fix(llama-cpp): let parallel:1 in the model options win over LLAMACPP_PARALLEL (#12426)

The environment fallback was applied whenever n_parallel was still 1
after option parsing. An explicit `parallel: 1` in the model YAML is
indistinguishable from the default that way, so it was replaced by
LLAMACPP_PARALLEL. The docs say options in the YAML take precedence
over environment variables; a single model could not be forced to one
slot while the global variable was set.

Track whether the options set the slot count and resolve it in a small
helper (parallel_params.h): option first, then LLAMACPP_PARALLEL, then
1. The helper gets a standalone unit test picked up by
`make test-backend-cpp`.

Assisted-by: Claude:claude-opus-5-5

Signed-off-by: Stefan Walcz <stefan.walcz@walcz.de>

backend/cpp/llama-cpp/CMakeLists.txt

@@ -126,6 +126,11 @@ if(LLAMA_GRPC_BUILD_TESTS)
     target_compile_features(thread_params_test PRIVATE cxx_std_17)
     add_test(NAME thread_params_test COMMAND thread_params_test)
 
+    add_executable(parallel_params_test parallel_params_test.cpp parallel_params.h)
+    target_include_directories(parallel_params_test PRIVATE ${CMAKE_CURRENT_SOURCE_DIR})
+    target_compile_features(parallel_params_test PRIVATE cxx_std_17)
+    add_test(NAME parallel_params_test COMMAND parallel_params_test)
+
     add_executable(model_load_error_test model_load_error_test.cpp model_load_error.h)
     target_include_directories(model_load_error_test PRIVATE ${CMAKE_CURRENT_SOURCE_DIR})
     target_compile_features(model_load_error_test PRIVATE cxx_std_17)

backend/cpp/llama-cpp/grpc-server.cpp

@@ -55,6 +55,7 @@
 #include "llama_compat.h" // fork-skew switches, generated by prepare.sh
 #include "model_load_error.h"
 #include "thread_params.h"
+#include "parallel_params.h"
 #include "message_content.h"
 #include "passthrough_options.h"
 #include "stream_peer.h"
@@ -545,6 +546,9 @@ static void params_parse(server_context& /*ctx_server*/, const backend::ModelOpt
     // slot state in host RAM and the backend grows without bound.
     // Initialize n_parallel to 1 by default (can be overridden by options)
     params.n_parallel = 1;
+    // Set only when the model options carry parallel/n_parallel, so an
+    // explicit 1 can be told apart from the default.
+    std::optional<int> parallel_option;
     // Initialize grpc_servers to empty (can be overridden by options)
     std::string grpc_servers_option = "";
 
@@ -664,12 +668,9 @@ static void params_parse(server_context& /*ctx_server*/, const backend::ModelOpt
         } else if (!strcmp(optname, "parallel") || !strcmp(optname, "n_parallel")) {
             if (optval != NULL) {
                 try {
-                    params.n_parallel = std::stoi(optval_str);
-                    if (params.n_parallel > 1) {
-                        params.cont_batching = true;
-                    }
+                    parallel_option = std::stoi(optval_str);
                 } catch (const std::exception& e) {
-                    // If conversion fails, keep default value (1)
+                    // If conversion fails, fall back to the environment/default
                 }
             }
         } else if (!strcmp(optname, "grpc_servers") || !strcmp(optname, "rpc_servers")) {
@@ -1238,19 +1239,12 @@ static void params_parse(server_context& /*ctx_server*/, const backend::ModelOpt
         }
     }
 
-    // Set params.n_parallel from environment variable if not set via options (fallback)
-    if (params.n_parallel == 1) {
-        const char *env_parallel = std::getenv("LLAMACPP_PARALLEL");
-        if (env_parallel != NULL) {
-            try {
-                params.n_parallel = std::stoi(env_parallel);
-                if (params.n_parallel > 1) {
-                    params.cont_batching = true;
-                }
-            } catch (const std::exception& e) {
-                // If conversion fails, keep default value (1)
-            }
-        }
+    // The model options win over LLAMACPP_PARALLEL, including an explicit
+    // parallel:1 (previously indistinguishable from the default and replaced
+    // by the environment value).
+    params.n_parallel = llama_grpc::resolve_n_parallel(parallel_option, std::getenv("LLAMACPP_PARALLEL"));
+    if (params.n_parallel > 1) {
+        params.cont_batching = true;
     }
 
     // Add RPC devices from option or environment variable (fallback)

backend/cpp/llama-cpp/parallel_params.h

@@ -0,0 +1,25 @@
+#pragma once
+
+#include <optional>
+#include <string>
+
+namespace llama_grpc {
+
+// resolve_n_parallel picks the slot count. A value from the model options
+// always wins, including an explicit 1: the YAML takes precedence over the
+// environment, as documented. LLAMACPP_PARALLEL is only the fallback when
+// the options do not set it; a value that does not parse is ignored.
+inline int resolve_n_parallel(const std::optional<int>& from_options, const char* env, int fallback = 1) {
+    if (from_options) {
+        return *from_options;
+    }
+    if (env != nullptr) {
+        try {
+            return std::stoi(env);
+        } catch (const std::exception&) {
+        }
+    }
+    return fallback;
+}
+
+} // namespace llama_grpc

backend/cpp/llama-cpp/parallel_params_test.cpp

@@ -0,0 +1,29 @@
+#include "parallel_params.h"
+
+#include <cstdio>
+
+int main() {
+    // parallel:1 in the model options must not be replaced by the environment.
+    if (llama_grpc::resolve_n_parallel(1, "4") != 1) {
+        std::fprintf(stderr, "explicit parallel:1 was overwritten by LLAMACPP_PARALLEL\n");
+        return 1;
+    }
+    if (llama_grpc::resolve_n_parallel(8, "4") != 8) {
+        std::fprintf(stderr, "explicit parallel was overwritten by LLAMACPP_PARALLEL\n");
+        return 1;
+    }
+    // Without an option the environment applies, otherwise the default.
+    if (llama_grpc::resolve_n_parallel(std::nullopt, "4") != 4) {
+        std::fprintf(stderr, "LLAMACPP_PARALLEL was not used as fallback\n");
+        return 1;
+    }
+    if (llama_grpc::resolve_n_parallel(std::nullopt, nullptr) != 1) {
+        std::fprintf(stderr, "default slot count is not 1\n");
+        return 1;
+    }
+    if (llama_grpc::resolve_n_parallel(std::nullopt, "many") != 1) {
+        std::fprintf(stderr, "unparsable LLAMACPP_PARALLEL was not ignored\n");
+        return 1;
+    }
+    return 0;
+}

backend/cpp/llama-cpp/prepare.sh

@@ -63,6 +63,10 @@ cp -r tts_request_options_test.cpp llama.cpp/tools/grpc-server/
 # Thread-count default normalization and its standalone regression test.
 cp -r thread_params.h llama.cpp/tools/grpc-server/
 cp -r thread_params_test.cpp llama.cpp/tools/grpc-server/
+# Slot-count resolution (option over LLAMACPP_PARALLEL) and its standalone
+# regression test.
+cp -r parallel_params.h llama.cpp/tools/grpc-server/
+cp -r parallel_params_test.cpp llama.cpp/tools/grpc-server/
 # Parent-death watcher (included by grpc-server.cpp) and its standalone unit
 # test (run via backend/cpp/run-unit-tests.sh; also buildable under ctest).
 cp -r parent_watch.h llama.cpp/tools/grpc-server/