diff --git a/Tests/BridgeJSRuntimeTests/TypedThrowsAsyncClosureBugTests.swift b/Tests/BridgeJSRuntimeTests/TypedThrowsAsyncClosureBugTests.swift
new file mode 100644
index 000000000..5512ac295
--- /dev/null
+++ b/Tests/BridgeJSRuntimeTests/TypedThrowsAsyncClosureBugTests.swift
@@ -0,0 +1,231 @@
+import XCTest
+import JavaScriptKit
+import JavaScriptEventLoop
+
+/// Reproducer for swiftlang/swift#89320 (fix in flight: swiftlang/swift#89715):
+/// on wasm32, a typed error thrown from a CAPTURELESS async closure value or
+/// function reference (lowered via `thin_to_thick_function`) is lost across the
+/// async unwind when the error type is too large for the direct error convention.
+/// Capturing closures, direct async function calls, and small error payloads are
+/// unaffected. The bug cases below assert the current (incorrect) behavior so the
+/// suite stays green; flip them once swiftlang/swift#89715 lands.
+
+// MARK: - Payload types
+
+struct PlainValuePayloadError: Error, Equatable {
+    let marker: String
+}
+
+/// Intentionally limited to trivially-copyable fields: the corrupted error produced by
+/// swiftlang/swift#89320 is unsafe to read through String or class-reference payloads.
+struct LargePayloadError: Error, Equatable {
+    let first: UInt64
+    let second: UInt64
+    let third: UInt64
+    let marker: UInt64
+}
+
+final class MarkerBox: @unchecked Sendable {
+    let marker: String
+    init(_ marker: String) { self.marker = marker }
+}
+struct ReferencePayloadError: Error {
+    let box: MarkerBox
+}
+
+// MARK: - Control "async function" forms (payload must survive)
+
+private func asyncFunctionThrowsValue() async throws(PlainValuePayloadError) {
+    throw PlainValuePayloadError(marker: "ALIVE")
+}
+
+private func asyncFunctionThrowsReference() async throws(ReferencePayloadError) {
+    throw ReferencePayloadError(box: MarkerBox("ALIVE"))
+}
+
+private func asyncFunctionThrowsJSException() async throws(JSException) {
+    throw JSException(JSError(message: "ALIVE").jsValue)
+}
+
+@inline(never)
+private func largePayloadAfterThrow(
+    _ closure: () async throws(LargePayloadError) -> Void
+) async -> LargePayloadError? {
+    do {
+        try await closure()
+        return nil
+    } catch {
+        return error
+    }
+}
+
+final class TypedThrowsAsyncClosureBugTests: XCTestCase {
+
+    func testControl_valuePayload_asyncFunction() async throws {
+        do {
+            try await asyncFunctionThrowsValue()
+            XCTFail("expected throw")
+        } catch let error as PlainValuePayloadError {
+            print("[REPRO] value / async function           payload: \(error.marker)")
+            XCTAssertEqual(error.marker, "ALIVE")
+        }
+    }
+
+    func testControl_valuePayload_typedThrowsAsyncClosure() async throws {
+        let closure: () async throws(PlainValuePayloadError) -> Void = {
+            () async throws(PlainValuePayloadError) -> Void in
+            throw PlainValuePayloadError(marker: "ALIVE")
+        }
+        do {
+            try await closure()
+            XCTFail("expected throw")
+        } catch let error as PlainValuePayloadError {
+            print("[REPRO] value / typed-throws async closure payload: \(error.marker)")
+            XCTAssertEqual(error.marker, "ALIVE")
+        }
+    }
+
+    func testControl_valuePayload_untypedAsyncClosure() async throws {
+        let closure: () async throws -> Void = {
+            throw PlainValuePayloadError(marker: "ALIVE")
+        }
+        do {
+            try await closure()
+            XCTFail("expected throw")
+        } catch let error as PlainValuePayloadError {
+            print("[REPRO] value / untyped async closure     payload: \(error.marker)")
+            XCTAssertEqual(error.marker, "ALIVE")
+        }
+    }
+
+    func testControl_referencePayload_asyncFunction() async throws {
+        do {
+            try await asyncFunctionThrowsReference()
+            XCTFail("expected throw")
+        } catch let error as ReferencePayloadError {
+            print("[REPRO] ref   / async function           payload: \(error.box.marker)")
+            XCTAssertEqual(error.box.marker, "ALIVE")
+        }
+    }
+
+    func testControl_referencePayload_typedThrowsAsyncClosure() async throws {
+        let closure: () async throws(ReferencePayloadError) -> Void = {
+            () async throws(ReferencePayloadError) -> Void in
+            throw ReferencePayloadError(box: MarkerBox("ALIVE"))
+        }
+        do {
+            try await closure()
+            XCTFail("expected throw")
+        } catch let error as ReferencePayloadError {
+            print("[REPRO] ref   / typed-throws async closure payload: \(error.box.marker)")
+            XCTAssertEqual(error.box.marker, "ALIVE")
+        }
+    }
+
+    func testControl_jsException_asyncFunction() async throws {
+        do {
+            try await asyncFunctionThrowsJSException()
+            XCTFail("expected throw")
+        } catch let error as JSException {
+            let alive = error.thrownValue.object != nil
+            print("[REPRO] jsexc / async function           object!=nil: \(alive)")
+            XCTAssertTrue(alive, "async function should preserve the thrown JS object")
+            XCTAssertEqual(error.thrownValue.object?.message.string, "ALIVE")
+        }
+    }
+
+    func testControl_jsException_untypedAsyncClosure() async throws {
+        let closure: () async throws -> Void = {
+            throw JSException(JSError(message: "ALIVE").jsValue)
+        }
+        do {
+            try await closure()
+            XCTFail("expected throw")
+        } catch let error as JSException {
+            let alive = error.thrownValue.object != nil
+            print("[REPRO] jsexc / untyped async closure     object!=nil: \(alive)")
+            XCTAssertTrue(alive, "untyped async closure should preserve the thrown JS object")
+            XCTAssertEqual(error.thrownValue.object?.message.string, "ALIVE")
+        } catch {
+            XCTFail("unexpected error type: \(error)")
+        }
+    }
+
+    func testControl_jsException_capturingTypedThrowsAsyncClosure() async throws {
+        let capturedMarker = "ALIVE"
+        let closure: () async throws(JSException) -> Void = { () async throws(JSException) -> Void in
+            throw JSException(JSError(message: capturedMarker).jsValue)
+        }
+        do {
+            try await closure()
+            XCTFail("expected throw")
+        } catch let error as JSException {
+            let alive = error.thrownValue.object != nil
+            print("[REPRO] jsexc / capturing typed-throws closure object!=nil: \(alive)")
+            XCTAssertTrue(alive, "capturing typed-throws async closure should preserve the thrown JS object")
+            XCTAssertEqual(error.thrownValue.object?.message.string, "ALIVE")
+        }
+    }
+
+    /// Asserts the current (buggy) behavior of swiftlang/swift#89320: the JSException payload is lost.
+    /// When the compiler is fixed this test will fail; flip to XCTAssertTrue(alive).
+    func testBug_jsException_typedThrowsAsyncClosure() async throws {
+        let closure: () async throws(JSException) -> Void = { () async throws(JSException) -> Void in
+            throw JSException(JSError(message: "ALIVE").jsValue)
+        }
+        do {
+            try await closure()
+            XCTFail("expected throw")
+        } catch let error as JSException {
+            let alive = error.thrownValue.object != nil
+            print("[REPRO] jsexc / typed-throws async closure object!=nil: \(alive)  <-- BUG: payload lost")
+
+            XCTAssertFalse(
+                alive,
+                """
+                swiftlang/swift#89320 is expected to LOSE the JSException payload \
+                thrown from a captureless typed-throws async closure. If this \
+                assertion now fails, the compiler bug appears fixed -- flip this to \
+                XCTAssertTrue(alive) and re-enable the message round-trip check.
+                """
+            )
+        }
+    }
+
+    /// Asserts the current (buggy) behavior of swiftlang/swift#89320 with a plain Swift
+    /// error larger than the direct error convention: no JavaScriptKit types involved.
+    func testBug_largePlainPayload_typedThrowsAsyncClosure() async throws {
+        let expected = LargePayloadError(
+            first: 0x1111_1111_1111_1111,
+            second: 0x2222_2222_2222_2222,
+            third: 0x3333_3333_3333_3333,
+            marker: 0xA11FE
+        )
+        let closure: () async throws(LargePayloadError) -> Void = { () async throws(LargePayloadError) -> Void in
+            throw LargePayloadError(
+                first: 0x1111_1111_1111_1111,
+                second: 0x2222_2222_2222_2222,
+                third: 0x3333_3333_3333_3333,
+                marker: 0xA11FE
+            )
+        }
+        let observed = await largePayloadAfterThrow(closure)
+        XCTAssertNotNil(observed, "expected throw")
+        let observedDescription = observed.map {
+            "first: \(String($0.first, radix: 16)) second: \(String($0.second, radix: 16)) "
+                + "third: \(String($0.third, radix: 16)) marker: \(String($0.marker, radix: 16))"
+        }
+        print("[REPRO] large / typed-throws async closure \(observedDescription ?? "nil")  <-- BUG: payload lost")
+
+        XCTAssertNotEqual(
+            observed,
+            expected,
+            """
+            swiftlang/swift#89320 is expected to LOSE the large plain-Swift error \
+            payload thrown from a captureless typed-throws async closure. If this \
+            assertion now fails, the compiler bug appears fixed -- flip this to \
+            assert the payload is preserved.
+            """
+        )
+    }
+}