diff --git a/Sources/Testing/Parameterization/Test.Case.Generator.swift b/Sources/Testing/Parameterization/Test.Case.Generator.swift
index c682be19a..477391ae4 100644
--- a/Sources/Testing/Parameterization/Test.Case.Generator.swift
+++ b/Sources/Testing/Parameterization/Test.Case.Generator.swift
@@ -57,7 +57,7 @@ extension Test.Case {
     /// - Parameters:
     ///   - testFunction: The test function called by the generated test case.
     init(
-      testFunction: @escaping @Sendable () async throws -> Void
+      testFunction: nonisolated(nonsending) @escaping @Sendable () async throws -> Void
     ) where S == CollectionOfOne<Void> {
       // A beautiful hack to give us the right number of cases: iterate over a
       // collection containing a single Void value.
@@ -85,7 +85,7 @@ extension Test.Case {
     init(
       arguments collection: S,
       parameters: [Test.Parameter],
-      testFunction: @escaping @Sendable (S.Element) async throws -> Void
+      testFunction: nonisolated(nonsending) @escaping @Sendable (S.Element) async throws -> Void
     ) where S: Collection {
       if parameters.count > 1 {
         self.init(sequence: collection) { element in
@@ -124,7 +124,7 @@ extension Test.Case {
     init<C1, C2>(
       arguments collection1: C1, _ collection2: C2,
       parameters: [Test.Parameter],
-      testFunction: @escaping @Sendable (C1.Element, C2.Element) async throws -> Void
+      testFunction: nonisolated(nonsending) @escaping @Sendable (C1.Element, C2.Element) async throws -> Void
     ) where S == CartesianProduct<C1, C2> {
       self.init(sequence: cartesianProduct(collection1, collection2)) { element in
         Test.Case(values: [element.0, element.1], parameters: parameters) {
@@ -154,7 +154,7 @@ extension Test.Case {
     private init<E1, E2>(
       sequence: S,
       parameters: [Test.Parameter],
-      testFunction: @escaping @Sendable ((E1, E2)) async throws -> Void
+      testFunction: nonisolated(nonsending) @escaping @Sendable ((E1, E2)) async throws -> Void
     ) where S.Element == (E1, E2), E1: Sendable, E2: Sendable {
       if parameters.count > 1 {
         self.init(sequence: sequence) { element in
@@ -192,7 +192,7 @@ extension Test.Case {
     init<E1, E2>(
       arguments collection: S,
       parameters: [Test.Parameter],
-      testFunction: @escaping @Sendable ((E1, E2)) async throws -> Void
+      testFunction: nonisolated(nonsending) @escaping @Sendable ((E1, E2)) async throws -> Void
     ) where S: Collection, S.Element == (E1, E2) {
       self.init(sequence: collection, parameters: parameters, testFunction: testFunction)
     }
@@ -210,7 +210,7 @@ extension Test.Case {
     init<C1, C2>(
       arguments zippedCollections: Zip2Sequence<C1, C2>,
       parameters: [Test.Parameter],
-      testFunction: @escaping @Sendable ((C1.Element, C2.Element)) async throws -> Void
+      testFunction: nonisolated(nonsending) @escaping @Sendable ((C1.Element, C2.Element)) async throws -> Void
     ) where S == Zip2Sequence<C1, C2>, C1: Collection, C2: Collection {
       self.init(sequence: zippedCollections, parameters: parameters, testFunction: testFunction)
     }
@@ -234,7 +234,7 @@ extension Test.Case {
     init<Key, Value>(
       arguments dictionary: Dictionary<Key, Value>,
       parameters: [Test.Parameter],
-      testFunction: @escaping @Sendable ((Key, Value)) async throws -> Void
+      testFunction: nonisolated(nonsending) @escaping @Sendable ((Key, Value)) async throws -> Void
     ) where S == Dictionary<Key, Value> {
       if parameters.count > 1 {
         self.init(sequence: dictionary) { element in
diff --git a/Sources/Testing/Parameterization/Test.Case.swift b/Sources/Testing/Parameterization/Test.Case.swift
index 18faaf4e2..f62ac8c1b 100644
--- a/Sources/Testing/Parameterization/Test.Case.swift
+++ b/Sources/Testing/Parameterization/Test.Case.swift
@@ -155,9 +155,9 @@ extension Test {
       }
     }
 
-    private init(kind: _Kind, body: @escaping @Sendable () async throws -> Void) {
-      self._kind = kind
-      self.body = body
+    private init(kind: _Kind, body: nonisolated(nonsending) @escaping @Sendable () async throws -> Void) {
+      _kind = kind
+      _body = body
     }
 
     /// Initialize a test case for a non-parameterized test function.
@@ -166,7 +166,7 @@ extension Test {
     ///   - body: The body closure of this test case.
     ///
     /// The resulting test case will have zero arguments.
-    init(body: @escaping @Sendable () async throws -> Void) {
+    init(body: nonisolated(nonsending) @escaping @Sendable () async throws -> Void) {
       self.init(kind: .nonParameterized, body: body)
     }
 
@@ -180,7 +180,7 @@ extension Test {
     init(
       values: [any Sendable],
       parameters: [Parameter],
-      body: @escaping @Sendable () async throws -> Void
+      body: nonisolated(nonsending) @escaping @Sendable () async throws -> Void
     ) {
       var isStable = true
 
@@ -229,10 +229,25 @@ extension Test {
     }
 
     /// The body closure of this test case.
+    private var _body: nonisolated(nonsending) @Sendable () async throws -> Void
+
+    /// Invoke the body closure of this test case.
+    ///
+    /// - Parameters:
+    ///   - configuration: The configuration to use for running.
     ///
-    /// Do not invoke this closure directly. Always use a ``Runner`` to invoke a
+    /// Do not call this function directly. Always use a ``Runner`` to invoke a
     /// test or test case.
-    var body: @Sendable () async throws -> Void
+    nonisolated(nonsending) func run(configuration: borrowing Configuration) async throws {
+      if let actor = configuration.defaultSynchronousIsolationContext {
+        func runIsolated(to actor: isolated some Actor) async throws {
+          try await _body()
+        }
+        try await runIsolated(to: actor)
+      } else {
+        try await _body()
+      }
+    }
   }
 
   /// A type representing a single parameter to a parameterized test function.
diff --git a/Sources/Testing/Running/Runner.swift b/Sources/Testing/Running/Runner.swift
index 849e06b26..31969b4f9 100644
--- a/Sources/Testing/Running/Runner.swift
+++ b/Sources/Testing/Running/Runner.swift
@@ -104,7 +104,7 @@ extension Runner {
   private static func _applyScopingTraits(
     for test: Test,
     testCase: Test.Case?,
-    _ body: @escaping @Sendable () async throws -> Void
+    _ body: nonisolated(nonsending) @escaping @Sendable () async throws -> Void
   ) async throws {
     // If the test does not have any traits, exit early to avoid unnecessary
     // heap allocations below.
@@ -140,7 +140,7 @@ extension Runner {
   ///
   /// - Throws: Whatever is thrown by `body` or by any of the traits' provide
   ///   scope function calls.
-  private static func _applyIssueHandlingTraits(for test: Test, _ body: @escaping @Sendable () async throws -> Void) async throws {
+  private static func _applyIssueHandlingTraits(for test: Test, _ body: nonisolated(nonsending) @escaping @Sendable () async throws -> Void) async throws {
     // If the test does not have any traits, exit early to avoid unnecessary
     // heap allocations below.
     if test.traits.isEmpty {
@@ -420,7 +420,7 @@ extension Runner {
 
         try await withTimeLimit(for: step.test, configuration: configuration) {
           try await _applyScopingTraits(for: step.test, testCase: testCase) {
-            try await testCase.body()
+            try await testCase.run(configuration: configuration)
           }
         } timeoutHandler: { timeLimit in
           let issue = Issue(
diff --git a/Sources/Testing/Test+Macro.swift b/Sources/Testing/Test+Macro.swift
index 97fe104de..1f235b63b 100644
--- a/Sources/Testing/Test+Macro.swift
+++ b/Sources/Testing/Test+Macro.swift
@@ -163,7 +163,7 @@ extension Test {
     traits: [any TestTrait],
     sourceBounds: __SourceBounds,
     parameters: [__Parameter] = [],
-    testFunction: @escaping @Sendable () async throws -> Void
+    testFunction: nonisolated(nonsending) @escaping @Sendable () async throws -> Void
   ) -> Self where S: ~Copyable & ~Escapable {
     // Don't use Optional.map here due to a miscompile/crash. Expand out to an
     // if expression instead. SEE: rdar://134280902
@@ -248,7 +248,7 @@ extension Test {
     arguments collection: @escaping @Sendable () async throws -> C,
     sourceBounds: __SourceBounds,
     parameters paramTuples: [__Parameter],
-    testFunction: @escaping @Sendable (C.Element) async throws -> Void
+    testFunction: nonisolated(nonsending) @escaping @Sendable (C.Element) async throws -> Void
   ) -> Self where S: ~Copyable & ~Escapable, C: Collection & Sendable, C.Element: Sendable {
     let containingTypeInfo: TypeInfo? = if let containingType {
       TypeInfo(describing: containingType)
@@ -395,7 +395,7 @@ extension Test {
     arguments collection1: @escaping @Sendable () async throws -> C1, _ collection2: @escaping @Sendable () async throws -> C2,
     sourceBounds: __SourceBounds,
     parameters paramTuples: [__Parameter],
-    testFunction: @escaping @Sendable (C1.Element, C2.Element) async throws -> Void
+    testFunction: nonisolated(nonsending) @escaping @Sendable (C1.Element, C2.Element) async throws -> Void
   ) -> Self where S: ~Copyable & ~Escapable, C1: Collection & Sendable, C1.Element: Sendable, C2: Collection & Sendable, C2.Element: Sendable {
     let containingTypeInfo: TypeInfo? = if let containingType {
       TypeInfo(describing: containingType)
@@ -423,7 +423,7 @@ extension Test {
     arguments collection: @escaping @Sendable () async throws -> C,
     sourceBounds: __SourceBounds,
     parameters paramTuples: [__Parameter],
-    testFunction: @escaping @Sendable ((E1, E2)) async throws -> Void
+    testFunction: nonisolated(nonsending) @escaping @Sendable ((E1, E2)) async throws -> Void
   ) -> Self where S: ~Copyable & ~Escapable, C: Collection & Sendable, C.Element == (E1, E2), E1: Sendable, E2: Sendable {
     let containingTypeInfo: TypeInfo? = if let containingType {
       TypeInfo(describing: containingType)
@@ -454,7 +454,7 @@ extension Test {
     arguments dictionary: @escaping @Sendable () async throws -> Dictionary<Key, Value>,
     sourceBounds: __SourceBounds,
     parameters paramTuples: [__Parameter],
-    testFunction: @escaping @Sendable ((Key, Value)) async throws -> Void
+    testFunction: nonisolated(nonsending) @escaping @Sendable ((Key, Value)) async throws -> Void
   ) -> Self where S: ~Copyable & ~Escapable, Key: Sendable, Value: Sendable {
     let containingTypeInfo: TypeInfo? = if let containingType {
       TypeInfo(describing: containingType)
@@ -479,7 +479,7 @@ extension Test {
     arguments zippedCollections: @escaping @Sendable () async throws -> Zip2Sequence<C1, C2>,
     sourceBounds: __SourceBounds,
     parameters paramTuples: [__Parameter],
-    testFunction: @escaping @Sendable (C1.Element, C2.Element) async throws -> Void
+    testFunction: nonisolated(nonsending) @escaping @Sendable (C1.Element, C2.Element) async throws -> Void
   ) -> Self where S: ~Copyable & ~Escapable, C1: Collection & Sendable, C1.Element: Sendable, C2: Collection & Sendable, C2.Element: Sendable {
     let containingTypeInfo: TypeInfo? = if let containingType {
       TypeInfo(describing: containingType)
@@ -547,7 +547,7 @@ extension Test {
 /// - Warning: This function is used to implement the `@Test` macro. Do not use
 ///   it directly.
 @_lifetime(copy value)
-@inlinable public func __requiringAwait<T>(_ value: consuming T, isolation: isolated (any Actor)? = #isolation) async -> T where T: ~Copyable & ~Escapable {
+@inlinable public nonisolated(nonsending) func __requiringAwait<T>(_ value: consuming T) async -> T where T: ~Copyable & ~Escapable {
   value
 }
 
diff --git a/Sources/TestingMacros/ConditionMacro.swift b/Sources/TestingMacros/ConditionMacro.swift
index 85f5ed227..5f9c4d345 100644
--- a/Sources/TestingMacros/ConditionMacro.swift
+++ b/Sources/TestingMacros/ConditionMacro.swift
@@ -507,7 +507,7 @@ extension ExitTestConditionMacro {
     }
     decls.append(
       """
-      @Sendable func \(bodyThunkName)(\(bodyThunkParameterList)) async throws {
+      @Sendable nonisolated(nonsending) func \(bodyThunkName)(\(bodyThunkParameterList)) async throws {
         _ = \(applyEffectfulKeywords([.try, .await, .unsafe], to: bodyArgumentExpr))()
       }
       """
diff --git a/Sources/TestingMacros/SuiteDeclarationMacro.swift b/Sources/TestingMacros/SuiteDeclarationMacro.swift
index dcf0a76bd..42de5f27c 100644
--- a/Sources/TestingMacros/SuiteDeclarationMacro.swift
+++ b/Sources/TestingMacros/SuiteDeclarationMacro.swift
@@ -130,8 +130,8 @@ public struct SuiteDeclarationMacro: PeerMacro, Sendable {
     let generatorName = context.makeUniqueName("generator")
     result.append(
       """
-      @available(*, deprecated, message: "This property is an implementation detail of the testing library. Do not use it directly.")
-      @Sendable private \(staticKeyword(for: containingType)) func \(generatorName)() async -> Testing.Test {
+      @available(*, deprecated, message: "This function is an implementation detail of the testing library. Do not use it directly.")
+      @Sendable private nonisolated(nonsending) \(staticKeyword(for: containingType)) func \(generatorName)() async -> Testing.Test {
         .__type(
           \(declaration.type.trimmed).self,
           \(raw: attributeInfo.functionArgumentList(in: context))
diff --git a/Sources/TestingMacros/Support/Additions/FunctionDeclSyntaxAdditions.swift b/Sources/TestingMacros/Support/Additions/FunctionDeclSyntaxAdditions.swift
index 8065d299e..8ea344754 100644
--- a/Sources/TestingMacros/Support/Additions/FunctionDeclSyntaxAdditions.swift
+++ b/Sources/TestingMacros/Support/Additions/FunctionDeclSyntaxAdditions.swift
@@ -27,13 +27,6 @@ extension FunctionDeclSyntax {
       .contains(.keyword(.mutating))
   }
 
-  /// Whether or not this function is a `nonisolated` function.
-  var isNonisolated: Bool {
-    modifiers.lazy
-      .map(\.name.tokenKind)
-      .contains(.keyword(.nonisolated))
-  }
-
   /// Whether or not this function declares an operator.
   var isOperator: Bool {
     switch name.tokenKind {
diff --git a/Sources/TestingMacros/Support/TestContentGeneration.swift b/Sources/TestingMacros/Support/TestContentGeneration.swift
index 3a139abf5..3802e052c 100644
--- a/Sources/TestingMacros/Support/TestContentGeneration.swift
+++ b/Sources/TestingMacros/Support/TestContentGeneration.swift
@@ -56,7 +56,7 @@ func makeTestContentRecordDecl(named name: TokenSyntax, in typeName: TypeSyntax?
   private nonisolated \(staticKeyword(for: typeName)) let \(name): Testing.__TestContentRecord = (
     \(kindExpr), \(kind.commentRepresentation)
     0,
-    \(accessorExpr),
+    \(raw: accessorExpr),
     \(contextExpr),
     0
   )
diff --git a/Sources/TestingMacros/TestDeclarationMacro.swift b/Sources/TestingMacros/TestDeclarationMacro.swift
index c3d9f64fd..39ae46535 100644
--- a/Sources/TestingMacros/TestDeclarationMacro.swift
+++ b/Sources/TestingMacros/TestDeclarationMacro.swift
@@ -298,46 +298,13 @@ public struct TestDeclarationMacro: PeerMacro, Sendable {
       thunkBody = "_ = \(forwardCall("\(functionDecl.name.trimmed)\(forwardedParamsExpr)"))"
     }
 
-    // If this function is synchronous, is not explicitly nonisolated, and is
-    // not explicitly isolated to some actor, it should run in the configured
-    // default isolation context. If the suite type is an actor, this will cause
-    // a hop off the actor followed by an immediate hop back on, but otherwise
-    // should be harmless. Note that we do not support specifying an `isolated`
-    // parameter on a test function at this time.
-    //
-    // We use a second, inner thunk function here instead of just adding the
-    // isolation parameter to the "real" thunk because adding it there prevents
-    // correct tuple desugaring of the "real" arguments to the thunk.
-    if functionDecl.signature.effectSpecifiers?.asyncSpecifier == nil && !isMainActorIsolated && !functionDecl.isNonisolated {
-      // Get a unique name for this secondary thunk. We don't need it to be
-      // uniqued against functionDecl because it's interior to the "real" thunk,
-      // so its name can't conflict with any other names visible in this scope.
-      let isolationThunkName = context.makeUniqueName("")
-
-      // Insert a (defaulted) isolated argument. If we emit a closure (or inner
-      // function) that captured the arguments to the "real" thunk, the compiler
-      // has trouble reasoning about the lifetime of arguments to that closure
-      // especially if those arguments are borrowed or consumed, which results
-      // in hard-to-avoid compile-time errors. Fortunately, forwarding the full
-      // argument list is straightforward.
-      let thunkParamsExprCopy = FunctionParameterClauseSyntax {
-        for thunkParam in thunkParamsExpr.parameters {
-          thunkParam
-        }
-        FunctionParameterSyntax(
-          firstName: .wildcardToken(),
-          type: "isolated (any _Concurrency.Actor)?" as TypeSyntax,
-          defaultValue: InitializerClauseSyntax(value: "Testing.__defaultSynchronousIsolationContext" as ExprSyntax)
-        )
-      }
-
-      thunkBody = """
-      @Sendable func \(isolationThunkName)\(thunkParamsExprCopy) async throws {
-        \(thunkBody)
-      }
-      try await \(isolationThunkName)\(forwardedParamsExpr)
-      """
-    }
+    // Forward the nonisolated keyword from the original function. If none is
+    // present, use `nonisolated(nonsending)` by default.
+    let existingNonisolatedKeyword = functionDecl.modifiers.first { $0.name.tokenKind == .keyword(.nonisolated) }
+    let nonisolatedKeyword = existingNonisolatedKeyword?.trimmed ?? DeclModifierSyntax(
+      name: .keyword(.nonisolated),
+      detail: DeclModifierDetailSyntax(detail: .keyword(.nonsending))
+    )
 
     // Add availability guards if needed.
     thunkBody = createSyntaxNode(
@@ -349,7 +316,7 @@ public struct TestDeclarationMacro: PeerMacro, Sendable {
     let thunkName = context.makeUniqueName(thunking: functionDecl)
     let thunkDecl: DeclSyntax = """
     @available(*, deprecated, message: "This function is an implementation detail of the testing library. Do not use it directly.")
-    @Sendable private \(staticKeyword(for: typeName)) func \(thunkName)\(thunkParamsExpr) async throws -> Void {
+    @Sendable private \(nonisolatedKeyword) \(staticKeyword(for: typeName)) func \(thunkName)\(thunkParamsExpr) async throws -> Void {
       \(thunkBody)
     }
     """
@@ -440,7 +407,7 @@ public struct TestDeclarationMacro: PeerMacro, Sendable {
       result.append(
         """
         @available(*, deprecated, message: "This property is an implementation detail of the testing library. Do not use it directly.")
-        private \(staticKeyword(for: typeName)) nonisolated func \(unavailableTestName)() async -> Testing.Test {
+        private nonisolated(nonsending) \(staticKeyword(for: typeName)) func \(unavailableTestName)() async -> Testing.Test {
           .__function(
             named: \(literal: functionDecl.completeName.trimmedDescription),
             in: \(typeNameExpr),
@@ -465,8 +432,8 @@ public struct TestDeclarationMacro: PeerMacro, Sendable {
     let generatorName = context.makeUniqueName(thunking: functionDecl, withPrefix: "generator")
     result.append(
       """
-      @available(*, deprecated, message: "This property is an implementation detail of the testing library. Do not use it directly.")
-      @Sendable private \(staticKeyword(for: typeName)) func \(generatorName)() async -> Testing.Test {
+      @available(*, deprecated, message: "This function is an implementation detail of the testing library. Do not use it directly.")
+      @Sendable private nonisolated(nonsending) \(staticKeyword(for: typeName)) func \(generatorName)() async -> Testing.Test {
         \(raw: testsBody)
       }
       """
diff --git a/Tests/TestingTests/TestSupport/TestingAdditions.swift b/Tests/TestingTests/TestSupport/TestingAdditions.swift
index 8758ac9c6..1ac4de803 100644
--- a/Tests/TestingTests/TestSupport/TestingAdditions.swift
+++ b/Tests/TestingTests/TestSupport/TestingAdditions.swift
@@ -175,7 +175,7 @@ extension Test {
     sourceLocation: SourceLocation = #_sourceLocation,
     sourceBounds: __SourceBounds? = nil,
     name: String = #function,
-    testFunction: @escaping @Sendable () async throws -> Void
+    testFunction: nonisolated(nonsending) @escaping @Sendable () async throws -> Void
   ) {
     let sourceBounds = sourceBounds ?? __SourceBounds(lowerBoundOnly: sourceLocation)
     let caseGenerator = Case.Generator(testFunction: testFunction)
@@ -207,7 +207,7 @@ extension Test {
     sourceBounds: __SourceBounds? = nil,
     column: Int = #column,
     name: String = #function,
-    testFunction: @escaping @Sendable (C.Element) async throws -> Void
+    testFunction: nonisolated(nonsending) @escaping @Sendable (C.Element) async throws -> Void
   ) where C: Collection & Sendable, C.Element: Sendable {
     let sourceBounds = sourceBounds ?? __SourceBounds(lowerBoundOnly: sourceLocation)
     let caseGenerator = Case.Generator(arguments: collection, parameters: parameters, testFunction: testFunction)
@@ -224,7 +224,7 @@ extension Test {
     sourceBounds: __SourceBounds? = nil,
     column: Int = #column,
     name: String = #function,
-    testFunction: @escaping @Sendable (C.Element) async throws -> Void
+    testFunction: nonisolated(nonsending) @escaping @Sendable (C.Element) async throws -> Void
   ) where C: Collection & Sendable, C.Element: Sendable {
     let sourceBounds = sourceBounds ?? __SourceBounds(lowerBoundOnly: sourceLocation)
     let caseGenerator = { @Sendable in
@@ -259,7 +259,7 @@ extension Test {
     sourceLocation: SourceLocation = #_sourceLocation,
     sourceBounds: __SourceBounds? = nil,
     name: String = #function,
-    testFunction: @escaping @Sendable (C1.Element, C2.Element) async throws -> Void
+    testFunction: nonisolated(nonsending) @escaping @Sendable (C1.Element, C2.Element) async throws -> Void
   ) where C1: Collection & Sendable, C1.Element: Sendable, C2: Collection & Sendable, C2.Element: Sendable {
     let sourceBounds = sourceBounds ?? __SourceBounds(lowerBoundOnly: sourceLocation)
     let caseGenerator = Case.Generator(arguments: collection1, collection2, parameters: parameters, testFunction: testFunction)
@@ -287,7 +287,7 @@ extension Test {
     sourceLocation: SourceLocation = #_sourceLocation,
     sourceBounds: __SourceBounds? = nil,
     name: String = #function,
-    testFunction: @escaping @Sendable ((C1.Element, C2.Element)) async throws -> Void
+    testFunction: nonisolated(nonsending) @escaping @Sendable ((C1.Element, C2.Element)) async throws -> Void
   ) where C1: Collection & Sendable, C1.Element: Sendable, C2: Collection & Sendable, C2.Element: Sendable {
     let sourceBounds = sourceBounds ?? __SourceBounds(lowerBoundOnly: sourceLocation)
     let caseGenerator = Case.Generator(arguments: zippedCollections, parameters: parameters, testFunction: testFunction)
diff --git a/Tests/TestingTests/ZipTests.swift b/Tests/TestingTests/ZipTests.swift
index bd767b222..71fe97c5f 100644
--- a/Tests/TestingTests/ZipTests.swift
+++ b/Tests/TestingTests/ZipTests.swift
@@ -26,3 +26,5 @@ struct ZipTests {
     #expect(i == j)
   }
 }
+
+