diff --git a/lambda/api_client_proxy_test.go b/lambda/api_client_proxy_test.go
index ec4230f8..c16de107 100644
--- a/lambda/api_client_proxy_test.go
+++ b/lambda/api_client_proxy_test.go
@@ -61,3 +61,42 @@ func TestExtensionAPIClientBypassesProxy(t *testing.T) {
 	proxy := proxyForClient(t, client, nonLoopbackAPIRequest(t))
 	require.Nil(t, proxy, "Extensions API client must not route through the configured proxy, got %v", proxy)
 }
+
+// wrappedRoundTripper is a RoundTripper that is not an *http.Transport, mirroring
+// how instrumentation libraries (for example otelhttp) replace http.DefaultTransport.
+type wrappedRoundTripper struct {
+	base http.RoundTripper
+}
+
+func (w *wrappedRoundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
+	return w.base.RoundTrip(req)
+}
+
+// When http.DefaultTransport is a non-*http.Transport wrapper, constructing the
+// clients must not panic on a type assertion, and the transport must still bypass
+// the proxy.
+func TestAPIClientsWithWrappedDefaultTransport(t *testing.T) {
+	t.Setenv("HTTP_PROXY", "http://192.0.2.100:3128")
+	t.Setenv("HTTPS_PROXY", "http://192.0.2.100:3128")
+	t.Setenv("http_proxy", "http://192.0.2.100:3128")
+	t.Setenv("https_proxy", "http://192.0.2.100:3128")
+
+	original := http.DefaultTransport
+	t.Cleanup(func() { http.DefaultTransport = original })
+	http.DefaultTransport = &wrappedRoundTripper{base: original}
+
+	for _, tc := range []struct {
+		name   string
+		client func() *http.Client
+	}{
+		{"runtime", func() *http.Client { return newRuntimeAPIClient("192.0.2.1:9001").httpClient }},
+		{"extension", func() *http.Client { return newExtensionAPIClient("192.0.2.1:9001").httpClient }},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			var client *http.Client
+			require.NotPanics(t, func() { client = tc.client() })
+			proxy := proxyForClient(t, client, nonLoopbackAPIRequest(t))
+			require.Nil(t, proxy, "%s API client must not route through the configured proxy, got %v", tc.name, proxy)
+		})
+	}
+}
diff --git a/lambda/runtime_api_client.go b/lambda/runtime_api_client.go
index 42fe5232..7893c176 100644
--- a/lambda/runtime_api_client.go
+++ b/lambda/runtime_api_client.go
@@ -43,7 +43,12 @@ type runtimeAPIClient struct {
 // newAPITransport returns an HTTP transport that never proxies, so calls to the
 // link-local Runtime/Extensions API bypass any customer-configured proxy.
 func newAPITransport() *http.Transport {
-	transport := http.DefaultTransport.(*http.Transport).Clone()
+	transport, ok := http.DefaultTransport.(*http.Transport)
+	if ok {
+		transport = transport.Clone()
+	} else {
+		transport = &http.Transport{}
+	}
 	transport.Proxy = nil
 	return transport
 }