llama.cpp C API Guide

Comprehensive reference for the llama.cpp C API, documenting all non-deprecated functions and common usage patterns.

Overview

llama.cpp is a C/C++ implementation for LLM inference with minimal dependencies and state-of-the-art performance. This skill provides:

• Complete API Reference: All non-deprecated functions organized by category
• Common Workflows: Working examples for typical use cases
• Best Practices: Patterns for efficient and correct API usage

Quick Start

See references/workflows.md for complete working examples. Basic workflow:

1. llama_backend_init() - Initialize backend
2. llama_model_load_from_file() - Load model
3. llama_init_from_model() - Create context
4. llama_tokenize() - Convert text to tokens
5. llama_decode() - Process tokens
6. llama_sampler_sample() - Sample next token
7. Cleanup in reverse order

When to Use This Skill

Use this skill when:

1. API Lookup: You need to find a specific function (e.g., "How do I load a model?", "What function creates a context?")
2. Code Generation: You're writing C code that uses llama.cpp
3. Workflow Guidance: You need to understand the steps for a task (e.g., text generation, embeddings, chat)
4. Advanced Features: You're working with batches, sequences, LoRA adapters, state management, or custom sampling
5. Migration: You're updating code from deprecated functions to current API

Core Concepts

Key Objects

• llama_model: Loaded model weights and architecture
• llama_context: Inference state (KV cache, compute buffers)
• llama_batch: Input tokens and positions for processing
• llama_batch_ext: Opaque extended batch builder, processed by llama_process() (b11284+)
• llama_sampler: Token sampling configuration
• llama_vocab: Vocabulary and tokenizer
• llama_memory_t: KV cache memory handle

Typical Flow

1. Initialize: llama_backend_init()
2. Load Model: llama_model_load_from_file()
3. Create Context: llama_init_from_model()
4. Tokenize: llama_tokenize()
5. Process: llama_encode() or llama_decode()
6. Sample: llama_sampler_sample()
7. Generate: Repeat steps 5-6
8. Cleanup: Free in reverse order

API Reference

For detailed API documentation, the complete API is split across 6 files for efficient targeted loading. Start with references/api-core.md which links to all other sections.

API Files:

• api-core.md (373 lines) - Initialization, parameters, model loading, quantization structs
• api-model-info.md (241 lines) - Model properties, architecture detection, metadata enums
• api-context.md (423 lines) - Context, memory (KV cache), state management
• api-inference.md (507 lines) - Batch operations (incl. extended batch API), inference, tokenization, chat
• api-sampling.md (524 lines) - All 20+ sampling strategies (incl. adaptive-p) + backend sampling API
• api-advanced.md (426 lines) - LoRA adapters, performance, training, constants, C++ RAII wrappers

Total: 219 active functions (b11284) across 6 organized files

Quick Function Lookup

Most common: llama_backend_init(), llama_model_load_from_file(), llama_init_from_model(), llama_tokenize(), llama_decode(), llama_sampler_sample(), llama_vocab_is_eog(), llama_memory_clear()

Extended batch (b11284+): llama_batch_ext_init(), llama_batch_ext_add_token(), llama_batch_ext_set_pos(), llama_batch_ext_set_output_logits(), llama_process()

See references/api-core.md for the full API index linking to all function signatures.

Common Workflows

See references/workflows.md for 16 complete working examples: basic text generation, chat, embeddings, batch processing, multi-sequence, LoRA, state save/load, custom sampling (XTC/DRY), encoder-decoder models, model detection, memory management patterns, streaming, and the extended batch API (llama_process).

Best Practices

See references/workflows.md for detailed best practices. Key points:

• Always use default parameter functions (llama_model_default_params(), etc.)
• Check return values for errors
• Free resources in reverse order of creation
• Handle dynamic buffer sizes for tokenization
• Query actual context size after creation (llama_n_ctx())
• Check for end-of-generation with llama_vocab_is_eog()

Common Patterns

End-of-generation check (llama_vocab_is_eog()), logits retrieval (llama_get_logits_ith()), batch creation (llama_batch_get_one()), tokenization buffer handling. See references/workflows.md for complete code examples.

Troubleshooting

Common Issues

Model loading fails:

• Verify file path and GGUF format validity
• Check available RAM/VRAM for model size
• Reduce n_gpu_layers if GPU memory insufficient

Tokenization returns negative value:

• Buffer too small; reallocate with -n size and retry
• See tokenization pattern in Common Patterns

Decode/encode returns non-zero:

• Verify batch initialization (llama_batch_get_one() or llama_batch_init())
• Check context capacity (llama_n_ctx())
• Ensure positions within context window

Silent failures / no output:

• Check if llama_vocab_is_eog() immediately returns true
• Verify sampler initialization
• Enable logging: llama_log_set()

Performance issues:

• Increase n_threads for CPU
• Set n_gpu_layers for GPU offloading
• Use larger n_batch for prompts
• See Performance & Utilities

Sliding Window Attention (SWA) issues:

• If using Mistral-style models with SWA, set ctx_params.swa_full = true to access beyond attention window
• Check: llama_model_n_swa(model) to detect SWA size and configuration needs
• Symptoms: Token positions beyond window size causing decode errors

Per-sequence state errors:

• Ensure sequence ID matches when loading: llama_state_seq_load_file(ctx, "file", dest_seq_id, ...)
• Verify token buffer is large enough for loaded tokens
• Check sequence wasn't cleared or removed before loading state

Model type detection:

• Use llama_model_has_encoder() before assuming decoder-only architecture
• For recurrent models (Mamba/RWKV), KV cache behavior differs from standard transformers
• Encoder-decoder models require llama_encode() then llama_decode() workflow

For advanced issues: https://github.com/ggerganov/llama.cpp/discussions

Resources

• API Reference (6 files, 2,494 lines total) - Complete API reference split by category for targeted loading:
  • api-core.md - Initialization, parameters, model loading, quantization structs
  • api-model-info.md - Model properties, architecture detection, metadata enums
  • api-context.md - Context, memory, state management
  • api-inference.md - Batch (incl. llama_batch_ext), inference, tokenization, chat
  • api-sampling.md - All 20+ sampling strategies (incl. adaptive-p) + backend sampling API
  • api-advanced.md - LoRA, performance, training, constants, C++ RAII (llama-cpp.h)
• references/workflows.md (1,706 lines) - 16 complete working examples: basic workflows (text generation, chat, embeddings, batching, sequences), intermediate (LoRA, state, sampling, encoder-decoder, memory), advanced features (XTC/DRY, per-sequence state, model detection), production applications (interactive chat, streaming), and the extended batch API.

What's New in b11284

b11284 (164 commits since b11120) — additive, non-breaking: 14 new functions, nothing removed, no signature changes.

• New extended batch API — opaque struct llama_batch_ext + llama_process(ctx, type, batch) with enum llama_process_type (LLAMA_PROCESS_TYPE_ENCODE/DECODE); return codes match llama_decode(). Builder: llama_batch_ext_init(ctx)/_free/_clear, _add(batch, seq_id)/_add_token/_add_embd (return batch index, or -1 full / -2 invalid token / -3 invalid seq id), _add_seq, _set_embd_token, _set_embd_state (stub), _set_output_embd/_set_output_logits (equivalent for now), _set_pos (caller must set positions; M-RoPE embedding entries take multiple positions). New struct llama_embd { data, n_rows, n_embd }. batch_ext get_logits/get_embeddings are TODO — read outputs with llama_get_logits_ith(ctx, idx). llama_decode() now converts llama_batch to this internally. See api-inference.md and workflow 16.
• C++: llama_batch_ext_ptr (+ llama_batch_ext_deleter) in llama-cpp.h.
• New: llama_get_causal_attn(ctx) — getter paired with llama_set_causal_attn().
• Heads-up: llama_numa_init() marked "TODO: deprecate and make part of llama_backend_init()" (not deprecated yet).

Previously (b11120, 252 commits since b10868) — small, non-breaking C API changes:

• New: llama_adapter_lora_init_from_file_ptr(model, FILE *) — load a LoRA adapter from an open FILE* at its current position (mirrors llama_model_load_from_file_ptr). See api-advanced.md.
• GGUF-in-a-larger-file: llama_model_load_from_file_ptr() reads from the current file position; the GGUF data section is now aligned relative to the GGUF start. For mmap, its absolute offset must be 32-byte aligned.
• llama_sampler_chain_n() now returns int32_t (was int) — ABI-identical on all mainstream platforms.
• New LLAMA_VOCAB_TYPE_TEST = 7 (dummy tokenizer for tests).

Previously (b10868, 203 commits since b10665) — no functions added or removed; the only public C API change is a rename:

BREAKING (rename, not behavior):

• enum llama_tensor_read_lazy → enum llama_lazy_mode; its constants LLAMA_TENSOR_READ_LAZY_OFF/AUTO/ON → LLAMA_LAZY_MODE_OFF/AUTO/ON; and llama_model_params.tensor_read_lazy → llama_model_params.lazy_mode. Behavior is unchanged — update call sites setting the old field/enum names. See api-core.md.

Previously (b10665, 249 commits since b10416) — no functions added, removed, or re-signatured. The public C API changed only in struct fields, one new enum, and the state-file versions:

BREAKING (data, not code):

• LLAMA_SESSION_VERSION 9 → 10 and LLAMA_STATE_SEQ_VERSION 2 → 3 (recurrent-state rollback in ggml_ssm_scan changed the serialized layout). Session/sequence-state files from older builds are rejected — llama_state_load_file() / llama_state_seq_load_file() fail on them. Regenerate cached sessions; check the return value and fall back to re-ingesting the prompt.

New lazy tensor reading (renamed in b10868, see above):

• Added enum llama_tensor_read_lazy (LLAMA_TENSOR_READ_LAZY_OFF/AUTO/ON) + llama_model_params.tensor_read_lazy. Faults in rows of arch-marked tensors on demand instead of reading them whole at load — cuts resident memory and load latency for models with very large sparsely-used tensors. AUTO applies only to marked tensors > 4 GiB; both AUTO and ON require an mmap load mode. See api-core.md.

New quantizer memory cap:

• llama_model_quantize_params.max_buf_size (size_t) — max bytes of tensor rows held in memory at once, 0 = default (8 GiB). Lower it to quantize very large models on memory-constrained machines.

Previously (b10416, 158 commits since b10258) — multi-output backend sampling, versioning, and a DRY sampler signature change:

BREAKING:

• llama_sampler_init_dry() no longer takes the int32_t n_ctx_train parameter (previously the 2nd argument, right after vocab). Update all call sites — see api-sampling.md.

New multi-output backend sampling [EXPERIMENTAL]:

• llama_context_params.n_outputs_max_per_seq (uint32_t) — max sampled outputs per sequence in a ubatch (0 = n_outputs_max).
• llama_sampler_i.backend_init() gained a uint32_t n_outputs_max_per_seq parameter; new backend_reset() / copy_state() callbacks for authors of custom backend samplers.
• llama_sampler_copy() — copy mutable sampler state between two same-type/config samplers without losing the destination's bound compute graph.
• llama_get_sampled_token_ith(): with multiple outputs, sampler state now advances on token acceptance, not on read; accept a contiguous prefix in output order (no gaps).

New load-mode default:

• LLAMA_LOAD_MODE_AUTO (-1) is the new default for llama_model_params.load_mode (was LLAMA_LOAD_MODE_MMAP). Auto-detects based on device capabilities — e.g. avoids mmap on iGPUs.

New:

• llama_version() — returns the llama.cpp library version string (from CMake's new semantic-versioning setup).

Doc correction (no code change): llama_sampler_init_penalties()'s penalty_last_n doc previously said "-1 = context size"; that was never accurate — negative values are clamped to 0 (disabled). History-based samplers (DRY, penalties) no longer resolve a "full-context window" from training context size, since backend sampling constructs samplers before a llama_context (and its resolved context length) exists.

Not in llama.h/llama-cpp.h, mentioned for awareness: mtmd gained Qwen3-TTS support — a breaking change to the llama-tts CLI binary (outside this skill's C API scope).

Previously (b10258, 183 commits since b10075):

BREAKING:

• llama_sampler_init_penalties() gained a new required first parameter n_vocab (source it via llama_vocab_n_tokens(vocab)). Update all call sites.

New load-mode API (replaces three booleans):

• enum llama_load_mode (LLAMA_LOAD_MODE_NONE/MMAP/MLOCK/MMAP_MLOCK/DIRECT_IO) + llama_load_mode_name() / llama_load_mode_from_str().
• llama_model_params.load_mode replaces the removed use_mmap, use_direct_io, and use_mlock boolean fields.
• llama_model_params.load_mtp (bool) — whether to load MTP layers.

New vocab function:

• llama_vocab_get_suppress_tokens() — model-specific suppress tokens (gguf key tokenizer.ggml.suppress_tokens).

Previously (b10075, ~205 commits since b9870): touched the public C API in exactly one place — new LLAMA_FTYPE_MOSTLY_Q2_0 = 41 quantization type (CPU backend), PR #24448. Everything else in that range (new Hy3/hy_v3 model + MTP speculative decoding, server reasoning_budget_tokens, mtmd NUL-truncation fix, deepseek-ocr v1 multi-tile, /responses streaming timings, etc.) was internal/server-side and did not change llama.h.

Recent (added in b9859→b9870, PR #25134):

• llama_model_ftype() — returns the model's file type as an enum llama_ftype (e.g. LLAMA_FTYPE_MOSTLY_Q8_0).
• llama_ftype_name() — converts an enum llama_ftype to a human-readable string (e.g. "Q8_0", "Q4_K - Medium"). Pair the two to display a loaded model's quantization.

Recent (added in b9840, from b9704):

• llama_model_n_layer_nextn() — returns the number of NextN (Multi-Token Prediction / MTP) layers in the model. These speculative next-token prediction layers power MTP-capable architectures such as DeepSeek V3/V4, GLM, Qwen3.5-MoE, and Step3.5. Returns 0 for non-MTP models. The total layer count equals llama_model_n_layer() (effective layers) + this value.

Recent (added in b9704) [EXPERIMENTAL]:

• llama_context_params.n_outputs_max (uint32_t) — max outputs in a ubatch (0 = n_batch). Cap it to reserve less output VRAM when you read only a few logits/embeddings per batch (e.g. one output per sequence during generation).
• llama_context_params.ctx_other (struct llama_context *) — a source/target/parent context for sharing inference results or llama_memory (KV cache) between two contexts; used by MTP setups such as Gemma4 MTP.
• llama_set_warmup() deprecated — perform warmup runs manually instead. It changed graph topology with MoE models (causing extra reallocations) and will be removed in a future release.

Still recent (added in b9246) [EXPERIMENTAL]:

• Multi-Token Prediction: enum llama_context_type (LLAMA_CONTEXT_TYPE_DEFAULT/LLAMA_CONTEXT_TYPE_MTP) + llama_context_params.ctx_type
• Recurrent-state rollback (Mamba/RWKV): llama_context_params.n_rs_seq + llama_n_rs_seq(ctx)
• Sequence-state flags: LLAMA_STATE_SEQ_FLAGS_NONE (0), LLAMA_STATE_SEQ_FLAGS_ON_DEVICE (2)

Stable Since b8809:

• Model loading: llama_model_load_from_file_ptr(), llama_model_init_from_user()
• Quantization types: MXFP4_MOE (38), NVFP4 (39), Q1_0 (40)
• Split mode: LLAMA_SPLIT_MODE_TENSOR (3) for backend-agnostic tensor parallelism
• Backend sampling API (EXPERIMENTAL): GPU-accelerated sampling via context params
• Adaptive-P sampler: llama_sampler_init_adaptive_p()

Key Differences from Deprecated API

If you're updating old code:

• Use llama_model_load_from_file() instead of llama_load_model_from_file()
• Use llama_model_free() instead of llama_free_model()
• Use llama_init_from_model() instead of llama_new_context_with_model()
• Use llama_vocab_*() functions instead of llama_token_*()
• Use llama_state_*() functions instead of deprecated state functions
• Use llama_set_adapters_lora() instead of the removed llama_set_adapter_lora() for LoRA adapters
• Use llama_vocab_bos() instead of llama_vocab_cls() (CLS is equivalent to BOS)
• Use llama_sampler_init_grammar_lazy_patterns() instead of llama_sampler_init_grammar_lazy()
• Perform warmup runs manually instead of calling deprecated llama_set_warmup() (deprecated in b9704)

See the API reference for complete mappings.