LLM Inference

How LucidPal streams tokens from llama.cpp to the SwiftUI UI.

Flow

User sends message
       ↓
ChatViewModel.sendMessage()
       ↓
Build system prompt (calendar context injected — see [System Prompt Builder](./system-prompt))
       ↓
llmService.generate() → AsyncThrowingStream<String>
       ↓
LlamaActor.generate() — serial actor, C FFI
       ↓
Token-by-token via AsyncThrowingStream
       ↓
applyStreamToken() — think/response split
       ↓
executeCalendarActions() — extract + execute JSON blocks
       ↓
Update messages array → SwiftUI re-renders

Token Streaming

LLMService.generate() returns an AsyncThrowingStream<String, Error> — each element is one token (~1 word fragment). ChatViewModel consumes this stream and applies each token live:

for try await token in llmService.generate(
    systemPrompt: systemPrompt,
    messages: historyMessages,
    thinkingEnabled: showThinking
) {
    guard let idx = messages.firstIndex(where: { $0.id == assistantID }) else { break }
    applyStreamToken(token, rawBuffer: &raw, thinkDone: &thinkDone, ...)
}

Token travel path (end-to-end)

llama.cpp C layer
  llama_sampler_sample()          ← samples next token ID from logits
  llama_token_to_piece()          ← converts token ID → raw [CChar] bytes
  decodeUTF8([CChar])             ← buffers incomplete UTF-8 sequences,
                                     flushes when valid string boundary found
  continuation.yield(str)         ← emits String into AsyncThrowingStream
        ↓
LLMService (actor bridge)
  for try await token in stream   ← re-yields each fragment upstream
        ↓
ChatViewModel.runGenerationLoop()
  applyStreamToken(token, ...)    ← routes token to think buffer or content
        ↓
@Published messages[idx].content  ← triggers SwiftUI diff
        ↓
MessageBubbleView / ThinkingDisclosure
  re-rendered on every token yield

decodeUTF8 is the only buffering step before the stream: it holds CChar bytes in a pending buffer until String(validatingUTF8:) succeeds, ensuring multi-byte characters (e.g. emoji, CJK) are never split across yields.

Stop Conditions

streamTokens (in LlamaActor+Generate.swift) exits the autoregressive loop on the first matching condition:

Condition	Check	Behaviour
EOS / EOG token	llama_vocab_is_eog(vocab, newTok)	Clean stop, stream finishes normally
maxNewTokens reached	currentCursor - startCur >= maxNew (default 768)	Clean stop
Context window full	currentCursor >= ctxLimit	Clean stop (last usable position)
Swift Task cancelled	Task.isCancelled checked each iteration	Breaks loop; partial content kept if non-empty
llama_decode failure	return value != 0	Throws LLMError.generateFailed
Timeout	withThrowingTaskGroup race in streamLLMResponse	Throws LLMError.timeout; partial content shown + notice appended

After any clean exit flushPending(continuation:) is called to emit any remaining buffered UTF-8 bytes before the stream is finished.

Action Block Detection

Action blocks are not parsed during streaming — they are extracted after generation completes, in finalizeResponse(). The pipeline is:

Generation stream ends
        ↓
finalizeResponse(assistantID:)
        ↓
  extractWebSearchQuery()     → if [WEB_SEARCH:{...}] found, re-generate with results
        ↓
  executeCalendarActions()    → [CALENDAR_ACTION:{...}]  → EventKit
        ↓
  executeNoteActions()        → [NOTE_ACTION:{...}]      → NotesStore
        ↓
  executeContactsSearch()     → [CONTACTS_SEARCH:{...}]  → ContactsService
        ↓
  executeHabitActions()       → [HABIT_ACTION:{...}]     → HabitStore
        ↓
  executeReminderActions()    → [REMINDER_ACTION:{...}]  → EventKit reminders
        ↓
messages[idx].content stripped of action tokens
messages[idx].calendarEventPreviews / notePreviews / … populated
        ↓
SwiftUI renders action result cards (CalendarEventCard, NoteCard, …)

Each execute* call receives the full final content string, scans for its action tag using a regex or string search, decodes the embedded JSON payload, executes the side-effect, and returns a cleaned content string with the raw action token removed.

Web search is special: if a [WEB_SEARCH:{...}] block is found, the assistant message is cleared and a second full generation pass runs with the search results injected as a user message, subject to the same timeout.

Thinking Mode (Qwen3.5 <think> Tags)

Qwen3.5 models emit a <think>...</think> block before answering. LucidPal handles this live:

<think>
The user wants to create an event...
</think>
Here's your event — tap confirm.

applyStreamToken() buffers the prefix and splits at </think>:

Buffer state	Action
Starts with <think> (no close yet)	Set isThinking = true, show in ThinkingDisclosure
</think> detected	Extract thinking text, reset to response mode
No <think> prefix	Treat entire output as response

applyStreamToken is called on every individual token during streaming. The rawBuffer accumulates the full output so far. Think-block detection happens live — isThinking and thinkingContent are updated on each token, so ThinkingDisclosure animates in real time while the model is still generating.

If the model emits text before any <think> tag (i.e. rawBuffer does not start with <think> and is not a prefix of it), thinkDone is set immediately and all subsequent tokens go directly to messages[idx].content.

Tokenize Path

LlamaActor+Tokenize.swift exposes three low-level helpers used exclusively within LlamaActor:

Function	Purpose	Called by
tokenize(text:addBOS:parseSpecial:vocab:)	Converts a prompt string → [llama_token] via llama_tokenize	generate() before prefill
tokenToPiece(token:vocab:)	Converts a sampled token ID → raw [CChar] bytes via llama_token_to_piece	streamTokens() each iteration
decodeUTF8([CChar])	Assembles valid UTF-8 strings from raw byte sequences, buffering incomplete multi-byte chars	streamTokens() after each tokenToPiece

tokenize is called once per generation turn to produce the full prompt token array, which is then truncated to contextSize - maxNewTokens if needed and passed to prefill. It is not called for every token during generation — only during the initial prompt encoding step.

For vision (mtmd) input, tokenisation is handled separately by mtmd_tokenize which interleaves text tokens with image embedding chunks; the tokenize helper is bypassed for that path.

Context Window

Context size is chosen based on device RAM at launch:

let ramGB = Int(ProcessInfo.processInfo.physicalMemory / 1_073_741_824)
let historyLimit = ramGB >= 6
    ? ChatConstants.largeHistoryLimit   // 50 messages ≈ 5 000 tokens
    : ChatConstants.smallHistoryLimit   // 20 messages ≈ 2 000 tokens

RAM	Context	History Limit
< 6 GB	4K tokens	20 messages
≥ 6 GB	8K tokens	50 messages

Sampler Configuration

Parameter	Value	Reason
Temperature	0.35	Low — reduces hallucinated JSON fields
Max new tokens	768	Prevents runaway generation

The sampler chain uses temperature + random distribution (llama_sampler_init_dist). No top-p/top-k sampler is applied.

LlamaActor

LlamaActor is a Swift actor wrapping llama.cpp's C API. All calls are serialized on a single actor executor — no concurrent inference, no data races on C pointers.

Dual-model architecture

LucidPal supports two model roles via the ModelType enum:

enum ModelType: Sendable {
    case text    // Qwen3.x text model
    case vision  // Qwen3.5 Vision 4B model (or integrated text+vision)
}

Each role has its own independent llama.cpp model/context/vocab/sampler pointers. A model can be loaded, unloaded, or swapped per role without affecting the other.

Model loading & unloading lifecycle

// Load a text model
try await actor.loadModel(at: path, contextSize: 8192, role: .text)

// Load a vision model — on low-RAM devices (<6 GB), the text model is
// automatically unloaded first to free memory
try await actor.loadModel(at: path, contextSize: 4096, role: .vision,
                          mmprojPath: mmprojPath)

// Unload a specific role
actor.unloadModel(role: .text)

// Unload all models
actor.unload()

On low-RAM devices (< 6 GB), loading a vision model when a text model is already active triggers an automatic unloadModel(role: .text) before loading.

Metal warm-up

After every model load, warmup(role:) decodes a single BOS token to pre-initialize Metal pipeline state, avoiding a cold-start latency spike on the first real generation.

Text-only generation

generate(prompt:role:continuation:) runs the full prefill → autoregressive decode loop:

1. Tokenize prompt via llama_tokenize
2. Truncate to contextSize - maxNewTokens if needed (keeps most-recent content)
3. Prefill: batch all prompt tokens → llama_decode
4. Stream: sample one token per step via llama_sampler_sample, yield via continuation.yield
5. Stop on EOS/EOG token, cancellation, or maxNewTokens (768) limit

Vision generation (mtmd / CLIP)

generateWithImages(prompt:imageDataList:role:continuation:) handles multimodal input:

1. Decode raw JPEG buffers into mtmd bitmaps (mtmd_helper_bitmap_init_from_buf)
2. Tokenize prompt + bitmaps together via mtmd_tokenize into input chunks
3. Evaluate chunks (text + image embeddings) via mtmd_helper_eval_chunks
4. Stream output tokens with the same streamTokens loop as text-only

If mtmdCtx is not available (no mmproj loaded), falls back to text-only generation automatically.

Integrated vision model

When a text model also handles vision (e.g. Qwen3.5 Vision 4B integrated), textModelSupportsVision = true. In this case generate(role: .vision) transparently uses the text model slot — no separate vision slot needed.

warning

LlamaActor requires a physical device — the llama.cpp Metal backend does not run in the Simulator. GPU layers are set to 0 in #if targetEnvironment(simulator).

Error Handling

Error	Source	Handling
LLMError.modelNotLoaded	LLMService.generate when model not ready	ChatViewModel shows error banner
CancellationError	User taps stop button	Partial content left visible, no error shown
Any other Error	llama.cpp runtime	Displayed in messages[idx].content