Runtime Parameters

Runtime behavior is primarily controlled by:

• ModelParams at model load time.
• GenerationParams per generation call.

For a strategy-focused walkthrough on how to change these knobs and what to measure, see Performance Tuning.

ModelParams essentials#

await engine.loadModel(
  '/path/to/model.gguf',
  modelParams: const ModelParams(
    contextSize: 4096,
    gpuLayers: ModelParams.maxGpuLayers,
    preferredBackend: GpuBackend.vulkan,
    splitMode: ModelSplitMode.layer,
    mainGpu: 0,
    numberOfThreads: 0,
    numberOfThreadsBatch: 0,
    batchSize: 0,
    microBatchSize: 0,
    maxParallelSequences: 1,
  ),
);

Important fields:

• contextSize: total context window.
• gpuLayers: number of layers offloaded to GPU.
• preferredBackend: backend preference (auto, vulkan, metal, etc).
• splitMode: model tensor distribution mode passed through to llama.cpp split_mode. Defaults to upstream layer behavior.
• mainGpu: primary GPU device index passed through to llama.cpp main_gpu. To select one GPU for the full model, use splitMode: ModelSplitMode.none with the desired mainGpu index.
• batchSize: context logical batch size (n_batch). When left at 0, llamadart uses the effective context size (n_ctx) on native and WebGPU backends, except for model-specific WebGPU safety tuning such as the bundled Qwen3.5-0.8B small-model preset.
• microBatchSize: context micro-batch size (n_ubatch). When left at 0, llamadart uses the resolved batchSize; explicit values are capped so n_ubatch <= n_batch <= n_ctx.
• maxParallelSequences: max sequence slots (n_seq_max) for parallel sequence workloads (for example, batched embeddings).
• chatTemplate: optional template override.
• preferMemory64 (web/WebGPU only): prefer the 64-bit (wasm64/mem64) bridge core. The default 32-bit core has a 4 GiB address-space limit, but large models need room for KV cache and intermediate buffers. null (default) lets llamadart decide from modelBytesHint using the current wasm32-safe ceiling (about 2 GiB of model bytes); true forces mem64; false forces wasm32. Ignored on non-web backends.
• modelBytesHint (web/WebGPU only): approximate model size in bytes, used to select the mem64 core up front instead of waiting for an out-of-memory retry. Ignored on non-web backends.
• liteRtLmActivationDataType, liteRtLmPrefillChunkSize, liteRtLmParallelFileSectionLoading, and liteRtLmDispatchLibDir: opt-in native LiteRT-LM .litertlm engine settings. Leave them unset to preserve runtime defaults; LiteRT-LM web rejects them as native-only.
• numberOfThreads: honored by native LiteRT-LM; 0 keeps automatic selection.
• loras: native LiteRT-LM accepts one default-scale initial text adapter at model load. Runtime LoRA control APIs, adapter stacking, and custom scales remain llama.cpp-only.

For runtime LoRA control (setLora, removeLora, clearLoras), see LoRA Adapters.

Embedding-oriented model params#

For high-throughput embedBatch(...), tune context batch fields together:

• Keep batchSize large enough for total tokens across your average batch.
• Set microBatchSize close to batchSize unless you need tighter memory bounds.
• Increase maxParallelSequences above 1 (for example 2, 4, 8) to enable true multi-sequence embedding batching.

See Embeddings for API usage and benchmark scripts.

GenerationParams essentials#

const params = GenerationParams(
  maxTokens: 512,
  temp: 0.7,
  topK: 40,
  topP: 0.9,
  minP: 0.0,
  penalty: 1.1,
  stopSequences: ['</s>'],
  speculativeDecoding: false,
  speculativeDecodingConfig: null,
);

Important fields:

• maxTokens: generation length cap.
• temp: randomness.
• topK, topP, minP: token filtering controls.
• penalty: repeat penalty.
• speculativeDecoding / speculativeDecodingConfig: opt-in backend-native speculative decoding. Native LiteRT-LM honors the legacy boolean flag. llama.cpp supports SpeculativeDecodingConfig.mtp(...) for compatible MTP GGUF models, including separate target/draft model pairs through draftModelPath. WebGPU and LiteRT-LM web reject speculative decoding until their speculative paths are implemented.
• seed: deterministic replay when set.
• grammar: constrained decoding with GBNF.

Practical tuning defaults#

• Deterministic extraction: lower temp (0.1-0.3) + explicit stops.
• General chat: temp around 0.6-0.9, topP around 0.9-0.95.
• Tool calling: stable temp and sufficient maxTokens for call payload.