fix: Respect cc_model_filenames in Python backend auto-fill

.gitignore

@@ -1,4 +1,5 @@
 /build
+/build_test
 /.vscode
 *.so
 *__pycache__/

src/model_config_utils.cc

@@ -1273,7 +1273,8 @@ AutoCompleteBackendFields(
     }
   }
   if (config->backend() == kPythonBackend) {
-    if (config->default_model_filename().empty()) {
+    if (config->default_model_filename().empty() &&
+        config->cc_model_filenames().empty()) {
       config->set_default_model_filename(kPythonFilename);
     }
     return Status::Success;

src/test/CMakeLists.txt

@@ -439,6 +439,56 @@ install(
   RUNTIME DESTINATION bin
 )
 
+#
+# Unit test for AutoCompleteBackendFields in model_config_utils
+#
+add_executable(
+  model_config_utils_test
+  model_config_utils_test.cc
+  ../model_config_utils.cc
+  ../status.cc
+  ../filesystem/api.cc
+  ../model_config_utils.h
+  ../status.h
+  ../filesystem/api.h
+)
+
+set_target_properties(
+  model_config_utils_test
+  PROPERTIES
+    SKIP_BUILD_RPATH TRUE
+    BUILD_WITH_INSTALL_RPATH TRUE
+    INSTALL_RPATH_USE_LINK_PATH FALSE
+    INSTALL_RPATH ""
+)
+
+target_include_directories(
+  model_config_utils_test
+  PRIVATE
+    ${CMAKE_CURRENT_SOURCE_DIR}/..
+    ${CMAKE_CURRENT_SOURCE_DIR}/../../include
+    ${GTEST_INCLUDE_DIRS}
+    ${Boost_INCLUDE_DIRS}
+)
+
+target_link_libraries(
+  model_config_utils_test
+  PRIVATE
+    triton-common-error        # from repo-common
+    triton-common-model-config # from repo-common
+    triton-common-json         # from repo-common
+    triton-common-logging      # from repo-common
+    proto-library              # from repo-common
+    GTest::gtest
+    GTest::gtest_main
+    protobuf::libprotobuf
+)
+
+install(
+  TARGETS model_config_utils_test
+  RUNTIME DESTINATION bin
+)
+
 
 if(${TRITON_ENABLE_METRICS})
   #

src/test/model_config_utils_test.cc

@@ -0,0 +1,178 @@
+// Copyright 2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions
+// are met:
+//  * Redistributions of source code must retain the above copyright
+//    notice, this list of conditions and the following disclaimer.
+//  * Redistributions in binary form must reproduce the above copyright
+//    notice, this list of conditions and the following disclaimer in the
+//    documentation and/or other materials provided with the distribution.
+//  * Neither the name of NVIDIA CORPORATION nor the names of its
+//    contributors may be used to endorse or promote products derived
+//    from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS ``AS IS'' AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+// PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+// CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+// EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+// PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
+// OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "model_config_utils.h"
+
+#include <sys/stat.h>
+
+#include <fstream>
+#include <string>
+
+#include "constants.h"
+#include "filesystem/api.h"
+#include "gtest/gtest.h"
+
+namespace tc = triton::core;
+
+namespace {
+
+// Helper to create a temporary model directory with a version subdirectory
+// and an optional file inside it.
+class TempModelDir {
+ public:
+  TempModelDir()
+  {
+    auto status =
+        tc::MakeTemporaryDirectory(tc::FileSystemType::LOCAL, &root_path_);
+    EXPECT_TRUE(status.IsOk()) << status.AsString();
+  }
+
+  ~TempModelDir()
+  {
+    // Best-effort cleanup
+    std::string cmd = "rm -rf " + root_path_;
+    (void)system(cmd.c_str());
+  }
+
+  // Create version subdir (e.g., "1") and optionally place a file in it.
+  void AddVersionWithFile(
+      const std::string& version, const std::string& filename)
+  {
+    std::string version_dir = tc::JoinPath({root_path_, version});
+    mkdir(version_dir.c_str(), S_IRWXU | S_IRWXG | S_IROTH | S_IXOTH);
+    if (!filename.empty()) {
+      std::ofstream f(tc::JoinPath({version_dir, filename}));
+      f << "# placeholder";
+    }
+  }
+
+  const std::string& Path() const { return root_path_; }
+
+ private:
+  std::string root_path_;
+};
+
+class AutoCompleteBackendFieldsTest : public ::testing::Test {};
+
+// When backend is "python" and default_model_filename is empty and
+// cc_model_filenames is empty, default_model_filename should be set to
+// "model.py".
+TEST_F(AutoCompleteBackendFieldsTest, PythonBackendSetsDefaultFilename)
+{
+  TempModelDir dir;
+  dir.AddVersionWithFile("1", "model.py");
+
+  inference::ModelConfig config;
+  config.set_backend("python");
+  // default_model_filename and cc_model_filenames are both empty
+
+  auto status =
+      tc::AutoCompleteBackendFields("test_model", dir.Path(), &config);
+  ASSERT_TRUE(status.IsOk()) << status.AsString();
+  EXPECT_EQ(config.default_model_filename(), "model.py");
+}
+
+// When backend is "python" and default_model_filename is empty but
+// cc_model_filenames is populated, default_model_filename should NOT be
+// auto-filled to "model.py".
+TEST_F(
+    AutoCompleteBackendFieldsTest,
+    PythonBackendSkipsDefaultFilenameWhenCcModelFilenamesSet)
+{
+  TempModelDir dir;
+  dir.AddVersionWithFile("1", "custom_model.py");
+
+  inference::ModelConfig config;
+  config.set_backend("python");
+  (*config.mutable_cc_model_filenames())["gpu"] = "custom_model.py";
+  // default_model_filename is empty, cc_model_filenames is set
+
+  auto status =
+      tc::AutoCompleteBackendFields("test_model", dir.Path(), &config);
+  ASSERT_TRUE(status.IsOk()) << status.AsString();
+  EXPECT_EQ(config.default_model_filename(), "")
+      << "default_model_filename should remain empty when cc_model_filenames "
+         "is set";
+}
+
+// When backend is "python" and default_model_filename is already set,
+// it should be preserved regardless of cc_model_filenames.
+TEST_F(
+    AutoCompleteBackendFieldsTest,
+    PythonBackendPreservesExplicitDefaultFilename)
+{
+  TempModelDir dir;
+  dir.AddVersionWithFile("1", "my_model.py");
+
+  inference::ModelConfig config;
+  config.set_backend("python");
+  config.set_default_model_filename("my_model.py");
+
+  auto status =
+      tc::AutoCompleteBackendFields("test_model", dir.Path(), &config);
+  ASSERT_TRUE(status.IsOk()) << status.AsString();
+  EXPECT_EQ(config.default_model_filename(), "my_model.py");
+}
+
+// When backend is "python" and both default_model_filename and
+// cc_model_filenames are set, both should be preserved as-is.
+TEST_F(
+    AutoCompleteBackendFieldsTest,
+    PythonBackendPreservesBothDefaultAndCcModelFilenames)
+{
+  TempModelDir dir;
+  dir.AddVersionWithFile("1", "my_model.py");
+
+  inference::ModelConfig config;
+  config.set_backend("python");
+  config.set_default_model_filename("my_model.py");
+  (*config.mutable_cc_model_filenames())["gpu"] = "gpu_model.py";
+
+  auto status =
+      tc::AutoCompleteBackendFields("test_model", dir.Path(), &config);
+  ASSERT_TRUE(status.IsOk()) << status.AsString();
+  EXPECT_EQ(config.default_model_filename(), "my_model.py");
+  EXPECT_EQ(config.cc_model_filenames().at("gpu"), "gpu_model.py");
+}
+
+// When backend is empty but version dir contains model.py, backend should be
+// auto-detected as "python" and default_model_filename set to "model.py".
+TEST_F(AutoCompleteBackendFieldsTest, AutoDetectPythonBackendFromModelFile)
+{
+  TempModelDir dir;
+  dir.AddVersionWithFile("1", "model.py");
+
+  inference::ModelConfig config;
+  // backend, platform, default_model_filename all empty
+
+  auto status =
+      tc::AutoCompleteBackendFields("test_model", dir.Path(), &config);
+  ASSERT_TRUE(status.IsOk()) << status.AsString();
+  EXPECT_EQ(config.backend(), "python");
+  EXPECT_EQ(config.default_model_filename(), "model.py");
+}
+
+}  // namespace