server: (router) move model downloading to dedicated process (#24834)

* server: real-time model load progress tracking via /models/sse

* update docs

* server: move model download to child process

* rm unused

* fix most problems

* clean up

* nit fixes

* fix test case

* do not detact() thread

* shorter MODEL_DOWNLOAD_TIMEOUT in test

* throttle

common/arg.cpp

@@ -396,7 +396,7 @@ static bool parse_bool_value(const std::string & value) {
 // CLI argument parsing functions
 //
 
-bool common_params_handle_models(common_params & params, llama_example curr_ex) {
+bool common_params_handle_models(common_params & params, llama_example curr_ex, common_download_callback * callback) {
     const bool spec_type_draft_mtp = std::find(params.speculative.types.begin(),
                                          params.speculative.types.end(),
                                          COMMON_SPECULATIVE_TYPE_DRAFT_MTP) != params.speculative.types.end();
@@ -408,6 +408,10 @@ bool common_params_handle_models(common_params & params, llama_example curr_ex)
     opts.download_mtp    = spec_type_draft_mtp;
     opts.download_mmproj = !params.no_mmproj && params.mmproj.path.empty() && params.mmproj.url.empty();
 
+    if (callback) {
+        opts.callback = callback;
+    }
+
     // sub-models (draft, mmproj, vocoder) are explicitly specified by the user,
     // so we should not auto-discover mtp/mmproj siblings for them
     common_download_opts sub_opts = opts;
@@ -584,8 +588,11 @@ static bool common_params_parse_ex(int argc, char ** argv, common_params_context
         throw std::invalid_argument("error: --prompt-cache-all not supported in interactive mode yet\n");
     }
 
+    const bool skip_model_download =
+        // server will call common_params_handle_models() later, so we skip it here
+        ctx_arg.ex == LLAMA_EXAMPLE_SERVER ||
         // export_graph_ops loads only metadata
-    const bool skip_model_download = ctx_arg.ex == LLAMA_EXAMPLE_EXPORT_GRAPH_OPS;
+        ctx_arg.ex == LLAMA_EXAMPLE_EXPORT_GRAPH_OPS;
 
     if (!skip_model_download) {
         // handle model and download
@@ -594,7 +601,6 @@ static bool common_params_parse_ex(int argc, char ** argv, common_params_context
         // model is required (except for server)
         // TODO @ngxson : maybe show a list of available models in CLI in this case
         if (params.model.path.empty()
-                && ctx_arg.ex != LLAMA_EXAMPLE_SERVER
                 && !params.usage
                 && !params.completion) {
             throw std::invalid_argument("error: --model is required\n");

common/arg.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "common.h"
+#include "download.h"
 
 #include <set>
 #include <map>
@@ -133,7 +134,10 @@ void common_params_add_preset_options(std::vector<common_arg> & args);
 // return true if the model is ready to use
 // throw an exception if there is an error that prevents the model from being used (e.g. network error, model not found, etc)
 // if params.skip_download is true, no downloads will be attempted. return false if the model is invalid or missing (e.g. ETag check failed)
-bool common_params_handle_models(common_params & params, llama_example curr_ex);
+bool common_params_handle_models(
+    common_params & params,
+    llama_example curr_ex,
+    common_download_callback * callback = nullptr);
 
 // initialize argument parser context - used by test-arg-parser and preset
 common_params_context common_params_parser_init(common_params & params, llama_example ex, void(*print_usage)(int, char **) = nullptr);

tools/server/README-dev.md

@@ -204,9 +204,9 @@ Instead of building everything from the ground up (like what most AI agents will
 
 The flow for downloading a new model:
 - POST request comes in --> `post_router_models` --> validation
-- `server_models::download()` is called
+- A new `llama-server` subprocess will be spawned with special `SERVER_CHILD_MODE_DOWNLOAD`
-    - Sets up a new thread `inst.th` and runs the download inside
+- Child process runs the download and report status back to router via stdin/out
-- If a stop request comes in, set `stop_download` to `true`
+- If a stop request comes in, the router asks the child process to stop (same mechanism as running a model in child process)
 - Otherwise, upon completion, we call `load_models()` to refresh the list of models
 
 ### Notable Related PRs

tools/server/server-context.cpp

@@ -931,6 +931,8 @@ private:
 
     bool sleeping = false;
 
+    int64_t t_last_load_progress_ms = 0;
+
     void destroy() {
         spec.reset();
         ctx_dft.reset();
@@ -1244,6 +1246,10 @@ private:
         }
 
         if (has_mmproj) {
+            if (callback_state) {
+                callback_state(SERVER_STATE_LOADING, {{"stage", "mmproj_model"}});
+            }
+
             if (!is_resume) {
                 mtmd_helper_log_set(common_log_default_callback, nullptr);
             }

tools/server/server-context.h

@@ -53,7 +53,7 @@ struct server_context_meta {
 };
 
 enum server_state {
-    // SERVER_STATE_DOWNLOADING,
+    SERVER_STATE_DOWNLOADING,
     SERVER_STATE_LOADING,
     SERVER_STATE_READY,
     SERVER_STATE_SLEEPING,
@@ -61,6 +61,7 @@ enum server_state {
 
 static std::string server_state_to_str(server_state state) {
     switch (state) {
+        case SERVER_STATE_DOWNLOADING: return "downloading";
         case SERVER_STATE_LOADING:     return "loading";
         case SERVER_STATE_READY:       return "ready";
         case SERVER_STATE_SLEEPING:    return "sleeping";
@@ -69,6 +70,7 @@ static std::string server_state_to_str(server_state state) {
 }
 
 static server_state server_state_from_str(const std::string & str) {
+    if (str == "downloading") return SERVER_STATE_DOWNLOADING;
     if (str == "loading")     return SERVER_STATE_LOADING;
     if (str == "ready")       return SERVER_STATE_READY;
     if (str == "sleeping")    return SERVER_STATE_SLEEPING;

tools/server/server-models.cpp

@@ -64,6 +64,17 @@ struct server_subproc {
         return sproc.has_value() && subprocess_alive(&sproc.value());
     }
 
+    void request_exit() {
+        if (sproc.has_value()) {
+            FILE * stdin_file = subprocess_stdin(&sproc.value());
+            if (stdin_file) {
+                fprintf(stdin_file, "%s\n", CMD_ROUTER_TO_CHILD_EXIT);
+                fflush(stdin_file);
+            }
+        }
+        stopped.store(true, std::memory_order_relaxed);
+    }
+
     void terminate() {
         if (!sproc.has_value()) {
             return;
@@ -323,7 +334,7 @@ void server_models::notify_sse(const std::string & event, const std::string & mo
 }
 
 void server_models::load_models() {
-    // Phase 1: load presets from all sources — pure I/O, no lock needed
+    // Phase 1: load presets from all sources - pure I/O, no lock needed
     // 1. cached models
     common_presets cached_models = ctx_preset.load_from_cache();
     SRV_INF("Loaded %zu cached model presets\n", cached_models.size());
@@ -376,7 +387,7 @@ void server_models::load_models() {
         return source_map.count(name) ? source_map.at(name) : SERVER_MODEL_SOURCE_PRESET;
     };
 
-    // Helpers that read `mapping` — must be called while holding the lock.
+    // Helpers that read `mapping` - must be called while holding the lock.
     std::unordered_set<std::string> custom_names;
     for (const auto & [name, preset] : custom_presets) custom_names.insert(name);
     auto join_set = [](const std::set<std::string> & s) {
@@ -523,7 +534,7 @@ void server_models::load_models() {
             }
         }
 
-        // join outside the lock — monitoring thread calls update_status (needs lock)
+        // join outside the lock - monitoring thread calls update_status (needs lock)
         lk.unlock();
         for (auto & th : threads_to_join) th.join();
         lk.lock();
@@ -622,7 +633,7 @@ void server_models::load_models() {
 
         apply_stop_timeout();
 
-        // clear reload flag before unlocking for autoload — load() blocks on !is_reloading,
+        // clear reload flag before unlocking for autoload - load() blocks on !is_reloading,
         // so clearing it here (while still locked) prevents a deadlock in the autoload calls below
         is_reloading = false;
         cv.notify_all();
@@ -815,17 +826,23 @@ void server_models::unload_lru() {
 }
 
 void server_models::load(const std::string & name) {
+    load(name, load_options{});
+}
+
+void server_models::load(const std::string & name, const load_options & opts) {
+    if (!opts.custom_meta.has_value()) {
         if (!has_model(name)) {
             throw std::runtime_error("model name=" + name + " is not found");
         }
         unload_lru();
+    }
 
     std::unique_lock<std::mutex> lk(mutex);
     // edge case: block until any in-progress reload has finished so we always load
     // against the freshest preset and a consistent mapping state
     cv.wait(lk, [this]() { return !is_reloading; });
 
-    auto meta = mapping[name].meta;
+    auto meta = opts.custom_meta.has_value() ? *opts.custom_meta : mapping[name].meta;
     if (meta.status != SERVER_MODEL_STATUS_UNLOADED) {
         SRV_INF("model %s is not ready\n", name.c_str());
         return;
@@ -869,6 +886,12 @@ void server_models::load(const std::string & name) {
         std::vector<std::string> child_env  = base_env; // copy
         child_env.push_back("LLAMA_SERVER_ROUTER_PORT=" + std::to_string(base_params.port));
 
+        if (opts.mode == SERVER_CHILD_MODE_DOWNLOAD) {
+            inst.meta.status = SERVER_MODEL_STATUS_DOWNLOADING;
+            child_env.push_back("LLAMA_SERVER_CHILD_MODE=download");
+            child_env.push_back("LLAMA_ARG_HF_REPO=" + name);
+        }
+
         SRV_INF("%s", "spawning server instance with args:\n");
         for (const auto & arg : child_args) {
             SRV_INF("  %s\n", arg.c_str());
@@ -886,13 +909,17 @@ void server_models::load(const std::string & name) {
         if (result != 0) {
             throw std::runtime_error("failed to spawn server instance");
         }
-
-        inst.stdin_file = subprocess_stdin(&inst.subproc->get());
     }
 
     // start a thread to manage the child process
     // captured variables are guaranteed to be destroyed only after the thread is joined
-    inst.th = std::thread([this, name, child_proc = inst.subproc, port = inst.meta.port, stop_timeout = inst.meta.stop_timeout]() {
+    inst.th = std::thread([
+        this, name,
+        child_proc = inst.subproc,
+        port = inst.meta.port,
+        stop_timeout = inst.meta.stop_timeout,
+        child_mode = opts.mode
+    ]() {
         FILE * stdin_file = subprocess_stdin(&child_proc->get());
         FILE * stdout_file = subprocess_stdout(&child_proc->get()); // combined stdout/stderr
 
@@ -925,7 +952,7 @@ void server_models::load(const std::string & name) {
                     return is_stopping() || child_proc->stopped.load(std::memory_order_acquire);
                 });
             }
-            // child crashed or finished on its own — skip graceful shutdown sequence
+            // child crashed or finished on its own, skip graceful shutdown sequence
             if (child_proc->stopped.load(std::memory_order_acquire)) {
                 return;
             }
@@ -973,10 +1000,14 @@ void server_models::load(const std::string & name) {
         subprocess_destroy(&child_proc->get());
 
         // update status and exit code
+        if (child_mode == SERVER_CHILD_MODE_DOWNLOAD) {
+            // instance will be cleaned up on next load_models() call
+        } else {
             this->update_status(name, {
                 SERVER_MODEL_STATUS_UNLOADED,
                 exit_code
             });
+        }
         SRV_INF("instance name=%s exited with status %d\n", name.c_str(), exit_code);
     });
 
@@ -984,7 +1015,7 @@ void server_models::load(const std::string & name) {
     {
         auto & old_instance = mapping[name];
         // old process should have exited already, but just in case, we clean it up here
-        if (old_instance.subproc->is_alive()) {
+        if (old_instance.subproc && old_instance.subproc->is_alive()) {
             SRV_WRN("old process for model name=%s is still alive, this is unexpected\n", name.c_str());
             old_instance.subproc->terminate(); // force kill
         }
@@ -1001,92 +1032,13 @@ void server_models::load(const std::string & name) {
     cv.notify_all();
 }
 
-// callback for model downloading functionality
-struct server_models_download_res : public common_download_callback {
-    common_params_model model;
-    common_download_opts opts;
-
-    std::function<bool()> should_stop;
-    std::function<void(const common_download_progress & p)> on_progress;
-
-    bool is_ok = false;
-
-    bool run() {
-        try {
-            common_download_model(model, opts);
-            is_ok = true;
-        } catch (const std::exception & e) {
-            auto model_name = model.get_name();
-            SRV_ERR("download failed for model name=%s: %s\n", model_name.c_str(), e.what());
-            is_ok = false;
-        }
-        return is_ok;
-    }
-    void on_start(const common_download_progress & p) override {
-        on_progress(p);
-    }
-    void on_update(const common_download_progress & p) override {
-        on_progress(p);
-    }
-    void on_done(const common_download_progress &, bool ok) override {
-        is_ok = ok;
-    }
-    bool is_cancelled() const override {
-        return should_stop();
-    }
-};
-
-void server_models::download(common_params_model && model, common_download_opts && opts) {
-    std::string name = model.get_name();
-    GGML_ASSERT(name == model.hf_repo);
-
-    std::unique_lock<std::mutex> lk(mutex);
-    if (mapping.find(name) != mapping.end()) {
-        throw std::runtime_error("model name=" + name + " already exists");
-    }
-
-    instance_t inst;
-    inst.meta.name   = name;
-    inst.meta.status = SERVER_MODEL_STATUS_DOWNLOADING;
-    inst.subproc     = std::make_shared<server_subproc>();
-
-    auto dl = std::make_unique<server_models_download_res>();
-    dl->model = model; // copy
-    dl->opts  = opts;  // copy
-
-    dl->should_stop = [sp = inst.subproc]() {
-        return sp->stopped.load(std::memory_order_relaxed);
-    };
-
-    dl->on_progress = [this, name](const common_download_progress & p) {
-        update_download_progress(name, p, false);
-    };
-
-    inst.th = std::thread([this, dl = std::move(dl)]() {
-        dl->opts.callback = dl.get();
-        bool ok = dl->run();
-        auto model_name = dl->model.get_name();
-        SRV_INF("download finished for model name=%s with status=%s\n",
-                    model_name.c_str(), ok ? "success" : "failure");
-        update_download_progress(model_name, {}, true, ok);
-        // need_reload is set inside update_download_progress under the mutex;
-        // the next load_models() call will clean up this instance
-    });
-
-    mapping[name] = std::move(inst);
-    notify_sse("status_update", name, {
-        {"status", server_model_status_to_string(SERVER_MODEL_STATUS_DOWNLOADING)},
-    });
-    cv.notify_all();
-}
-
 void server_models::unload(const std::string & name) {
     std::unique_lock<std::mutex> lk(mutex);
     auto it = mapping.find(name);
     if (it != mapping.end()) {
         if (it->second.meta.status == SERVER_MODEL_STATUS_DOWNLOADING) {
             SRV_INF("cancelling download for model name=%s\n", name.c_str());
-            it->second.subproc->stopped.store(true, std::memory_order_relaxed);
+            it->second.subproc->request_exit();
             // for convenience, we wait the status change here
             wait(lk, name, [](const server_model_meta & new_meta) {
                 return new_meta.status != SERVER_MODEL_STATUS_DOWNLOADING;
@@ -1198,37 +1150,65 @@ void server_models::update_download_progress(const std::string & name, const com
 }
 
 bool server_models::remove(const std::string & name) {
-    auto meta = get_meta(name);
+    // do everything under one lock acquisition; avoid get_meta() /
+    // unload() because they can trigger load_models() which erases
+    // transient DOWNLOADING / DOWNLOADED entries as a side-effect
+    std::unique_lock<std::mutex> lk(mutex);
 
-    if (!meta.has_value()) {
+    auto it = mapping.find(name);
+    if (it == mapping.end()) {
         throw std::runtime_error("model name=" + name + " is not found");
     }
-    if (meta->source != SERVER_MODEL_SOURCE_CACHE) {
+    if (it->second.meta.source != SERVER_MODEL_SOURCE_CACHE) {
         throw std::runtime_error("model name=" + name + " is not removable (not from cache)");
     }
 
-    unload(name); // cancel download or stop running instance
+    if (it->second.meta.status == SERVER_MODEL_STATUS_DOWNLOADING) {
-    {
+        // cancel in-flight download
-        std::unique_lock<std::mutex> lk(mutex);
+        SRV_INF("cancelling download for model name=%s\n", name.c_str());
-        // a cancelled download lands on DOWNLOADED; a stopped instance lands on UNLOADED
+        it->second.subproc->request_exit();
-        wait(lk, name, [](const server_model_meta & new_meta) {
+    } else if (it->second.meta.is_running()) {
-            return new_meta.status == SERVER_MODEL_STATUS_UNLOADED
+        // stop running instance
-                || new_meta.status == SERVER_MODEL_STATUS_DOWNLOADED;
+        SRV_INF("stopping model instance name=%s\n", name.c_str());
+        stopping_models.insert(name);
+        if (it->second.meta.status == SERVER_MODEL_STATUS_LOADING) {
+            it->second.subproc->terminate();
+        }
+        cv_stop.notify_all();
+    }
+
+    // wait until the monitoring thread finishes
+    wait(lk, name, [](const server_model_meta & meta) {
+        return meta.status == SERVER_MODEL_STATUS_UNLOADED
+            || meta.status == SERVER_MODEL_STATUS_DOWNLOADED;
     });
-        // join before erasing - after status reaches UNLOADED/DOWNLOADED the thread no
+
-        // longer acquires this mutex, so joining while holding it is safe
+    // re-find after wait - load_models() may have erased the entry during the wait
-        if (mapping[name].th.joinable()) {
+    it = mapping.find(name);
-            mapping[name].th.join();
+    if (it == mapping.end()) {
-        }
+        // load_models() already joined the thread and erased the entry;
-        // remove the model from disk (hold lock to prevent concurrent load)
+        // we just need to clean up the cached files on disk
+        lk.unlock();
         bool ok = common_download_remove(name);
-        if (ok) {
+        SRV_INF("removing model name=%s from cache (%s)\n", name.c_str(), ok ? "succeeded" : "partial");
-            mapping.erase(name);
-        }
-        SRV_INF("removing model name=%s from cache (%s)\n", name.c_str(), ok ? "succeeded" : "failed");
         notify_sse("model_remove", name, {});
-        return ok;
+        return true;
     }
+
+    // join before erasing - thread no longer acquires this mutex
+    if (it->second.th.joinable()) {
+        it->second.th.join();
+    }
+
+    // remove from disk (best-effort: cancelled downloads may have no cached files)
+    bool ok = common_download_remove(name);
+    mapping.erase(name);
+    if (!ok) {
+        SRV_WRN("removing model name=%s from disk returned false (no cached files?)\n", name.c_str());
+    }
+    SRV_INF("removing model name=%s from cache (%s)\n", name.c_str(), ok ? "succeeded" : "partial");
+    notify_sse("model_remove", name, {});
+    return true;
 }
 
 void server_models::wait(const std::string & name, std::function<bool(const server_model_meta &)> predicate) {
@@ -1243,7 +1223,9 @@ void server_models::wait(std::unique_lock<std::mutex> & lk, const std::string &
             return predicate(it->second.meta);
 
         }
-        return false;
+        // model was removed from mapping by another code path (e.g. load_models()).
+        // nothing left to wait for - tell the caller to proceed.
+        return true;
     });
 }
 
@@ -1328,6 +1310,31 @@ void server_models::handle_child_state(const std::string & name, const std::stri
     }
 
     switch (state) {
+        case SERVER_STATE_DOWNLOADING:
+            {
+                std::string result = json_value(payload, "result", std::string());
+                std::string url    = json_value(payload, "url",    std::string());
+                auto request_exit = [&]() {
+                    std::lock_guard<std::mutex> lk(mutex);
+                    auto it = mapping.find(name);
+                    if (it != mapping.end()) {
+                        return it->second.subproc->request_exit();
+                    }
+                };
+                if (result == "download_finished") {
+                    update_download_progress(name, {}, true, true);
+                    request_exit();
+                } else if (result == "download_failed") {
+                    update_download_progress(name, {}, true, false);
+                    request_exit();
+                } else if (!url.empty()) {
+                    common_download_progress p;
+                    p.url        = url;
+                    p.downloaded = json_value(payload, "downloaded", (size_t)0);
+                    p.total      = json_value(payload, "total", (size_t)0);
+                    update_download_progress(name, p, false);
+                }
+            } break;
         case SERVER_STATE_LOADING:
             {
                 update_status(name, {
@@ -1366,6 +1373,90 @@ bool server_child::is_child() {
     return router_port != nullptr;
 }
 
+server_child_mode server_child::get_mode() {
+    const char * mode = std::getenv("LLAMA_SERVER_CHILD_MODE");
+    std::string mode_str(mode ? mode : "");
+    if (mode_str == "download") {
+        return SERVER_CHILD_MODE_DOWNLOAD;
+    } else {
+        return SERVER_CHILD_MODE_NORMAL;
+    }
+}
+
+struct server_download_state : public common_download_callback {
+    server_child * self;
+    std::function<bool()> should_stop;
+    std::atomic<int64_t> last_progress_time{0}; // multiple files downloading in different threads
+    bool is_ok = false;
+
+    server_download_state(server_child * s) : self(s) {}
+
+    bool run(common_params & params) {
+        try {
+            common_params_handle_models(params, LLAMA_EXAMPLE_SERVER, this);
+            is_ok = true;
+        } catch (const std::exception & e) {
+            auto model_name = params.model.get_name();
+            SRV_ERR("download failed for model name=%s: %s\n", model_name.c_str(), e.what());
+            is_ok = false;
+        }
+        return is_ok;
+    }
+    void on_progress(const common_download_progress & p) {
+        json data = {
+            {"url", p.url},
+            {"downloaded", p.downloaded},
+            {"total", p.total},
+        };
+        self->notify_to_router(server_state_to_str(SERVER_STATE_DOWNLOADING), data);
+    }
+    void on_start(const common_download_progress & p) override {
+        on_progress(p);
+    }
+    void on_update(const common_download_progress & p) override {
+        int64_t now = ggml_time_ms();
+        // throttle progress updates to avoid flooding logs
+        if (now - last_progress_time.load(std::memory_order_relaxed) >= 100) {
+            on_progress(p);
+            last_progress_time.store(now, std::memory_order_relaxed);
+        }
+    }
+    void on_done(const common_download_progress & p, bool) override {
+        on_progress(p);
+    }
+    bool is_cancelled() const override {
+        return should_stop ? should_stop() : false;
+    }
+};
+
+int server_child::run_download(common_params & params) {
+    auto cancelled = std::make_shared<std::atomic<bool>>(false);
+
+    // monitor stdin for cancellation command from the router
+    std::thread signal_thread = setup([cancelled](int) {
+        cancelled->store(true, std::memory_order_relaxed);
+    });
+
+    server_download_state dl(this);
+    dl.should_stop = [cancelled]() {
+        return cancelled->load(std::memory_order_relaxed);
+    };
+
+    bool ok = dl.run(params);
+
+    notify_to_router(server_state_to_str(SERVER_STATE_DOWNLOADING), {
+        {"result", ok ? "download_finished" : "download_failed"},
+    });
+
+    // router should send CMD_ROUTER_TO_CHILD_EXIT after receiving the result
+    if (signal_thread.joinable()) {
+        signal_thread.join();
+    }
+
+    SRV_INF("download completed %s\n", ok ? "successfully" : "with errors");
+    return 0;
+}
+
 std::thread server_child::setup(const std::function<void(int)> & shutdown_handler) {
     // setup thread for monitoring stdin
     return std::thread([shutdown_handler]() {
@@ -1639,7 +1730,7 @@ void server_models_routes::init_routes() {
             res_err(res, format_error_response("model is not found", ERROR_TYPE_INVALID_REQUEST));
             return res;
         }
-        if (!model->is_running()) {
+        if (!model->is_running() && model->status != SERVER_MODEL_STATUS_DOWNLOADING) {
             res_err(res, format_error_response("model is not running", ERROR_TYPE_INVALID_REQUEST));
             return res;
         }
@@ -1680,8 +1771,9 @@ void server_models_routes::init_routes() {
 
         model.hf_repo        = name;
         opts.bearer_token    = params.hf_token;
-        opts.download_mmproj = true;
+        // note: we only check main model, no need sidecar here
-        opts.download_mtp    = true;
+        opts.download_mmproj = false;
+        opts.download_mtp    = false;
 
         // first, only check if the model is valid and can be downloaded
         opts.skip_download = true;
@@ -1702,10 +1794,21 @@ void server_models_routes::init_routes() {
             throw std::invalid_argument("model validation failed, unable to download");
         }
 
+        // reject if model already exists
+        if (models.has_model(name)) {
+            throw std::invalid_argument("model '" + name + "' already exists");
+        }
+
         // then, proceed with the actual download
-        opts.skip_download = false;
         SRV_INF("starting download for model '%s'\n", name.c_str());
-        models.download(std::move(model), std::move(opts));
+        {
+            server_models::load_options load_opts;
+            load_opts.mode = SERVER_CHILD_MODE_DOWNLOAD;
+            load_opts.custom_meta = server_model_meta{};
+            load_opts.custom_meta->source = SERVER_MODEL_SOURCE_CACHE;
+            load_opts.custom_meta->name   = name;
+            models.load(name, load_opts);
+        }
 
         res_ok(res, {{"success", true}});
         return res;
@@ -1719,10 +1822,7 @@ void server_models_routes::init_routes() {
             throw std::invalid_argument("model must be a non-empty string");
         }
 
-        bool ok = models.remove(name);
+        models.remove(name); // throws on error
-        if (!ok) {
-            throw std::runtime_error("failed to remove model '" + name + "'");
-        }
 
         res_ok(res, {{"success", true}});
         return res;

tools/server/server-models.h

@@ -40,6 +40,11 @@ enum server_model_source {
     SERVER_MODEL_SOURCE_CACHE,
 };
 
+enum server_child_mode {
+    SERVER_CHILD_MODE_NORMAL,   // load the model and run normally
+    SERVER_CHILD_MODE_DOWNLOAD, // download the model and exit
+};
+
 static std::string server_model_status_to_string(server_model_status status) {
     switch (status) {
         case SERVER_MODEL_STATUS_DOWNLOADING: return "downloading";
@@ -105,7 +110,6 @@ private:
         std::shared_ptr<server_subproc> subproc; // shared between main thread and monitoring thread
         std::thread th;
         server_model_meta meta;
-        FILE * stdin_file = nullptr;
     };
 
     std::mutex mutex;
@@ -161,16 +165,19 @@ public:
     // return a copy of all model metadata (thread-safe)
     std::vector<server_model_meta> get_all_meta();
 
+    struct load_options {
+        server_child_mode mode = SERVER_CHILD_MODE_NORMAL;
+        // used for spawning a downloading child process
+        std::optional<server_model_meta> custom_meta = std::nullopt;
+    };
+
     // load and unload model instances
     // these functions are thread-safe
     void load(const std::string & name);
+    void load(const std::string & name, const load_options & opts);
     void unload(const std::string & name);
     void unload_all();
 
-    // download a new model, progress is reported via SSE
-    // to stop the download, call unload()
-    void download(common_params_model && model, common_download_opts && opts);
-
     struct update_status_args {
         server_model_status status;
         int exit_code = 0; // only valid if status == UNLOADED
@@ -213,9 +220,12 @@ public:
 struct server_child {
     // serializes the notify_to_router writes
     std::mutex mtx_stdout;
+    std::atomic<bool> is_finished_downloading = false; // set by run_download
 
     // return true if the current process is a child server instance
     bool is_child();
+    server_child_mode get_mode();
+    int run_download(common_params & params);
 
     // register the shutdown_handler to be called by the router
     // return the monitoring thread (to be joined by the caller)

tools/server/server.cpp

@@ -134,6 +134,7 @@ int llama_server(int argc, char ** argv) {
     //
 
     // register API routes
+    server_child child; // only used in non-router mode
     server_routes routes(params, ctx_server);
     server_tools tools;
 
@@ -254,11 +255,21 @@ int llama_server(int argc, char ** argv) {
         ctx_http.post("/tools",           ex_wrapper(tools.handle_post));
     }
 
+    //
+    // Handle downloading model
+    //
+
+    if (child.is_child() && child.get_mode() == SERVER_CHILD_MODE_DOWNLOAD) {
+        return child.run_download(params);
+    } else if (!is_router_server) {
+        // single-model mode (NOT spawned by router)
+        common_params_handle_models(params, LLAMA_EXAMPLE_SERVER);
+    }
+
     //
     // Start the server
     //
 
-    server_child child; // only used in non-router mode
     std::function<void()> clean_up;
 
     if (is_router_server) {

tools/server/tests/unit/test_router.py

@@ -257,14 +257,25 @@ def test_router_reload_models():
 
 
 MODEL_DOWNLOAD_ID = "ggml-org/test-model-router-download:F16"
-MODEL_DOWNLOAD_TIMEOUT = 300
+MODEL_DOWNLOAD_TIMEOUT = 30
 
 
-def _listen_sse(server: ServerProcess, collected: list, stop: threading.Event):
+def _listen_sse(
-    """Collect /models/sse events into `collected` until `stop` is set."""
+    server: ServerProcess, collected: list, stop: threading.Event, ready: threading.Event | None = None
+):
+    """Collect /models/sse events into `collected` until `stop` is set.
+
+    When `ready` is provided, it is set once the streaming response is open,
+    i.e. the server has accepted the connection and registered us as a
+    subscriber. Callers that trigger one-shot events (e.g. download_finished)
+    must wait on `ready` before acting, otherwise the event can be broadcast
+    before this client is subscribed and be lost.
+    """
     url = f"http://{server.server_host}:{server.server_port}/models/sse"
     try:
         with requests.get(url, stream=True, timeout=MODEL_DOWNLOAD_TIMEOUT) as resp:
+            if ready is not None:
+                ready.set()
             for line_bytes in resp.iter_lines():
                 if stop.is_set():
                     break
@@ -294,11 +305,17 @@ def test_router_download_model():
 
     sse_events: list = []
     stop = threading.Event()
+    sse_ready = threading.Event()
     sse_thread = threading.Thread(
-        target=_listen_sse, args=(server, sse_events, stop), daemon=True
+        target=_listen_sse, args=(server, sse_events, stop, sse_ready), daemon=True
     )
     sse_thread.start()
 
+    # wait for the SSE client to be subscribed before triggering the download,
+    # otherwise the one-shot download_finished event can be broadcast before
+    # this client is registered and be lost
+    assert sse_ready.wait(10), "SSE client failed to connect"
+
     # Trigger the download
     res = server.make_request("POST", "/models", data={"model": MODEL_DOWNLOAD_ID})
     assert res.status_code == 200
@@ -328,13 +345,17 @@ def test_router_delete_model():
 
     # Ensure the model exists (download it if needed)
     if MODEL_DOWNLOAD_ID not in _get_model_ids(is_reload=False):
-        res = server.make_request("POST", "/models", data={"model": MODEL_DOWNLOAD_ID})
-        assert res.status_code == 200
         sse_events: list = []
         stop = threading.Event()
+        sse_ready = threading.Event()
         threading.Thread(
-            target=_listen_sse, args=(server, sse_events, stop), daemon=True
+            target=_listen_sse, args=(server, sse_events, stop, sse_ready), daemon=True
         ).start()
+        # subscribe before triggering the download so the one-shot
+        # download_finished event is not lost (see test_router_download_model)
+        assert sse_ready.wait(10), "SSE client failed to connect"
+        res = server.make_request("POST", "/models", data={"model": MODEL_DOWNLOAD_ID})
+        assert res.status_code == 200
         finished = _wait_for_sse_event(
             sse_events, "download_finished", MODEL_DOWNLOAD_ID, MODEL_DOWNLOAD_TIMEOUT
         )