diff --git a/curl-sys/build.rs b/curl-sys/build.rs
index 7d028d410..b4ad9707d 100644
--- a/curl-sys/build.rs
+++ b/curl-sys/build.rs
@@ -342,12 +342,6 @@ fn main() {
         cfg.include(path);
     }
 
-    if cfg!(feature = "spnego") {
-        cfg.define("USE_SPNEGO", None)
-            .file("curl/lib/http_negotiate.c")
-            .file("curl/lib/vauth/vauth.c");
-    }
-
     // Configure TLS backend. Since Cargo does not support mutually exclusive
     // features, make sure we only compile one vtls.
     if cfg!(feature = "rustls") {
@@ -380,14 +374,7 @@ fn main() {
         if windows {
             // For windows, spnego feature is auto on in case ssl feature is on.
             // Please see definition of USE_SPNEGO in curl_setup.h for more info.
-            cfg.define("USE_WINDOWS_SSPI", None)
-                .define("USE_SCHANNEL", None)
-                .file("curl/lib/http_negotiate.c")
-                .file("curl/lib/curl_sspi.c")
-                .file("curl/lib/socks_sspi.c")
-                .file("curl/lib/vauth/krb5_sspi.c")
-                .file("curl/lib/vauth/spnego_sspi.c")
-                .file("curl/lib/vauth/vauth.c")
+            cfg.define("USE_SCHANNEL", None)
                 .file("curl/lib/vtls/schannel.c")
                 .file("curl/lib/vtls/schannel_verify.c")
                 .file("curl/lib/vtls/x509asn1.c");
@@ -407,6 +394,34 @@ fn main() {
         }
     }
 
+    // Compile Windows SSPI alongside any TLS backend so libcurl's integrated
+    // Windows authentication (NTLM/Negotiate) stays available.
+    // `USE_WINDOWS_SSPI` unconditionally triggers `USE_SPNEGO` in
+    // `curl_setup.h`, which causes `http.c` (always compiled) to reference
+    // symbols from `http_negotiate.c` and `spnego_sspi.c`. Those files must
+    // therefore be compiled whenever `windows_sspi` is true, regardless of
+    // whether the `spnego` cargo feature was explicitly requested.
+    let windows_sspi = windows
+        && cfg!(any(
+            feature = "ssl",
+            feature = "rustls",
+            feature = "windows-static-ssl"
+        ));
+
+    if !windows && cfg!(feature = "spnego") {
+        cfg.define("USE_SPNEGO", None);
+    }
+    if windows_sspi || cfg!(feature = "spnego") {
+        cfg.file("curl/lib/http_negotiate.c");
+    }
+    if windows_sspi {
+        cfg.define("USE_WINDOWS_SSPI", None)
+            .file("curl/lib/curl_sspi.c")
+            .file("curl/lib/socks_sspi.c")
+            .file("curl/lib/vauth/krb5_sspi.c")
+            .file("curl/lib/vauth/spnego_sspi.c");
+    }
+
     // Configure platform-specific details.
     if windows {
         cfg.define("HAVE_CONFIG_H", None)
@@ -421,10 +436,6 @@ fn main() {
             .file("curl/lib/curlx/multibyte.c")
             .file("curl/lib/curlx/version_win32.c")
             .file("curl/lib/curlx/winapi.c");
-
-        if cfg!(feature = "spnego") {
-            cfg.file("curl/lib/vauth/spnego_sspi.c");
-        }
     } else {
         cfg.define("RECV_TYPE_ARG1", "int")
             .define("HAVE_PTHREAD_H", None)