)
+    }
+    guard fallbackEventHandler.pointee == nil else {
+      return false
+    }
+    fallbackEventHandler.pointee = handler
+    return true
+  }
+#else
+  let handler = unsafeBitCast(handler, to: UnsafeRawPointer.self)
+  return _fallbackEventHandler.compareExchange(expected: nil, desired: handler, ordering: .sequentiallyConsistent).exchanged
+#endif
+}
+#endif