server: return 400 instead of 500 on validation error with X-Conversation-Id (#25760 )

* server: return 400 instead of 500 on validation error with X-Conversation-Id set_req() attaches the spipe as soon as the header is present, before the request body is parsed. When params validation throws, set_next() never runs and next_orig stays empty, so on_complete() called it and crashed with std::bad_function_call, turning the prepared 400 JSON into a generic 500. on_complete() now treats an empty next_orig as "streaming never started" and evicts the session installed by set_req(), so a failed request leaves nothing behind for discovery or replay. This also covers valid requests that carry the header but do not stream, which previously left an empty finalized session in the map until the GC TTL. * ui: do not send the backend_sampling placeholder On a fresh profile the syncable settings hold the empty string placeholder meaning "let the server decide". Every neighbor field goes through the hasValue() guard that filters it, except backend_sampling, which sent the placeholder verbatim and made every default settings completion fail validation. Guard the field with hasValue() like its neighbors. hasValue(false) is true, so an explicit false still reaches the server and the intent of #18781 (send both true and false) is preserved. Only the placeholder is filtered.
server : properly handle null llama_context (#25868 )
2026-07-27 08:08:14 +02:00 · 2026-07-21 17:47:54 +02:00 · 2026-07-21 17:47:17 +02:00 · 2026-07-21 17:40:45 +02:00
4 changed files with 153 additions and 57 deletions
@@ -156,6 +156,24 @@ typedef struct VkPhysicalDeviceShaderFloat8FeaturesEXT {
 } VkPhysicalDeviceShaderFloat8FeaturesEXT;
 #endif

+#ifndef VK_KHR_INTERNALLY_SYNCHRONIZED_QUEUES_EXTENSION_NAME
+#define VK_KHR_INTERNALLY_SYNCHRONIZED_QUEUES_EXTENSION_NAME "VK_KHR_internally_synchronized_queues"
+#define VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_INTERNALLY_SYNCHRONIZED_QUEUES_FEATURES_KHR ((VkStructureType)1000504000)
+#define VK_DEVICE_QUEUE_CREATE_INTERNALLY_SYNCHRONIZED_BIT_KHR ((VkDeviceQueueCreateFlagBits)0x00000004)
+
+// Compile-time constant guaranteed; no runtime initialization overhead
+static constexpr vk::DeviceQueueCreateFlagBits eInternallySynchronizedKHR =
+    static_cast<vk::DeviceQueueCreateFlagBits>(0x00000004);
+
+typedef struct VkPhysicalDeviceInternallySynchronizedQueuesFeaturesKHR {
+    VkStructureType    sType;
+    void* pNext;
+    VkBool32           internallySynchronizedQueues;
+} VkPhysicalDeviceInternallySynchronizedQueuesFeaturesKHR;
+#else
+static constexpr vk::DeviceQueueCreateFlagBits eInternallySynchronizedKHR = vk::DeviceQueueCreateFlagBits::eInternallySynchronizedKHR;
+#endif
+
 #define ROUNDUP_POW2(M, N) (((M) + (N) - 1) & ~((N) - 1))
 #define CEIL_DIV(M, N) (((M) / (N)) + (((M) % (N)) != 0))
 static bool is_pow2(uint32_t x) { return x > 1 && (x & (x-1)) == 0; }
@@ -285,27 +303,41 @@ struct vk_command_pool {
 };

 // Prevent simultaneous submissions to the same queue.
-// This could be per vk_queue if we stopped having two vk_queue structures
-// sharing the same vk::Queue.
-static std::mutex queue_mutex;
+struct vk_queue_handle {
+    vk::Queue queue;
+    virtual void submit(vk::ArrayProxy<const vk::SubmitInfo> submits, vk::Fence fence) = 0;
+    virtual void lock()   {}   // no-op by default (internally synchronized case)
+    virtual void unlock() {}
+    virtual ~vk_queue_handle() = default;
+};
+
+struct vk_queue_handle_synchronized : vk_queue_handle {
+    std::mutex mutex;
+    void submit(vk::ArrayProxy<const vk::SubmitInfo> submits, vk::Fence fence) override {
+        std::lock_guard<std::mutex> guard(mutex);
+        queue.submit(submits, fence);
+    }
+    void lock()   override { mutex.lock(); }
+    void unlock() override { mutex.unlock(); }
+};
+
+struct vk_queue_handle_unsynchronized : vk_queue_handle {
+    void submit(vk::ArrayProxy<const vk::SubmitInfo> submits, vk::Fence fence) override {
+        // Driver guarantees internal synchronization via VK_KHR_internally_synchronized_queues
+        queue.submit(submits, fence);
+    }
+    // lock()/unlock() inherited no-ops
+};

 struct vk_queue {
    uint32_t queue_family_index;
-    vk::Queue queue;
+    std::shared_ptr<vk_queue_handle> handle;

    vk_command_pool cmd_pool;

    vk::PipelineStageFlags stage_flags;

    bool transfer_only;
-
-    // copy everything except the cmd_pool
-    void copyFrom(vk_queue &other) {
-        queue_family_index = other.queue_family_index;
-        queue = other.queue;
-        stage_flags = other.stage_flags;
-        transfer_only = other.transfer_only;
-    }
 };

 static const char * ggml_backend_vk_buffer_type_name(ggml_backend_buffer_type_t buft);
@@ -712,11 +744,12 @@ struct vk_device_struct {
    uint32_t vendor_id;
    vk::DriverId driver_id;
    vk_device_architecture architecture;
-    vk_queue compute_queue;
-    vk_queue transfer_queue;
+    std::unique_ptr<vk_queue> compute_queue;
+    std::unique_ptr<vk_queue> transfer_queue;
    bool single_queue;
    bool support_async;
    bool async_use_transfer_queue;
+    bool has_internally_synchronized_queues = false;
    uint32_t subgroup_size;
    uint32_t subgroup_size_log2;
    uint32_t shader_core_count;
@@ -1019,8 +1052,13 @@ struct vk_device_struct {

        ggml_vk_destroy_buffer(sync_staging);

-        compute_queue.cmd_pool.destroy(device);
-        transfer_queue.cmd_pool.destroy(device);
+        if (compute_queue) compute_queue->cmd_pool.destroy(device);
+        if (transfer_queue) transfer_queue->cmd_pool.destroy(device);
+
+        // Explicitly clear to ensure queues drop their shared_ptrs to handles
+        // before the Vulkan logical device instance is destroyed
+        compute_queue.reset();
+        transfer_queue.reset();

        for (auto& pipeline : all_pipelines) {
            if (pipeline.expired()) {
@@ -2909,8 +2947,7 @@ static vk_command_buffer* ggml_vk_create_cmd_buffer(vk_device& device, vk_comman
 static void ggml_vk_submit(vk_context& ctx, vk::Fence fence) {
    if (ctx->seqs.empty()) {
        if (fence) {
-            std::lock_guard<std::mutex> guard(queue_mutex);
-            ctx->p->q->queue.submit({}, fence);
+            ctx->p->q->handle->submit({}, fence);
        }
        return;
    }
@@ -2979,8 +3016,7 @@ static void ggml_vk_submit(vk_context& ctx, vk::Fence fence) {
        }
    }

-    std::lock_guard<std::mutex> guard(queue_mutex);
-    ctx->p->q->queue.submit(submit_infos, fence);
+    ctx->p->q->handle->submit(submit_infos, fence);

    ctx->seqs.clear();
 }
@@ -3031,18 +3067,44 @@ static uint32_t ggml_vk_find_queue_family_index(std::vector<vk::QueueFamilyPrope
    abort();
 }

-static void ggml_vk_create_queue(vk_device& device, vk_queue& q, uint32_t queue_family_index, uint32_t queue_index, vk::PipelineStageFlags&& stage_flags, bool transfer_only) {
+static std::unique_ptr<vk_queue> ggml_vk_create_queue(vk_device& device, uint32_t queue_family_index, uint32_t queue_index, vk::PipelineStageFlags&& stage_flags, bool transfer_only) {
    VK_LOG_DEBUG("ggml_vk_create_queue()");
    std::lock_guard<std::recursive_mutex> guard(device->mutex);

-    q.queue_family_index = queue_family_index;
-    q.transfer_only = transfer_only;
+    auto q = std::make_unique<vk_queue>();
+    q->queue_family_index = queue_family_index;
+    q->transfer_only = transfer_only;

-    q.cmd_pool.init(device, &q);
+    std::shared_ptr<vk_queue_handle> h;
+    vk::DeviceQueueInfo2 queue_info2{};
+    queue_info2.queueFamilyIndex = queue_family_index;
+    queue_info2.queueIndex = queue_index;

-    q.queue = device->device.getQueue(queue_family_index, queue_index);
+    if (device->has_internally_synchronized_queues) {
+        h = std::make_shared<vk_queue_handle_unsynchronized>();
+        queue_info2.flags = eInternallySynchronizedKHR;
+    } else {
+        h = std::make_shared<vk_queue_handle_synchronized>();
+    }

-    q.stage_flags = stage_flags;
+    h->queue = device->device.getQueue2(queue_info2);
+    q->handle = h;
+
+    q->cmd_pool.init(device, q.get());
+
+    q->stage_flags = stage_flags;
+    return q;
+}
+
+static std::unique_ptr<vk_queue> ggml_vk_create_aliased_queue(vk_device& device, const std::unique_ptr<vk_queue>& source) {
+    std::lock_guard<std::recursive_mutex> guard(device->mutex);
+    auto q = std::make_unique<vk_queue>();
+    q->handle = source->handle;
+    q->queue_family_index = source->queue_family_index;
+    q->stage_flags = source->stage_flags;
+    q->transfer_only = source->transfer_only;
+    q->cmd_pool.init(device, q.get());
+    return q;
 }

 static vk_context ggml_vk_create_context(ggml_backend_vk_context * ctx, vk_command_pool& p) {
@@ -3107,11 +3169,11 @@ static void ggml_vk_queue_command_pools_cleanup(vk_device& device) {
    // Arbitrary frequency to cleanup/reuse command buffers
    static constexpr uint32_t cleanup_frequency = 10;

-    if (device->compute_queue.cmd_pool.buffers_in_use() >= cleanup_frequency) {
-        ggml_vk_command_pool_cleanup(device, device->compute_queue.cmd_pool);
+    if (device->compute_queue->cmd_pool.buffers_in_use() >= cleanup_frequency) {
+        ggml_vk_command_pool_cleanup(device, device->compute_queue->cmd_pool);
    }
-    if (device->transfer_queue.cmd_pool.buffers_in_use() >= cleanup_frequency) {
-        ggml_vk_command_pool_cleanup(device, device->transfer_queue.cmd_pool);
+    if (device->transfer_queue->cmd_pool.buffers_in_use() >= cleanup_frequency) {
+        ggml_vk_command_pool_cleanup(device, device->transfer_queue->cmd_pool);
    }
 }

@@ -5886,6 +5948,7 @@ static vk_device ggml_vk_get_device(size_t idx) {
        bool coopmat2_support = false;
        bool coopmat2_decode_vector_support = false;
        bool pipeline_executable_properties_support = false;
+        bool internally_sync_support = false;
        device->coopmat_support = false;
        device->integer_dot_product = false;
        device->shader_64b_indexing = false;
@@ -5957,6 +6020,8 @@ static vk_device ggml_vk_get_device(size_t idx) {
            } else if (strcmp("VK_EXT_shader_64bit_indexing", properties.extensionName) == 0) {
                device->shader_64b_indexing = true;
 #endif
+            } else if (strcmp(VK_KHR_INTERNALLY_SYNCHRONIZED_QUEUES_EXTENSION_NAME, properties.extensionName) == 0) {
+                internally_sync_support = true;
            }
        }

@@ -6143,14 +6208,6 @@ static vk_device ggml_vk_get_device(size_t idx) {
        device->single_queue = compute_queue_family_index == transfer_queue_family_index && queue_family_props[compute_queue_family_index].queueCount == 1;

        std::vector<vk::DeviceQueueCreateInfo> device_queue_create_infos;
-        if (compute_queue_family_index != transfer_queue_family_index) {
-            device_queue_create_infos.push_back({vk::DeviceQueueCreateFlags(), compute_queue_family_index, 1, priorities});
-            device_queue_create_infos.push_back({vk::DeviceQueueCreateFlags(), transfer_queue_family_index, 1, priorities + 1});
-        } else if(!device->single_queue) {
-            device_queue_create_infos.push_back({vk::DeviceQueueCreateFlags(), compute_queue_family_index, 2, priorities});
-        } else {
-            device_queue_create_infos.push_back({vk::DeviceQueueCreateFlags(), compute_queue_family_index, 1, priorities});
-        }
        vk::DeviceCreateInfo device_create_info{};
        std::vector<const char *> device_extensions;
        vk::PhysicalDeviceFeatures device_features = device->physical_device.getFeatures();
@@ -6172,6 +6229,17 @@ static vk_device ggml_vk_get_device(size_t idx) {

        last_struct = (VkBaseOutStructure *)&vk12_features;

+        VkPhysicalDeviceInternallySynchronizedQueuesFeaturesKHR internally_synchronized_queues_features{};
+        internally_synchronized_queues_features.sType = VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_INTERNALLY_SYNCHRONIZED_QUEUES_FEATURES_KHR;
+        internally_synchronized_queues_features.pNext = nullptr;
+        internally_synchronized_queues_features.internallySynchronizedQueues = VK_FALSE;
+
+        if (internally_sync_support) {
+            last_struct->pNext = (VkBaseOutStructure *)&internally_synchronized_queues_features;
+            last_struct = (VkBaseOutStructure *)&internally_synchronized_queues_features;
+            device_extensions.push_back(VK_KHR_INTERNALLY_SYNCHRONIZED_QUEUES_EXTENSION_NAME);
+        }
+
        VkPhysicalDevicePipelineRobustnessFeaturesEXT pl_robustness_features;
        pl_robustness_features.pNext = nullptr;
        pl_robustness_features.sType = VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_PIPELINE_ROBUSTNESS_FEATURES_EXT;
@@ -6310,6 +6378,23 @@ static vk_device ggml_vk_get_device(size_t idx) {

        vkGetPhysicalDeviceFeatures2(device->physical_device, &device_features2);

+        device->has_internally_synchronized_queues = internally_synchronized_queues_features.internallySynchronizedQueues;
+
+        // Build queue create infos only after querying whether internally synchronized queues are enabled.
+        // getQueue2() later uses the same flag, so creation/retrieval must stay consistent.
+        vk::DeviceQueueCreateFlags queue_flags = device->has_internally_synchronized_queues ?
+                                                eInternallySynchronizedKHR :
+                                                vk::DeviceQueueCreateFlags();
+
+        if (compute_queue_family_index != transfer_queue_family_index) {
+            device_queue_create_infos.push_back({queue_flags, compute_queue_family_index, 1, priorities});
+            device_queue_create_infos.push_back({queue_flags, transfer_queue_family_index, 1, priorities + 1});
+        } else if(!device->single_queue) {
+            device_queue_create_infos.push_back({queue_flags, compute_queue_family_index, 2, priorities});
+        } else {
+            device_queue_create_infos.push_back({queue_flags, compute_queue_family_index, 1, priorities});
+        }
+
        device->pipeline_executable_properties_support = pipeline_executable_properties_support;

        device->fp16 = device->fp16 && vk12_features.shaderFloat16;
@@ -6592,7 +6677,7 @@ static vk_device ggml_vk_get_device(size_t idx) {
        device->device = device->physical_device.createDevice(device_create_info);

        // Queues
-        ggml_vk_create_queue(device, device->compute_queue, compute_queue_family_index, 0, { vk::PipelineStageFlagBits::eComputeShader | vk::PipelineStageFlagBits::eTransfer }, false);
+        device->compute_queue = ggml_vk_create_queue(device, compute_queue_family_index, 0, { vk::PipelineStageFlagBits::eComputeShader | vk::PipelineStageFlagBits::eTransfer }, false);

        // Shaders
        // Disable matmul tile sizes early if performance low or not supported
@@ -6694,13 +6779,11 @@ static vk_device ggml_vk_get_device(size_t idx) {

        if (!device->single_queue) {
            const uint32_t transfer_queue_index = compute_queue_family_index == transfer_queue_family_index ? 1 : 0;
-            ggml_vk_create_queue(device, device->transfer_queue, transfer_queue_family_index, transfer_queue_index, { vk::PipelineStageFlagBits::eTransfer }, true);
+            device->transfer_queue = ggml_vk_create_queue(device, transfer_queue_family_index, transfer_queue_index, { vk::PipelineStageFlagBits::eTransfer }, true);

            device->async_use_transfer_queue = prefers_transfer_queue || (getenv("GGML_VK_ASYNC_USE_TRANSFER_QUEUE") != nullptr);
        } else {
-            // TODO: Use pointer or reference to avoid copy
-            device->transfer_queue.copyFrom(device->compute_queue);
-            device->transfer_queue.cmd_pool.init(device, &device->transfer_queue);
+            device->transfer_queue = ggml_vk_create_aliased_queue(device, device->compute_queue);

            device->async_use_transfer_queue = false;
        }
@@ -7263,7 +7346,7 @@ static void ggml_vk_init(ggml_backend_vk_context * ctx, size_t idx) {
    ctx->fence = ctx->device->device.createFence({});
    ctx->almost_ready_fence = ctx->device->device.createFence({});

-    ctx->compute_cmd_pool.init(ctx->device, &ctx->device->compute_queue);
+    ctx->compute_cmd_pool.init(ctx->device, ctx->device->compute_queue.get());
    if (ctx->device->async_use_transfer_queue) {
        vk::SemaphoreTypeCreateInfo tci{ vk::SemaphoreType::eTimeline, 0 };
        vk::SemaphoreCreateInfo ci{};
@@ -7271,7 +7354,7 @@ static void ggml_vk_init(ggml_backend_vk_context * ctx, size_t idx) {
        ctx->transfer_semaphore.s = ctx->device->device.createSemaphore(ci);
        ctx->transfer_semaphore.value = 0;

-        ctx->transfer_cmd_pool.init(ctx->device, &ctx->device->transfer_queue);
+        ctx->transfer_cmd_pool.init(ctx->device, ctx->device->transfer_queue.get());
    }

    if (vk_perf_logger_enabled) {
@@ -8126,7 +8209,7 @@ static void ggml_vk_buffer_write_2d(vk_buffer& dst, size_t offset, const void *
    } else {
        std::lock_guard<std::recursive_mutex> guard(dst->device->mutex);

-        vk_context subctx = ggml_vk_create_temporary_context(dst->device->transfer_queue.cmd_pool);
+        vk_context subctx = ggml_vk_create_temporary_context(dst->device->transfer_queue->cmd_pool);
        ggml_vk_ctx_begin(dst->device, subctx);
        bool ret = ggml_vk_buffer_write_2d_async(subctx, dst, offset, src, spitch, dpitch, width, height, true);
        GGML_ASSERT(ret);
@@ -8241,7 +8324,7 @@ static void ggml_vk_buffer_read_2d(vk_buffer& src, size_t offset, void * dst, si
        GGML_ASSERT(src->memory_property_flags & vk::MemoryPropertyFlagBits::eHostCoherent);

        std::lock_guard<std::recursive_mutex> guard(src->device->mutex);
-        vk_context subctx = ggml_vk_create_temporary_context(src->device->compute_queue.cmd_pool);
+        vk_context subctx = ggml_vk_create_temporary_context(src->device->compute_queue->cmd_pool);
        ggml_vk_ctx_begin(src->device, subctx);
        subctx->s->buffer->buf.pipelineBarrier(
            vk::PipelineStageFlagBits::eComputeShader | vk::PipelineStageFlagBits::eTransfer,
@@ -8267,7 +8350,7 @@ static void ggml_vk_buffer_read_2d(vk_buffer& src, size_t offset, void * dst, si
    } else {
        std::lock_guard<std::recursive_mutex> guard(src->device->mutex);

-        vk_context subctx = ggml_vk_create_temporary_context(src->device->transfer_queue.cmd_pool);
+        vk_context subctx = ggml_vk_create_temporary_context(src->device->transfer_queue->cmd_pool);
        ggml_vk_ctx_begin(src->device, subctx);
        bool ret = ggml_vk_buffer_read_2d_async(subctx, src, offset, dst, spitch, dpitch, width, height, true);
        GGML_ASSERT(ret);
@@ -8304,7 +8387,7 @@ static void ggml_vk_buffer_copy(vk_buffer& dst, size_t dst_offset, vk_buffer& sr
        std::lock_guard<std::recursive_mutex> guard(src->device->mutex);
        VK_LOG_DEBUG("ggml_vk_buffer_copy(SINGLE_DEVICE, " << size << ")");
        // Copy within the device
-        vk_context subctx = ggml_vk_create_temporary_context(src->device->transfer_queue.cmd_pool);
+        vk_context subctx = ggml_vk_create_temporary_context(src->device->transfer_queue->cmd_pool);
        ggml_vk_ctx_begin(src->device, subctx);
        ggml_vk_buffer_copy_async(subctx, dst, dst_offset, src, src_offset, size);
        ggml_vk_ctx_end(subctx);
@@ -8347,7 +8430,7 @@ static void ggml_vk_buffer_memset(vk_buffer& dst, size_t offset, uint32_t c, siz
    }

    std::lock_guard<std::recursive_mutex> guard(dst->device->mutex);
-    vk_context subctx = ggml_vk_create_temporary_context(dst->device->transfer_queue.cmd_pool);
+    vk_context subctx = ggml_vk_create_temporary_context(dst->device->transfer_queue->cmd_pool);
    ggml_vk_ctx_begin(dst->device, subctx);
    subctx->s->buffer->buf.fillBuffer(dst->buffer, offset, size, c);
    ggml_vk_ctx_end(subctx);
@@ -15875,19 +15958,17 @@ static void ggml_vk_synchronize(ggml_backend_vk_context * ctx) {
                1, &ctx->transfer_semaphore.value,
                0, nullptr,
            };
-            vk::PipelineStageFlags stage = ctx->device->transfer_queue.stage_flags;
+            vk::PipelineStageFlags stage = ctx->device->transfer_queue->stage_flags;
            vk::SubmitInfo si{
                1, &ctx->transfer_semaphore.s, &stage,
                0, nullptr,
                0, nullptr,
            };
            si.setPNext(&tl_info);
-            std::lock_guard<std::mutex> guard(queue_mutex);
-            ctx->device->compute_queue.queue.submit({ si }, ctx->fence);
+            ctx->device->compute_queue->handle->submit({ si }, ctx->fence);
            ctx->transfer_semaphore_last_submitted = ctx->transfer_semaphore.value;
        } else {
-            std::lock_guard<std::mutex> guard(queue_mutex);
-            ctx->device->compute_queue.queue.submit({}, ctx->fence);
+            ctx->device->compute_queue->handle->submit({}, ctx->fence);
        }
        ggml_vk_wait_for_fence(ctx);
        ctx->submit_pending = false;
@@ -16449,7 +16530,9 @@ static ggml_status ggml_backend_vk_graph_compute(ggml_backend_t backend, ggml_cg
        vk::DebugUtilsLabelEXT dul = {};
        dul.pLabelName = "ggml_backend_vk_graph_compute";
        dul.color = std::array<float,4>{1.0f, 1.0f, 1.0f, 1.0f};
-        vk_instance.pfn_vkQueueBeginDebugUtilsLabelEXT(ctx->device->compute_queue.queue, reinterpret_cast<VkDebugUtilsLabelEXT*>(&dul));
+
+        std::lock_guard<vk_queue_handle> guard(*ctx->device->compute_queue->handle);
+        vk_instance.pfn_vkQueueBeginDebugUtilsLabelEXT(ctx->device->compute_queue->handle->queue, reinterpret_cast<VkDebugUtilsLabelEXT*>(&dul));
    }

    ctx->prealloc_size_add_rms_partials_offset = 0;
@@ -1152,6 +1152,11 @@ private:
            return false;
        }

+        if (ctx_tgt == nullptr) {
+            SRV_ERR("failed to create_context with model '%s'\n", params_base.model.path.c_str());
+            return false;
+        }
+
        vocab = llama_model_get_vocab(model_tgt);

        n_ctx = llama_n_ctx(ctx_tgt);
@@ -632,6 +632,13 @@ void server_res_spipe::on_complete() {
    if (!spipe || next_finished) {
        return;
    }
+    // an empty next_orig means set_next() never ran: the request failed before streaming
+    // started, typically a params validation throw. evict the session installed by set_req()
+    // so the failed request leaves nothing behind for discovery or replay
+    if (!next_orig) {
+        g_stream_sessions.evict(server_stream_conv_id_from_headers(req->headers));
+        return;
+    }
    std::string chunk;
    while (!spipe->is_cancelled()) {
        chunk.clear();
@@ -2428,7 +2428,8 @@ class ChatStore {

 		if (currentConfig.samplers) apiOptions.samplers = currentConfig.samplers;

-		apiOptions.backend_sampling = currentConfig.backend_sampling;
+		if (hasValue(currentConfig.backend_sampling))
+			apiOptions.backend_sampling = currentConfig.backend_sampling;

 		if (currentConfig.customJson) apiOptions.custom = currentConfig.customJson;