---
license: mit
language:
- en
- pt
tags:
- llama.cpp
- gguf
- text-diffusion
- block-diffusion
- diffusion-language-model
- gemma
- openai-api
- server
- cpu-inference
- offline
pipeline_tag: text-generation
---
# ΩFFΣLLIα — diffusion-gemma-http
**A native, single-file, OpenAI-compatible HTTP server for DiffusionGemma block text-diffusion models (GGUF / llama.cpp).**
`diffusion-gemma-http.cpp` makes a block text-diffusion model behave like a regular `llama-server`: load the GGUF once, listen on a port, answer `/v1/chat/completions`. Everything — tokenization, chat template, the block-diffusion denoising loop, and the HTTP API — runs in a single C++ process on top of the llama.cpp diffusion fork. No Python, no external tokenizer files, no per-step IPC.
Validated end-to-end on **CPU-only consumer hardware** (AMD Ryzen 5 5625U, 8 GB shared UMA, Kali Linux), serving a **DiffusionGemma 26B-A4B MoE (NVFP4 GGUF)** to a local chat UI over `:8080`.
---
## Why this exists
Diffusion language models cannot be served by the standard `llama-server`: generation is not autoregressive token-by-token decoding, but iterative **denoising of a fixed-length canvas** (`[prompt | canvas]` bidirectional forwards, region-aware masks, self-conditioning). Until now the options were a raw logits server driven by an external Python loop, or a one-shot CLI. This tool closes the gap: a persistent HTTP server speaking the OpenAI Chat Completions protocol, with the entire diffusion decode loop in-process.
## Features
- **OpenAI-compatible API**: `POST /v1/chat/completions` (streaming via SSE and non-streaming), `GET /v1/models`, `GET /health`, `GET /`
- **Native tokenization** from the GGUF vocabulary (control tokens parsed atomically; no tokenizer.json needed)
- **Model chat template built in**: `<|turn>role\n … ` turns with `<|channel>thought … ` reasoning channels; thinking can be disabled per request (`"enable_thinking": false`, the default) by pre-filling an empty thought channel, or enabled globally with `--thinking`
- **Entropy-bound block-diffusion sampler** (see below), with all parameters read from GGUF metadata
- **Prompt-KV caching** (`DG_KVCACHE=1` / `--kvcache`): the prompt is prefilled once per block; each denoising step forwards only the canvas
- **Self-conditioning** across denoising steps (previous-step logits fed back from step 2 onward)
- **Multi-block generation**: when a block fills without an end-of-turn, it is appended to the prompt and a new canvas is denoised, until `max_tokens` or end of content
- **Streaming per block** with correct `finish_reason` (`stop` vs `length`)
- Memory-conscious: per-position statistics are computed in streaming passes over the logits — the probability matrix (`canvas × 262k vocab`) is never materialized
## Requirements
1. A **llama.cpp fork with the `diffusion-gemma` architecture** (the `DIFFUSION_GEMMA` model class providing `llama_diffusion_set_phase` / `llama_diffusion_set_sc`).
2. A **DiffusionGemma GGUF** carrying the diffusion metadata:
| GGUF key | Meaning |
|---|---|
| `diffusion.canvas_length` | Canvas size per block (required; the C++ graph splits `[prompt \| canvas]` on it) |
| `diffusion.eb_max_steps` | Max denoising steps per block |
| `diffusion.eb_t_min` / `diffusion.eb_t_max` | Temperature schedule (linear, `t_max → t_min`) |
| `diffusion.eb_entropy_bound` | Per-position entropy bound for locking |
| `diffusion.eb_stability_threshold` | Consecutive stable-argmax steps required to lock |
| `diffusion.eb_confidence_threshold` | Reference-decoder parameter (read, reported, not used by this sampler — see Limitations) |
3. The vendored headers already shipped with the llama.cpp tree: `vendor/cpp-httplib/httplib.h` (+ `httplib.cpp`) and `vendor/nlohmann/json.hpp`.
## Build
Place the file at `tools/diffusion-gemma-http/diffusion-gemma-http.cpp` inside the fork, then:
```cmake
# tools/diffusion-gemma-http/CMakeLists.txt
set(TARGET llama-diffusion-gemma-http)
add_executable(${TARGET}
diffusion-gemma-http.cpp
${CMAKE_SOURCE_DIR}/vendor/cpp-httplib/httplib.cpp)
target_include_directories(${TARGET} PRIVATE
${CMAKE_SOURCE_DIR}/vendor/cpp-httplib
${CMAKE_SOURCE_DIR}/vendor)
target_link_libraries(${TARGET} PRIVATE llama Threads::Threads)
target_compile_features(${TARGET} PRIVATE cxx_std_17)
install(TARGETS ${TARGET} RUNTIME)
```
```bash
echo 'add_subdirectory(diffusion-gemma-http)' >> tools/CMakeLists.txt
cmake -B build
cmake --build build --target llama-diffusion-gemma-http -j4
```
## Usage
```bash
DG_KVCACHE=1 ./build/bin/llama-diffusion-gemma-http \
-m model.gguf --port 8080 [--host 0.0.0.0] [-ngl N] [-c MAXTOK] [--thinking]
```
| Flag / env | Default | Description |
|---|---|---|
| `-m, --model` | — | GGUF path (positional also accepted) |
| `--port` / `--host` | `8080` / `0.0.0.0` | Bind address |
| `-c, --ctx` / `MAXTOK` | `2304` | Context budget = prompt + accumulated blocks. Non-causal forwards require the whole sequence in one ubatch, so the compute buffer scales with this — raise gradually on small-RAM machines |
| `-ngl` / `NGL` | `0` | GPU layers |
| `--kvcache` / `DG_KVCACHE=1` | off | Prompt-KV caching (strongly recommended on CPU) |
| `--thinking` / `DG_THINKING=1` | off | Enable the reasoning channel by default |
| `DG_MASK_ID` | auto (``) | Override the canvas mask token id |
| `FA=1` | off | Flash attention |
### API
```bash
curl -s http://127.0.0.1:8080/v1/chat/completions -H 'Content-Type: application/json' -d '{
"messages": [{"role": "user", "content": "Explique em duas frases o que é um número primo."}],
"max_tokens": 200,
"stream": false
}'
```
Any OpenAI-compatible client or front-end pointed at `http://host:8080` works unchanged. Per-request fields: `messages` (system merged into the first user turn), `max_tokens` / `max_completion_tokens`, `stream`, `enable_thinking`.
## The sampler
The decode loop implements an **entropy-bound block-diffusion sampler**:
1. The canvas starts fully masked. Each step runs one bidirectional forward and computes, per position, the best **real** token (mask excluded), its confidence, and the entropy of the mask-excluded distribution — in streaming passes, without materializing probabilities.
2. A position **locks** when its argmax has been stable for `stability_threshold` consecutive steps **and** its entropy is below `entropy_bound`.
3. A prediction of `` never locks: it means "not ready yet / end of content". Positions still masked when the model proposes nothing new for 2 consecutive steps signal **end of content** (the model's native length control).
4. Minimum progress per step is guaranteed by confidence ranking; **adjacent positions never lock in the same step**, and an **anti-echo guard** defers locking a token identical to an already-locked neighbor (legitimate repetition persists and passes; denoising echoes dissolve).
5. Temperature follows a linear `t_max → t_min` schedule; self-conditioning on the previous step's logits is active from step 2.
### Honest limitations
- This sampler is a **validated approximation**, not a byte-exact port of the reference entropy-bound decoder: in particular, `diffusion.eb_confidence_threshold` is read and reported but plays no role in the locking rule, whose reference semantics differ from a naive confidence cutoff. Residual artifacts of parallel unmasking (rare token echoes) are mitigated by the guards above but not formally eliminated.
- Single-flight inference: concurrent requests are serialized by a mutex.
- Diffusion on CPU is compute-heavy: every denoising step is a dense forward over the canvas (plus the prompt without KV caching). Expect minutes, not seconds, for long answers on laptop-class CPUs.
## Provenance
Developed iteratively against a live DiffusionGemma 26B-A4B (MoE, 30 layers, 262k vocab, canvas 256, Harmony-style `<|turn>`/`<|channel>` template) quantized to NVFP4 GGUF, debugged end-to-end from raw logits to a working chat UI. Part of the **ΩFFΣLLIα** local-first, zero-telemetry tooling line.
- Author: **Bruno Becker** — [huggingface.co/Brunobkr](https://huggingface.co/Brunobkr)
- Research: [doi.org/10.5281/zenodo.20026837](https://doi.org/10.5281/zenodo.20026837)
- Built on [llama.cpp](https://github.com/ggml-org/llama.cpp) (MIT) with a diffusion-gemma architecture fork; HTTP via [cpp-httplib](https://github.com/yhirose/cpp-httplib), JSON via [nlohmann/json](https://github.com/nlohmann/json).
## License
MIT, following the llama.cpp ecosystem it extends.