Text generation and streaming

llamadart exposes three generation entry points:

• engine.generate(prompt) for raw prompt strings.
• engine.create(messages) for stateless, chat-template aware completions.
• ChatSession.create(parts) for stateful, multi-turn chat with automatic history management.

Choosing the right API#

For one-shot instructions, prefer engine.create(...): it applies the chat template without session state, and a follow-up turn sees only the messages you pass again. For chat apps, prefer ChatSession unless your app already stores the transcript. session.addMessage(...) restores history or inserts tool results, and session.reset() starts over. See First Chat Session for a multi-turn example.

Generation pipeline (visual)#

sequenceDiagram
    autonumber
    participant App as App/ChatSession
    participant Engine as LlamaEngine
    participant Template as Template engine
    participant Backend as Native/Web backend
    participant Parser as Stream parser

    App->>Engine: generate(prompt), create(messages), or ChatSession.create(parts)
    alt ChatSession.create(parts)
        App->>App: append turn to session history
        App->>Engine: create(full session messages)
    end
    alt create(messages)
        Engine->>Template: detect format + render template
        Template-->>Engine: prompt + stops + grammar
    end

    Engine->>Backend: start generation
    loop token stream
        Backend-->>Engine: token bytes
        Engine->>Parser: UTF-8 decode + partial parse
        Parser-->>App: streaming chunk delta
    end

    Backend-->>Engine: generation finished
    Engine->>Parser: finalize parse
    Parser-->>App: final chunk (finish reason/tool calls)

Low-level generation API#

await for (final token in engine.generate(
  'List two advantages of local LLM inference.',
  params: const GenerationParams(maxTokens: 64, temp: 0.4),
)) {
  print(token);
}

Chat completion API#

final messages = [
  LlamaChatMessage.fromText(
    role: LlamaChatRole.user,
    text: 'Explain top-p in plain language.',
  ),
];

await for (final chunk in engine.create(
  messages,
  params: const GenerationParams(maxTokens: 128, topP: 0.95),
)) {
  if (chunk.thinking.isNotEmpty) stdout.write('[thinking] ${chunk.thinking}');
  stdout.write(chunk.text);
}

chunk.text and chunk.thinking are the first choice's answer and reasoning deltas, or empty strings. chunk.toolCalls lists its tool-call deltas, and chunk.finishReason is a LlamaFinishReason (stop, length or toolCalls) on the final chunk and null on every other one. The raw OpenAI-style fields stay available under chunk.choices.

Collect a whole reply#

When you do not need to render tokens as they arrive, collect the stream:

// Just the answer text.
final answer = await engine.create(messages).text();

// Only the non-empty text deltas, for a sink that takes strings.
await engine.create(messages).textDeltas().forEach(stdout.write);

// Text, reasoning, assembled tool calls, finish reason and usage.
final completion = await engine.create(messages).collect();
if (completion.finishReason == LlamaFinishReason.length) {
  print('Reply was cut off at maxTokens.');
}
messages.add(completion.message); // Assistant turn for the next request.

engine.complete(messages, ...) is engine.create(messages, ...).collect(), and session.send('...') sends one text turn through a ChatSession and collects the reply.

finishReason does not tell you that a generation was cancelled. On native llama.cpp and LiteRT-LM, a stream stopped by engine.cancelGeneration(), before or during generation, usually still ends with LlamaFinishReason.stop and whatever text it produced. ChatSession instead throws LlamaStateException and rolls back its turn when cancelled before any reply content, including through createStructuredJson. On WebGPU, cancelGeneration() can instead fail the stream with a generation error. Track cancels in the code that issues them; on native llama.cpp and LiteRT-LM you can also read LlamaOperationResult.cancelled from an operation observer.

chunk.model is the last path segment of the source the model was loaded from, such as qwen.gguf: a local path's file name, or the last segment of a URL path without its query or fragment. A local file name is reported as written, % included. It is llama_model when that segment could carry more than a file name, and for data: and blob: URLs. It leaves out directories and hosts, but not the segment itself: a URL whose last segment is a token reports that token, unless it repeats the URL's userinfo credential, which reports llama_model. An OpenAI-compatible server that exposes its own model id should set that id on its responses instead.

Token usage and timings#

On native llama.cpp, and on WebGPU with bridge assets v0.1.54+, the final create chunk carries the request's usage whenever the backend reports it. The backend can report none, for example for a request cancelled while it is queued. Usage is null on LiteRT-LM, on older bridge assets and on every earlier chunk. On WebGPU, completionTokens can include tokens generated after a stop sequence, before the stop reached the bridge.

final usage = (await engine.complete(messages)).usage;
if (usage != null) {
  print('prompt ${usage.promptTokens} '
      '(cached ${usage.cachedPromptTokens}), '
      'completion ${usage.completionTokens}, '
      'first token ${usage.timeToFirstToken}, total ${usage.duration}');
}

The backend times timeToFirstToken and duration from when it starts the request. They exclude template rendering and time spent queued behind another request, and timeToFirstToken excludes stream batching.

Observing operations#

Pass observers to LlamaEngine to trace, measure or log its work. An observer sees chat completions (create, createStructuredJson and ChatSession.create), generate, embed, embedBatch and model loads.

final class TimingObserver extends LlamaEngineObserver {
  @override
  LlamaOperationObserver? onStart(LlamaOperation operation) {
    final name = switch (operation) {
      LlamaChatOperation() => 'chat',
      LlamaTextCompletionOperation() => 'text_completion',
      LlamaEmbeddingsOperation() => 'embeddings',
      LlamaModelLoadOperation() => 'model_load',
      _ => 'other',
    };
    return _Timing('$name ${operation.model}', Stopwatch()..start());
  }
}

final class _Timing extends LlamaOperationObserver {
  _Timing(this.name, this.watch);

  final String name;
  final Stopwatch watch;

  @override
  void onEnd(LlamaOperationResult result) {
    print('$name: ${watch.elapsed}, finish ${result.finishReason}, '
        'tokens ${result.usage?.totalTokens}');
  }
}

final engine = LlamaEngine(LlamaBackend(), observers: [TimingObserver()]);

• A create or generate operation starts when its stream is listened to; the others start when their method is called. Every callback runs in the zone that called the engine method, so a tracer can read its parent context there.
• onChunk receives each create chunk and onText each generate piece.
• onEnd runs once, with the error, the cancel, or the finish reason and usage. Usage is reported where the final create chunk carries it. A chat subscription cancelled after the final chunk ends completed, unless cancelGeneration stopped it first.
• LlamaOperation.model is the model's general.name metadata, or else the last segment of the path or URL it was loaded from. It is null when that segment is empty or contains one of / \ ? # @ ; & =, so the segment is never a directory path, URL query, fragment or userinfo, and for data: and blob: URLs. On the built-in backends, runtime is LlamaRuntime.llamaCpp or LlamaRuntime.liteRtLm for operations after a model load, and null for the load itself.
• Operations carry copies of the prompts and messages. Record them only when your users opt in.
• Extend the observer classes rather than implementing them, and give a switch over operations a default case: later versions may add callbacks and operation types.
• An exception an observer throws is reported to the library logger as a warning and never reaches the caller. Without observers the engine does no observation work.

For a runnable observer-to-OpenTelemetry adapter and Langfuse/Grafana recipes, see Observability.

Thinking budget (native llama.cpp)#

For GGUF models with a thinking channel, ThinkingBudget maps to llama.cpp's reasoning-budget sampler. It counts generated tokens inside each reasoning block independently of maxTokens, which remains the cap for the entire completion.

await for (final chunk in engine.create(
  messages,
  enableThinking: true,
  params: const GenerationParams(
    maxTokens: 512,
    thinkingBudget: ThinkingBudget(maxTokens: 128),
  ),
)) {
  // Render chunk.thinking and chunk.text independently.
}

engine.create(...) fills the start and end delimiters from recognized chat templates. Raw engine.generate(...) calls must provide both delimiters in ThinkingBudget. A budget of 0 immediately forces the end delimiter. This is supported by native llama.cpp text generation only; LiteRT-LM and WebGPU reject it explicitly, and it cannot be combined with speculative decoding.

Structured JSON output#

Use LlamaStructuredOutput when you want strict JSON plus final validation and typed decoding. The helper builds the responseFormat map for grammar-capable backends and validates the completed model output before returning your value.

class TicketClassification {
  TicketClassification({required this.priority, required this.category});

  final String priority;
  final String category;

  static TicketClassification fromJson(Map<String, dynamic> json) {
    return TicketClassification(
      priority: json['priority'] as String,
      category: json['category'] as String,
    );
  }
}

final output = LlamaStructuredOutput<TicketClassification>.jsonSchema(
  schema: const {
    'type': 'object',
    'properties': {
      'priority': {
        'type': 'string',
        'enum': ['low', 'medium', 'high'],
      },
      'category': {'type': 'string'},
    },
    'required': ['priority', 'category'],
    'additionalProperties': false,
  },
  decoder: TicketClassification.fromJson,
);

final classification = await engine.createStructuredJson(
  [
    LlamaChatMessage.fromText(
      role: LlamaChatRole.user,
      text: 'Classify this ticket: checkout fails with card declined.',
    ),
  ],
  output: output,
  params: const GenerationParams(maxTokens: 96, temp: 0),
);

Without the helper, pass responseFormat: {'type': 'json_object'} or {'type': 'json_schema', 'json_schema': {'schema': <JSON schema>}} to engine.create(...); json_schema may also carry name, description and strict, and {'type': 'text'} requests unconstrained text. A key whose value is null counts as absent. Any other type or key, such as a misspelled json_shema or schma, throws LlamaUnsupportedException before generation on every backend.

For live rendering, keep the stream returned by engine.create(..., responseFormat: output.responseFormat) and finalize it with await stream.parseStructuredJson(output). Validation is a final-output step because partial stream chunks are often not valid JSON yet.

Supported schema features match the built-in JSON-schema-to-GBNF subset: primitive types, objects with properties, required, and additionalProperties, arrays with items or fixed prefixItems, enum/const, local $ref, anyOf, oneOf, allOf, minLength, maxLength, minItems, and maxItems. Unsupported schemas fail before generation. Annotation metadata such as title, description, and default is preserved but not enforced as a decoding constraint. Backends without grammar constraints, including current LiteRT-LM native and web paths, still fail early for strict structured output.

ChatSession takes the same responseFormat on session.create(...) and has session.createStructuredJson(parts, output: output) for multi-turn structured output; the JSON reply is kept in the session history like any other turn. An unrecognised format, or a strict one on a backend without grammar constraints, throws before the user message joins the history. A request that fails or is cancelled before any reply content takes back its own user message (and any turns its context trimming dropped, if the history is otherwise unchanged), so a retry does not repeat the user turn. Empty terminal chunks do not count as reply content. Generation cancellation before content throws LlamaStateException; structured JSON helpers preserve that error instead of trying to parse an empty reply. After content, the partial reply is kept as an assistant turn, unless the session was reset or its initiating message was removed. A model change during draft-model resolution throws LlamaStateException and rolls back the turn. A history edit while the context is being prepared also throws LlamaStateException, preserving the changed conversation instead of trimming it with stale offsets.

create(...) flow at a glance#

1. Build your List<LlamaChatMessage>.
2. engine.create(...) runs template rendering/parity logic.
3. Effective stop sequences and grammar are applied to generation params.
4. Backend token bytes are decoded and emitted as streaming chunks.
5. Final parse resolves tool calls and stop reason.

Cancellation#

engine.cancelGeneration();

This cancels every create, generate and ChatSession.create stream that has been listened to, including one still rendering its template or checking its input: that stream ends without generating. On native llama.cpp and LiteRT-LM the stream ends normally, and its final chunk's finishReason is usually stop, so it does not mark the cancel. ChatSession throws LlamaStateException if cancelled before reply content and rolls back the turn; it keeps a partial reply only while its original turn remains in history. On WebGPU, the cancel can instead surface as a generation error on the stream. A stream listened to after the call is not affected. How quickly a running generation stops depends on the backend.

Cancelling a stream's subscription also sends the cancel to its backend at once, even before the first token.

On native llama.cpp, a cancel during text prompt evaluation takes effect at the next prompt micro-batch (ModelParams.microBatchSize tokens), or at the next batch (ModelParams.batchSize tokens) with speculative decoding. A generation started while a cancelled one is still stopping waits for it to stop, then runs. Starting one while another is running and not cancelled throws LlamaStateException.

Tokenization helpers#

final tokens = await engine.tokenize('hello world');
final text = await engine.detokenize(tokens);
final count = await engine.getTokenCount('hello world');

These helpers are useful for context budgeting and prompt diagnostics.

Next-token scores#

engine.scoreNextToken(...) evaluates a prompt and returns the log-probabilities of the token that would follow it, without generating. Ask for specific token ids with candidates, the most probable tokens with topK, or both. Reading the probabilities of answer letters turns an instruction-tuned model into a classifier:

final prompt = (await engine.chatTemplate([
  const LlamaChatMessage.fromText(
    role: LlamaChatRole.user,
    text: 'Is "remind me to call mom at 5" a (A) reminder or (B) search? '
        'Answer with the letter only.',
  ),
], enableThinking: false)).prompt;
final letters = [
  for (final letter in ['A', 'B'])
    (await engine.tokenize(letter, addSpecial: false)).single,
];

final scores = await engine.scoreNextToken(prompt, candidates: letters);
final probabilities = [for (final t in scores.candidates) t.probability];

The values are a softmax over the raw logits at the last prompt position, the same as llama-server's n_probs; sampling settings do not apply. The prompt is tokenized like a generate prompt, and a prefix shared with the previous prompt is reused unless reusePromptPrefix is false. Check engine.supportsNextTokenScoring first: native llama.cpp and WebGPU bridge assets v0.1.52+ support it; LiteRT-LM and older bridge assets report false and throw LlamaUnsupportedException.