Fix: Skip CPU provider in SetProviderSessionOptions

src/models/session_options.cpp

@@ -3,6 +3,8 @@
 
 #include "session_options.h"
 
+#include <algorithm>
+#include <cctype>
 #include <functional>
 #include <unordered_map>
 
@@ -173,6 +175,27 @@ DeviceInterface* SetProviderSessionOptions(OrtSessionOptions& session_options,
   }
 
   for (const auto& provider : providers_list) {
+    // CPU EP is always implicitly registered and cannot be appended via
+    // ORT's AppendExecutionProvider API. Throw if users attempt to set
+    // CPU-specific provider options (which would be silently lost), but
+    // accept bare "cpu" as a no-op for backward compatibility.
+    {
+      std::string lower_provider(provider);
+      std::transform(lower_provider.begin(), lower_provider.end(), lower_provider.begin(),
+                     [](unsigned char c) { return static_cast<unsigned char>(std::tolower(c)); });
+      if (lower_provider == "cpu" || lower_provider == "cpuexecutionprovider") {
+        auto provider_options_it = std::find_if(provider_options_list.begin(), provider_options_list.end(),
+                                                [&provider](const Config::ProviderOptions& po) { return po.name == provider; });
+        if (provider_options_it != provider_options_list.end() && !provider_options_it->options.empty()) {
+          throw std::runtime_error(
+              "CPU execution provider does not support provider options. "
+              "CPU is always available as the default fallback and does not need to be explicitly registered. "
+              "Remove the CPU provider entry and its options from your configuration.");
+        }
+        continue;
+      }
+    }
+
     auto provider_options_it = std::find_if(provider_options_list.begin(), provider_options_list.end(),
                                             [&provider](const Config::ProviderOptions& po) { return po.name == provider; });
 

test/c_api_tests.cpp

@@ -48,6 +48,31 @@ TEST(CAPITests, Config) {
 #endif
 }
 
+// Regression test: appending CPU provider should not throw.
+// See https://github.com/microsoft/onnxruntime-genai/pull/2179
+TEST(CAPITests, AppendCpuProvider) {
+#if TEST_PHI2
+  auto config = OgaConfig::Create(PHI2_PATH);
+  config->ClearProviders();
+  config->AppendProvider("cpu");
+  auto model = OgaModel::Create(*config);
+  ASSERT_NE(model.get(), nullptr);
+
+  // Also test other case variants
+  auto config2 = OgaConfig::Create(PHI2_PATH);
+  config2->ClearProviders();
+  config2->AppendProvider("CPU");
+  auto model2 = OgaModel::Create(*config2);
+  ASSERT_NE(model2.get(), nullptr);
+
+  auto config3 = OgaConfig::Create(PHI2_PATH);
+  config3->ClearProviders();
+  config3->AppendProvider("CPUExecutionProvider");
+  auto model3 = OgaModel::Create(*config3);
+  ASSERT_NE(model3.get(), nullptr);
+#endif
+}
+
 TEST(CAPITests, TokenizerCAPI) {
 #if TEST_PHI2
   auto config = OgaConfig::Create(PHI2_PATH);