@@ -90,6 +90,8 @@ def run_build(build, python_versions, dry_run=False):
     wheelhousereq = must_exist(build.get("wheelhousereq", ""), build, "wheelhousereq")
     # "pip" (default, what ships) | "none" | "parallel" | "parallel-N"
     my_bytecode = build.get("bytecode", "pip")
+    # wheels only, unless a flavor still has a package with no wheel for its target
+    my_only_binary = "yes" if build.get("only_binary", True) else "no"
 
     # Get Python release info from TOML [pythons]
     py_target = my_python_target
@@ -134,6 +136,7 @@ def run_build(build, python_versions, dry_run=False):
         "--find-links", my_find_links,
         "--wheelhousereq", wheelhousereq,
         "--bytecode", my_bytecode,
+        "--only-binary", my_only_binary,
         "--create-installer", my_create_installer,
     ]
 
 
@@ -230,6 +230,11 @@ def main():
     parser.add_argument('--log-dir', default='WinPython_build_logs', help='Directory for logs')
     parser.add_argument('--mandatory-req', help='Mandatory requirements file')
     parser.add_argument('--wheelhousereq', help='Wheelhouse requirements file')
+    parser.add_argument('--only-binary', default='yes', choices=['yes', 'no'],
+                        help="'yes' (default) installs from wheels only, so a missing "
+                             "wheel fails here instead of quietly compiling third-party "
+                             "code on the build machine; 'no' for a flavor that still "
+                             "needs an sdist fallback")
     parser.add_argument('--bytecode', default='pip',
                         help="byte-compilation: 'pip' (inline, the default and what ships), "
                              "'none' (no .pyc -- for throwaway builds such as pylock generation "
@@ -277,6 +282,8 @@ def main():
 
     no_compile, compile_jobs = parse_bytecode_mode(args.bytecode)
     pip_options = ["--force-reinstall"] + (["--no-compile"] if no_compile else [])
+    if args.only_binary == "yes":
+        pip_options.append("--only-binary=:all:")
     for label, req in [
         ("Mandatory", args.mandatory_req),
         ("Main", args.requirements),
 
@@ -78,6 +78,11 @@ builds = ["dot", "dotf", "slim", "slimf"]
 
 [pythons."314".overrides.slimf]
 bytecode = "none"
+# tornado ships one cp39-abi3 wheel, which no free-threaded interpreter matches,
+# so this flavor alone still needs the sdist fallback. The maintainer expects
+# free-threaded wheels during the 2026-04 cycle (tornadoweb/tornado#3501):
+# drop this line then, and every build is wheels-only.
+only_binary = false
 
 # [pythons."314".overrides.slim]
 # create_installer = ".7z-mx7"