vllm-omni/docs/contributing/model/adding_tts_model.md

# Adding a TTS Model

This guide walks through adding a new TTS model to vLLM-Omni. Two patterns are
supported:

- **Two-stage pipeline** (e.g. Qwen3-TTS, Fish Speech): an AR code-predictor stage
  feeds an audio decoder stage via the `async_chunk` framework. This is the standard
  pattern for maximum streaming performance.
- **Single-stage AR model** (e.g. MOSS-TTS-Nano): the model runs entirely inside one
  AR worker and streams audio chunks directly from its own `inference_stream()` generator.

Qwen3-TTS is used as the reference for the two-stage pattern. For the single-stage
pattern, refer to MOSS-TTS-Nano.

## Table of Contents

1. [Overview](#overview)
2. [Cross-Cutting Invariants](#cross-cutting-invariants)
3. [Directory Structure](#directory-structure)
4. [Step-by-Step Implementation](#step-by-step-implementation)
5. [Key Components](#key-components)
6. [Model Registration](#model-registration)
7. [Pipeline and Deploy Configuration](#pipeline-and-deploy-configuration)
8. [Stage Input Processors](#stage-input-processors)
9. [Online Serving Integration](#online-serving-integration)
10. [Single-Stage Models](#single-stage-models)
11. [Testing](#testing)
12. [Pre-commit and DCO](#pre-commit-and-dco)
13. [Summary](#summary)

## Cross-Cutting Invariants

These rules apply to every TTS model regardless of architecture (AR vs AR+diffusion,
single-stage vs two-stage, codec-based vs VAE-based). Each has surfaced as a silent
bug in a shipped PR — check them at the end of every phase, not just at the start.

**I1. Streaming output contract.** Pick one per-step semantics for `forward()` and
document it in the docstring:

- *Delta*: yield only new audio samples produced this step. Preferred — linear cost.
- *Cumulative*: re-decode from step 0 every call. O(N²); only acceptable when the
  codec exposes no streaming decode.

If you choose delta, audit the full chain: `forward()` returns the new chunk →
`_consolidate_multimodal_tensors()` in `vllm_omni/engine/output_processor.py`
concatenates the audio key into a single tensor at finish → streaming consumers
receive per-step chunks, offline consumers receive the concatenated tensor. A
mismatch (consolidator skips the key with `continue`, or consumers expect a list
but receive a tensor) is invisible in offline RTF benchmarks — users hear replays
or truncation only under live playback.

**I2. Multimodal output consumer hygiene.** `outputs[0].outputs[0].multimodal_output[key]`
can be `Tensor`, `list[Tensor]` (pre-consolidation snapshot), `np.ndarray`, or
scalar. In every test, example, and benchmark:

- Never write `dict.get("a") or dict.get("b")` on tensor values — Python evaluates
  the tensor's truthiness and raises `Boolean value of Tensor with more than one
  value is ambiguous`. Use explicit `if x is None` chains.
- Defensively handle the list form:
  `if isinstance(x, list): x = torch.cat([t.reshape(-1) for t in x], dim=0)`.
- Assert `shape` / `dtype` / `duration` explicitly — do not rely on truthiness for
  presence checks.

**I3. Hot-loop GPU discipline.** Inside any per-step model loop (AR decode,
diffusion solver, CFM Euler step, per-frame vocoder):

- No `tensor.item()`, `.cpu()`, or `.tolist()` — each triggers a GPU→CPU sync; a
  10-step × 60-frame × 4-op loop creates 2400 syncs per request.
- Prefer `dst.copy_(src)` over `dst.fill_(src.item())` for scalar-into-buffer writes.
- Whole-model `torch.compile(Model.forward, fullgraph=False)` usually outperforms
  per-submodule compile — fewer dispatch boundaries, larger fusion regions. Measure
  before choosing granularity.
- No Python control flow that depends on tensor values; use `torch.where` or masking.

Profile before optimizing.

**I4. Validation pyramid.** Offline RTF alone is necessary but not sufficient. A
new TTS model must pass all three levels:

| Layer | Catches | Tool |
| ------- | --------- | ------ |
| Offline RTF / duration | Throughput regressions, missing audio, wrong sample rate | `end2end.py`, pytest e2e |
| Browser streaming playback | Delta-vs-cumulative bugs, chunk boundary glitches, TTFP regressions | Gradio demo over `/v1/audio/speech?stream=true` |
| Concurrent requests | Per-request state leaks, codec window round-robin gaps | `max_num_seqs>1` smoke with 4+ parallel prompts |

**I5. Per-request state belongs to the request.** If the model caches anything
across `forward()` calls (streaming generators, codec buffers, sliding-window pads,
CUDA graph state), key it by `info.get("_omni_req_id")` and free the entry on
request finish. A shared buffer silently corrupts audio across concurrent requests —
the symptom is crosstalk or truncation under load, nothing in single-request tests.

## Overview

vLLM-Omni supports TTS models as multi-stage pipelines where each stage runs independently
and can be placed on different devices. Qwen3-TTS has two stages:

| Stage | Name                | Input           | Output                   |
| ----- | ------------------- | --------------- | ------------------------ |
| 0     | Code Predictor (AR) | Text tokens     | Discrete RVQ codec codes |
| 1     | Code2Wav (Decoder)  | RVQ codec codes | Audio waveform           |

Each stage is a separate model class configured independently via YAML. The two stages
are connected by the `async_chunk` framework, which enables inter-stage streaming for
low first-packet latency (see [Async Chunk Design](../../design/feature/async_chunk.md)).

### Without async_chunk (batch mode)

Stage 0 runs to completion before Stage 1 starts, resulting in long first-packet latency:

```mermaid
flowchart TB
    subgraph stage0["Stage 0: AR Code Predictor"]
        direction LR
        P[Prefill] --> D1[Decode 1]
        D1 --> D2[Decode 2]
        D2 --> Dots1["..."]
        Dots1 --> DN[Decode N]
    end

    subgraph stage1["Stage 1: Code2Wav"]
        direction LR
        DEC[Decode all codes at once]
    end

    stage0 -- "all N codes" --> stage1
    stage1 --> FPL["First Packet Latency = Stage 0 + Stage 1"]

    style stage0 fill:#dae8fc,stroke:#6c8ebf
    style stage1 fill:#f8d7c8,stroke:#d4856a
    style FPL fill:#e8f0fe,stroke:#3366CC,stroke-width:2px
```

### With async_chunk (streaming mode)

Stage 0 sends codec codes to Stage 1 every `chunk_size=25` tokens. Stage 1 begins decoding
immediately, reducing first-packet latency from the full AR time to just the first chunk:

```mermaid
flowchart TB
    subgraph stage0["Stage 0: Code Predictor (AR)"]
        direction LR
        P[Prefill] --> D1["Decode 1-25"]
        D1 --> D2["Decode 26-50"]
        D2 --> Dots1["..."]
        Dots1 --> DN["Decode N"]
    end

    subgraph stage1["Stage 1: Code2Wav"]
        direction LR
        C1["Chunk 1\n(25 frames)"] --> C2["Chunk 2\n(context + 25)"]
        C2 --> Dots2["..."]
        Dots2 --> CN["Final chunk"]
    end

    D1 -. "chunk 1 (25 codes)" .-> C1
    D2 -. "chunk 2 (context + 25)" .-> C2
    DN -. "final" .-> CN

    stage0 --> FPL["⏱ First Packet Latency = Prefill + 25 decode steps only"]

    style stage0 fill:#dae8fc,stroke:#6c8ebf
    style stage1 fill:#e8d4f8,stroke:#8a6cad
    style FPL fill:#e8f0fe,stroke:#3366CC,stroke-width:2px
```

Key parameters: `chunk_size=25`, `left_context_size=25` (validated defaults from Qwen3-TTS
and Qwen3-Omni).

## Directory Structure

When adding a new TTS model, create the following structure:

```text
vllm_omni/model_executor/models/
  your_model_name/
    __init__.py
    your_model.py                    # Unified class (stage dispatch)
    pipeline.py                      # Pipeline topology registry entry
    your_model_ar_stage.py           # Stage 0: AR stage
    your_model_decoder.py            # Stage 1: audio decoder

vllm_omni/model_executor/stage_input_processors/
  your_model_name.py                 # Stage 0 -> Stage 1 transition

vllm_omni/deploy/
  your_model_name.yaml               # Batch/deploy config
  your_model_name_async_chunk.yaml   # Streaming/deploy config
```

### Example placement

TTS examples live in the consolidated text-to-speech hub, **not** in their
own top-level directory. Place per-model scripts under
`examples/offline_inference/text_to_speech/<your_model>/` and
`examples/online_serving/text_to_speech/<your_model>/`, and add a section
to the hub `README.md` files (table row + per-model section) instead of a
new per-model `README.md`. The mkdocs `generate_examples` hook treats the
`text_to_speech/` parent as a single example, so per-model READMEs inside
it would not be picked up — the hub README is the documented surface.

**Qwen3-TTS reference files:**

| File | Purpose |
| ------ | --------- |
| `models/qwen3_tts/qwen3_tts.py` | Unified model class |
| `models/qwen3_tts/qwen3_tts_code_predictor_vllm.py` | Stage 0 - optimized AR |
| `models/qwen3_tts/qwen3_tts_code2wav.py` | Stage 1 - decoder |
| `deploy/qwen3_tts.yaml` (new schema) | Deploy config (async_chunk enabled) — paired with `models/qwen3_tts/pipeline.py` for the frozen topology |

> **Chunked vs end-to-end modes**: `qwen3_tts` registers a single
> pipeline whose stage 1 declares alternate processor functions — an
> `async_chunk_process_next_stage_input_func` (per-chunk streaming, used
> when `deploy.async_chunk=True`) and a `sync_process_input_func`
> (batch-end, used when `deploy.async_chunk=False`). The loader selects
> one at merge time based on the bool, so `--no-async-chunk` alone
> switches modes — no variant yaml or variant pipeline registration is
> needed. Pipelines that only make sense in one mode (e.g.
> `qwen3_omni_moe` is always chunked) can keep using the unconditional
> `custom_process_*` fields.
| `stage_input_processors/qwen3_tts.py` | Stage transition processors |

## Step-by-Step Implementation

### Step 1: Implement Stage 0 - AR Stage

Stage 0 is the autoregressive stage that generates intermediate audio representations.
**It must use vLLM's native decoder layers with fused ops and PagedAttention** for the LLM
backbone - this is the primary source of speedup over HuggingFace inference.

#### 1.1 Use vLLM Decoder Layers Directly

Build your transformer layers from the corresponding vLLM decoder layer class (e.g.
`Qwen3DecoderLayer` for Qwen3-based backbones, or the equivalent for LLaMA, Qwen2, etc.).
Do not wrap the HuggingFace model directly - that bypasses PagedAttention and fused kernels.

```python
# your_model_ar_stage.py

from vllm.model_executor.models.qwen3 import Qwen3DecoderLayer

class YourTTSARStage(nn.Module):

    def __init__(self, config, vllm_config, prefix):
        self.layers = nn.ModuleList([
            Qwen3DecoderLayer(
                config, vllm_config=vllm_config, prefix=f"{prefix}.layers.{i}"
            )
            for i in range(config.num_hidden_layers)
        ])
        self.lm_head = ParallelLMHead(config.codec_size, config.hidden_size)
```

See `qwen3_tts_code_predictor_vllm.py` for the full implementation.

#### 1.2 Forward Pass

Implement `forward()` to return an `OmniOutput` with intermediate data for Stage 1:

```python
def forward(self, input_ids, positions, intermediate_tensors=None,
            inputs_embeds=None, **kwargs) -> OmniOutput:
    hidden_states = self.run_layers(input_ids, positions, intermediate_tensors, inputs_embeds)
    logits = self.lm_head(hidden_states)

    return OmniOutput(
        text_hidden_states=hidden_states,
        multimodal_outputs={
            "audio_codes": self.extract_codes(logits),
        },
    )
```

The keys in `multimodal_outputs` are what your stage input processor will read to build
Stage 1 inputs.

#### 1.3 Weight Loading with Fused QKV

When using vLLM's fused `QKVParallelLinear`, pack the HF `q_proj`/`k_proj`/`v_proj` weights
into `qkv_proj` using `stacked_params_mapping`. See the `load_weights()` method in
`qwen3_tts_code_predictor_vllm.py` for the standard pattern - it can be reused as-is
for any Qwen-family backbone.

#### 1.4 Custom Stop Condition (if needed)

Some TTS models use a learned stop head rather than an EOS token. If your model does this,
implement it inside `sample()`:

```python
def sample(self, logits, sampling_metadata) -> SamplerOutput | None:
    output = self.sampler(logits, sampling_metadata)
    if self._stop_head_fired():
        output = mark_as_finished(output)
    return output
```

### Step 2: Implement Stage 1 - Decoder

Stage 1 decodes Stage 0 output into audio. It runs outside the scheduler (no PagedAttention
needed). Implement `chunked_decode_streaming()` to support async_chunk streaming:

```python
# your_model_decoder.py

class YourTTSDecoder(nn.Module):

    def __init__(self, *, vllm_config: VllmConfig, prefix: str = ""):
        super().__init__()
        # Initialize your audio decoder (SpeechTokenizer, HiFiGAN, etc.)

    def forward(self, codes: torch.Tensor, **kwargs) -> torch.Tensor:
        return self.decoder(codes)

    def chunked_decode_streaming(self, codes, chunk_size=25,
                                  left_context_size=25) -> torch.Tensor:
        """Decode with a sliding context window for smooth chunk boundaries."""
        end_index = codes.shape[-1]
        context_size = 0 if end_index <= chunk_size else left_context_size
        wav_chunk = self(codes)
        # Trim left context to avoid duplicate audio
        return wav_chunk[..., context_size * self.total_upsample:]
```

### Step 3: Implement the Unified Model Class

The unified class dispatches to the correct stage based on `model_stage` in the config:

```python
# your_model.py

class YourTTSModelForConditionalGeneration(nn.Module, SupportsPP):

    def __init__(self, *, vllm_config: VllmConfig, prefix: str = ""):
        super().__init__()
        self.model_stage = vllm_config.model_config.model_stage

        if self.model_stage == "ar_stage":
            ar_vllm_config = vllm_config.with_hf_config(
                vllm_config.model_config.hf_config.ar_config,
                architectures=["YourTTSARStageForConditionalGeneration"],
            )
            self.ar_stage = init_vllm_registered_model(
                vllm_config=ar_vllm_config,
                prefix=maybe_prefix(prefix, "ar"),
                hf_config=ar_vllm_config.model_config.hf_config,
                architectures=["YourTTSARStageForConditionalGeneration"],
            )
            self.model = self.ar_stage

        elif self.model_stage == "decoder":
            self.decoder = YourTTSDecoder(vllm_config=vllm_config, prefix=prefix)
            self.model = self.decoder
```

### Step 4: Create `__init__.py`

```python
# vllm_omni/model_executor/models/your_model_name/__init__.py
from .your_model import YourTTSModelForConditionalGeneration

__all__ = ["YourTTSModelForConditionalGeneration"]
```

## Key Components

### Model Interfaces

Your unified model class should implement the appropriate interfaces:

- **`SupportsPP`**: Required for pipeline parallelism support (all models should implement this)
- **`SupportsMultiModal`**: Only if your model accepts multimodal inputs (e.g. reference audio for voice cloning)

### Output Format

Use `OmniOutput` so the orchestrator can route intermediate data between stages:

```python
from vllm_omni.model_executor.models.output_templates import OmniOutput

return OmniOutput(
    text_hidden_states=hidden_states,
    multimodal_outputs={
        "audio_codes": codec_codes,
    },
)
```

### Weight Loading from a Single Checkpoint

If both stages load from one checkpoint, separate them by prefix in the unified class:

```python
def load_weights(self, weights: Iterable[tuple[str, torch.Tensor]]) -> set[str]:
    ar_weights, decoder_weights = [], []
    for name, tensor in weights:
        if name.startswith("decoder."):
            decoder_weights.append((name, tensor))
        else:
            ar_weights.append((name, tensor))

    if self.model_stage == "ar_stage":
        return self.ar_stage.load_weights(ar_weights)
    elif self.model_stage == "decoder":
        return self.decoder.load_weights(decoder_weights)
```

## Model Registration

Register all stage classes in `vllm_omni/model_executor/models/registry.py`:

```python
_OMNI_MODELS = {
    # (package_name, module_name, class_name)
    "YourTTSModelForConditionalGeneration": (
        "your_model_name", "your_model",
        "YourTTSModelForConditionalGeneration",
    ),
    "YourTTSARStageForConditionalGeneration": (
        "your_model_name", "your_model_ar_stage",
        "YourTTSARStageForConditionalGeneration",
    ),
    "YourTTSDecoder": (
        "your_model_name", "your_model_decoder",
        "YourTTSDecoder",
    ),
}
```

The registry uses lazy loading - model classes are only imported when needed.

## Pipeline and Deploy Configuration

Define fixed topology in `models/<model>/pipeline.py`. The execution type
selects the scheduler and worker family:

- `StageExecutionType.LLM_AR` uses the autoregressive runtime with PagedAttention.
- `StageExecutionType.LLM_GENERATION` uses the generation runtime for stages such as audio decoders.

Key topology fields belong to `StagePipelineConfig`:

| Field | Description |
|-------|-------------|
| `model_stage` | Logical stage name (`ar_stage`, `decoder`, etc.). |
| `execution_type` | Runtime family for this stage. |
| `input_sources` | Upstream stage IDs that provide input. |
| `model_arch` | Stage-specific architecture name, when it differs from the pipeline default. |
| `engine_output_type` | Output representation such as `latent` or `audio`. |
| `custom_process_next_stage_input_func` | Processor for full-payload stage handoff. |
| `async_chunk_process_next_stage_input_func` | Processor for streaming chunk handoff. |
| `final_output` / `final_output_type` | Whether this stage produces user-facing output and its modality. |
| `owns_tokenizer` | Whether this stage supplies the pipeline tokenizer. |

For example, a two-stage TTS topology can be defined as:

```python
from vllm_omni.config.stage_config import (
    PipelineConfig,
    StageExecutionType,
    StagePipelineConfig,
)

_PROC = "vllm_omni.model_executor.stage_input_processors.your_model_name"

YOUR_TTS_PIPELINE = PipelineConfig(
    model_type="your_model_name",
    default_deploy_config_name="your_model_name.yaml",
    model_arch="YourTTSModelForConditionalGeneration",
    stages=(
        StagePipelineConfig(
            stage_id=0,
            model_stage="ar_stage",
            execution_type=StageExecutionType.LLM_AR,
            input_sources=(),
            owns_tokenizer=True,
            engine_output_type="latent",
            custom_process_next_stage_input_func=f"{_PROC}.ar2decoder",
            async_chunk_process_next_stage_input_func=f"{_PROC}.ar2decoder_async_chunk",
        ),
        StagePipelineConfig(
            stage_id=1,
            model_stage="decoder",
            execution_type=StageExecutionType.LLM_GENERATION,
            input_sources=(0,),
            model_arch="YourTTSDecoder",
            engine_output_type="audio",
            final_output=True,
            final_output_type="audio",
        ),
    ),
)
```

Register the pipeline in `vllm_omni/config/pipeline_registry.py`. Runtime
placement and sizing belong in `vllm_omni/deploy/your_model_name.yaml`:

```yaml
async_chunk: true

stages:
  - stage_id: 0
    devices: "0"
    max_num_seqs: 64
    default_sampling_params:
      temperature: 0.9
      top_k: 50
      max_tokens: 2048

  - stage_id: 1
    devices: "0"
    max_num_seqs: 1
```

Set `async_chunk: false` for end-to-end handoff. The registered pipeline
selects the full-payload or async chunk processor; processor paths and topology
must not be moved into the deploy YAML. Use `--deploy-config` to load a custom
deployment overlay.

## Stage Input Processors

Stage input processors convert Stage 0 outputs into Stage 1 inputs. Create yours in
`vllm_omni/model_executor/stage_input_processors/your_model_name.py`.

See `stage_input_processors/qwen3_tts.py` for the full reference implementation.

### Data structures

Understanding what's available in stage outputs:

- `stage_list[source_id].engine_outputs` - list of `EngineCoreOutput` objects
- Each `EngineCoreOutput` has `outputs` - list of `RequestOutput` objects
- Each `RequestOutput` has:
    - `token_ids` - generated token IDs
    - `multimodal_output` - dict with keys matching your model's `OmniOutput.multimodal_outputs`
    - `prompt_token_ids` - original prompt token IDs

### Batch mode (non-streaming)

Collects all Stage 0 outputs and forwards them to Stage 1 in one shot:

```python
def ar2decoder(
    stage_list: list[Any],
    engine_input_source: list[int],
    prompt: OmniTokensPrompt | TextPrompt | None = None,
    requires_multimodal_data: bool = False,
) -> list[OmniTokensPrompt]:
    source_id = engine_input_source[0]
    decoder_inputs = []

    for output in stage_list[source_id].engine_outputs:
        result = output.outputs[0]
        codes = result.multimodal_output["audio_codes"].cpu()
        decoder_inputs.append(
            OmniTokensPrompt(prompt_token_ids=codes.reshape(-1).tolist())
        )

    return decoder_inputs
```

### Streaming mode (async_chunk)

Buffers Stage 0 outputs and forwards a chunk to Stage 1 once `chunk_size` frames
have accumulated. The function signature follows the `OmniChunkTransferAdapter` protocol:

```python
def ar2decoder_async_chunk(
    transfer_manager: Any,
    pooling_output: dict[str, Any] | None,
    request: Any,
    is_finished: bool = False,
) -> dict[str, Any] | None:
    """Forward chunks of AR output to the decoder stage."""
    request_id = request.external_req_id
    finished = bool(is_finished or request.is_finished())

    # Extract and buffer the latest frame
    if isinstance(pooling_output, dict):
        frame = extract_frame(pooling_output)
        if frame is not None:
            transfer_manager.code_prompt_token_ids[request_id].append(
                frame.cpu().tolist()
            )
    elif not finished:
        return None

    # Read chunk config from connector
    chunk_size = 25
    left_context_size = 25

    length = len(transfer_manager.code_prompt_token_ids[request_id])
    if length <= 0:
        if finished:
            return {"codes": [], "finished": torch.tensor(True, dtype=torch.bool)}
        return None

    # Wait until a full chunk is ready (or request is finished)
    chunk_length = length % chunk_size
    if chunk_length != 0 and not finished:
        return None

    # Build context window: left_context + chunk
    context_length = chunk_length if chunk_length != 0 else chunk_size
    end_index = min(length, left_context_size + context_length)
    window = transfer_manager.code_prompt_token_ids[request_id][-end_index:]

    return {
        "codes": torch.tensor(window).transpose(0, 1).reshape(-1).tolist(),
        "left_context_size": max(0, int(end_index - context_length)),
        "finished": torch.tensor(finished, dtype=torch.bool),
    }
```

Key points:

- `transfer_manager` is the `OmniChunkTransferAdapter` that owns the chunk lifecycle
- Each call appends one AR decode step's output; a chunk is emitted every `chunk_size` steps
- The final (possibly partial) chunk is flushed when `is_finished` is true
- `left_context_size` frames of overlap are included for smooth audio boundaries

## Testing

For general testing conventions, see [Test Writing Guide](../ci/test_writing_guide.md).

Recommended test cases for a new TTS model:

1. **Single request** - verify waveform output shape and sample rate
2. **Batched requests** - verify each request in the batch finishes independently
3. **async_chunk streaming** - verify audio chunks arrive incrementally and decode correctly
4. **Speaker conditioning** (if applicable) - verify different speaker inputs produce different outputs

Reference test: `tests/model_executor/stage_input_processors/test_qwen3_tts_async_chunk.py`

### E2E Online Serving Tests (`tests/e2e/online_serving/test_<your_model>.py`)

The `omni_server` fixture in `tests/conftest.py` is **module-scoped**. Each distinct
`OmniServerParams` id in the same test file forces the fixture to tear the server
down and spawn a new one mid-module. A few rules that save real CI debugging time:

- **Prefer a single `OmniServerParams` set per file.** If you need to exercise two
  deploy variants (e.g. `model.yaml` and `model_async_chunk.yaml`), either use one
  variant and exercise streaming via request args, or split into two test files so
  each file does exactly one server lifecycle. Mid-module teardown/restart is the
  fragile path and surfaces startup races first.
- **Never depend on server-side fetching of external URLs** for reference audio or
  other fixture data. CI runners (and China-hosted dev boxes) routinely fail on
  SSL/DNS for public URLs. Inline the payload as a `data:audio/wav;base64,...`
  ref_audio value — the serving layer accepts both forms.
- **Don't roll your own readiness probe.** The harness already waits for HTTP 200
  on `/health` before releasing the server to the test. If your model needs extra
  warmup signals, expose them through `/health` rather than adding `time.sleep(...)`
  inside the test. (Bare TCP `connect_ex` probes were insufficient; see
  `tests/conftest.py` `OmniServer.wait_for_ready`.)
- **Use `core_model` marker + H100 hardware_test** to match the `test-ready.yml`
  pipeline so your test is picked up by the `ready` label, not only nightly.

## Online Serving Integration

To expose your model through the `/v1/audio/speech` OpenAI-compatible endpoint, write
one adapter under `vllm_omni/entrypoints/openai/tts_adapters/`. The adapter declares
how your model is recognized and owns its request handling; `serving_speech.py` should
not need an edit.

### 1. Write the adapter

Create `vllm_omni/entrypoints/openai/tts_adapters/your_model.py`:

```python
# SPDX-License-Identifier: Apache-2.0
# SPDX-FileCopyrightText: Copyright contributors to the vLLM-Omni project
"""YourModel serving adapter."""

from typing import TYPE_CHECKING

from vllm_omni.entrypoints.openai.tts_adapters import register_tts_adapter
from vllm_omni.entrypoints.openai.tts_adapters.base import ARTTSAdapter, PreparedRequest

if TYPE_CHECKING:
    from vllm_omni.entrypoints.openai.protocol.audio import OpenAICreateSpeechRequest


@register_tts_adapter
class YourModelAdapter(ARTTSAdapter):
    # The registry key, and the model-type string used in logs.
    name = "your_model"
    # The engine ``model_stage`` value(s) your deployment yaml sets. Detection,
    # stage discovery and `/v1/audio/speech` routing all derive from this.
    stage_keys = frozenset({"your_model_stage_key"})

    def validate(self, request: "OpenAICreateSpeechRequest") -> str | None:
        """Return an error string, or None if the request is valid."""
        if not request.input or not request.input.strip():
            return "Input text cannot be empty"
        return None

    async def build(
        self,
        request: "OpenAICreateSpeechRequest",
        sampling_params_list: list,
        has_inline_ref_audio: bool,
    ) -> PreparedRequest:
        """Build the engine prompt and per-model parameters."""
        params: dict[str, list] = {"text": [request.input]}
        if request.voice is not None:
            params["voice"] = [request.voice]
        return PreparedRequest(
            prompt={"prompt": request.input},
            tts_params=params,
            model_type=self.name,
        )
```

Then add it to the import block at the bottom of
`vllm_omni/entrypoints/openai/tts_adapters/__init__.py` so it registers on import.
That import line is the only shared file a new model has to touch.

> **Pure-diffusion TTS does not go through adapters yet.** When the server is built
> by `OmniOpenAIServingSpeech.for_diffusion()`, `create_speech()` routes straight to
> `_create_diffusion_speech()` and never calls `validate()` or `build()`.
> `DiffusionTTSAdapter` is scaffolding for that migration and has no production
> subclass today, so validation or prompt construction placed in one would silently
> never run. If your model is served by the diffusion engine rather than
> `engine_client`, follow the existing diffusion path and say so on #4855 — wiring
> that path through the adapter layer is open work, not something to do inside a
> new-model PR.

### 2. If a stage key alone cannot identify your model

Two escape hatches, in order of preference:

```python
# The architecture is authoritative (e.g. your model has no dedicated
# model_stage value, or shares one with another model).
model_archs = frozenset({"YourModelForConditionalGeneration"})

# Set this too if your model owns NO stage key, so stage discovery has to find
# your AR entry stage by architecture.
arch_identifies_entry_stage = True
```

If your rule is not a set-membership test, override `matches()` — see
`covo_audio.py`, which shares the generic `fused_thinker_talker` stage key with
non-CoVo deployments and confirms with the architecture:

```python
@classmethod
def matches(cls, model_stage: str | None, model_arch: str | None) -> bool:
    return super().matches(model_stage, model_arch) and bool(model_arch) and "CovoAudio" in model_arch
```

If your model can match the same deployment as another adapter, give one of them an
explicit `detect_priority` (lower runs first). `tests/entrypoints/openai_api/
test_tts_detection.py` fails if two same-priority detectors can both match one input,
so an unordered overlap cannot be merged.

Models that are speech-capable only in some deployment topologies override
`stage_serves_speech()` — see `audex.py`, whose omni thinker is text-final unless the
speech decoder is deployed alongside it.

### 3. Reuse the shared helpers

Adapters reach shared serving helpers through `self.ctx.server`: reference-audio
resolution (`_resolve_ref_audio`), uploaded-speaker handling
(`_apply_uploaded_speaker`), format validation (`_validate_ref_audio_format`), and
prompt-length limits (`_max_instructions_length`). Read a comparable adapter before
writing your own — `fish_speech.py` for voice cloning, `higgs_audio_v3.py` for
parameter-heavy models, `moss_tts.py` for a family sharing one base class.

### 4. Test it

`tests/entrypoints/openai_api/test_tts_adapter.py` covers registry invariants and
`test_tts_detection.py` covers detection. Add your model to
`EXPECTED_MODEL_TYPES` in the former, and a case to the latter if you added a
`model_archs` or `matches()` rule.

### Do not add branches to `serving_speech.py`

Older models predate the adapter framework and still have per-model branches in
`serving_speech.py`; they are being migrated out (RFC #4327, #4855). Do not copy
that pattern. `tools/pre_commit/check_tts_adapter.py` is a ratchet on the remaining
`self._tts_model_type == ...` comparisons and fails the commit if the count grows.

If your model genuinely needs behaviour that no adapter hook can express, that is a
missing hook — propose it on the RFC rather than adding a branch.

### Merge conflicts

Your adapter is a new file, so it cannot conflict. The one shared line is your entry
in the import block at the bottom of `tts_adapters/__init__.py`, which conflicts only
when another model lands at the same time. The resolution is always to **keep both**
imports — never discard either side. After resolving:

```bash
git add vllm_omni/entrypoints/openai/tts_adapters/__init__.py
git rebase --continue
```

If you find yourself resolving a conflict inside `serving_speech.py`, something in
your model is still going through the legacy path; see the note above.

## Single-Stage Models

Some TTS models (e.g. MOSS-TTS-Nano) do not use a two-stage pipeline. Instead the
entire AR LM and audio decoder run inside a single AR worker, streaming audio chunks
directly from the model's own generator.

### Directory structure

```text
vllm_omni/model_executor/models/your_model_name/
    __init__.py
    modeling_your_model_name.py    # unified class: load_weights + forward + streaming
    pipeline.py                    # single-stage pipeline topology

vllm_omni/deploy/your_model_name.yaml
```

No stage input processor is needed.

### Pipeline and deploy config

Use a single `LLM_AR` stage that owns the tokenizer and produces final audio:

```python
YOUR_TTS_PIPELINE = PipelineConfig(
    model_type="your_model_name",
    default_deploy_config_name="your_model_name.yaml",
    model_arch="YourModelForCausalLM",
    stages=(
        StagePipelineConfig(
            stage_id=0,
            model_stage="your_model_stage_key",
            execution_type=StageExecutionType.LLM_AR,
            input_sources=(),
            owns_tokenizer=True,
            engine_output_type="audio",
            final_output=True,
            final_output_type="audio",
        ),
    ),
)
```

The deploy YAML controls runtime behavior. Set `max_num_seqs` to at least 4
for concurrent production use:

```yaml
async_chunk: false

stages:
  - stage_id: 0
    devices: "0"
    max_num_seqs: 4
```

### Generator-based streaming pattern

This is the MOSS-TTS-Nano pattern, distinct from VoxCPM2's vLLM-native AR pattern
(see `plan/voxcpm2_native_ar_design.md` for that variant). Load model weights in
`load_weights()` (not `__init__`) so vLLM finishes distributed initialisation before
any CUDA allocations. Stream via a per-request generator stored in an instance dict:

```python
class YourModelForCausalLM(nn.Module):

    def __init__(self, *, vllm_config: VllmConfig, prefix: str = ""):
        super().__init__()
        self._lm: nn.Module | None = None          # populated in load_weights()
        self._stream_gens: dict[str, Any] = {}     # request_key → generator

    def load_weights(self, weights):
        # Load self._lm here, after vLLM distributed init
        ...

    def forward(
        self,
        input_ids,
        positions,
        intermediate_tensors=None,
        inputs_embeds=None,
        runtime_additional_information: list[dict] | None = None,  # one dict per request
        **kwargs,
    ) -> OmniOutput:
        infos = runtime_additional_information or [{}]
        # Return empty output during dummy/profiling calls
        if not runtime_additional_information or all(i.get("_is_dummy") for i in infos):
            self._ar_emit_stop_token = True
            return OmniOutput(...)

        outputs, last_flags = [], []
        for info in infos:
            request_key = str(info.get("_omni_req_id", "0"))  # set by vLLM, not user code
            if request_key not in self._stream_gens:
                self._stream_gens[request_key] = self._create_stream_gen(info)
            try:
                chunk, is_last = next(self._stream_gens[request_key])
            except StopIteration:
                chunk, is_last = torch.zeros(0), True
            if is_last:
                del self._stream_gens[request_key]
            outputs.append(chunk)
            last_flags.append(is_last)

        self._ar_emit_stop_token = all(last_flags)
        return OmniOutput(multimodal_outputs={"model_outputs": outputs, "is_last": last_flags})

    def _create_stream_gen(self, info: dict):
        """Yield (waveform_tensor, is_last) from the model's inference_stream().

        Handle both incremental ("audio" events) and batch ("result" event) models:
        some upstream implementations emit one "result" event with the full waveform
        instead of incremental "audio" events. Both paths must be covered.
        """
        for event in self._lm.inference_stream(...):
            if event["type"] == "audio":
                yield event["waveform"], False
            elif event["type"] == "result":
                # Fallback for models that don't emit incremental audio events
                yield event.get("waveform", torch.zeros(0)), True
                return
        yield torch.zeros(0), True

    def compute_logits(self, hidden_states, sampling_metadata):
        # Emit EOS only when the last chunk has been yielded so the AR
        # scheduler ends the request at the right time.
        ...
```

For an in-tree reference, look for any single-stage AR model under
`vllm_omni/model_executor/models/` (for example
`moss_tts_nano/modeling_moss_tts_nano.py` once its integration has landed).

## Pre-commit and DCO

All contributions must pass the pre-commit checks and the Developer Certificate of
Origin (DCO) sign-off before merging.

### Running pre-commit

Install the hooks once with `pre-commit install`. Then run before committing:

```bash
pre-commit run --files \
  vllm_omni/model_executor/models/your_model_name/*.py \
  vllm_omni/entrypoints/openai/serving_speech.py \
  vllm_omni/model_executor/models/registry.py \
  tests/e2e/offline_inference/test_your_model_name.py \
  tests/e2e/online_serving/test_your_model_name.py
```

When pre-commit **modifies files**, it exits with a non-zero code but the reformatting
is correct. Stage the modified files and commit again — do not revert the changes.

Common failures and fixes:

| Check | Cause | Fix |
| ------- | ------- | ----- |
| `ruff F841` | Local variable assigned but never used | Remove the extraction or forward it to the model call |
| `ruff E402` | Module-level import not at top of file | Move import to the top-level import block |
| `ruff format` | Line length, spacing, or quote style | Accept the auto-fix, stage, and re-commit |
| `check-spdx-header` | Missing header, or copyright still says `vLLM project` | Use `Copyright contributors to the vLLM-Omni project`; restage the rewrite |
| `check-forbidden-imports` | Stdlib `re`/`base64`, pickle, Hugging Face Hub API, or direct Triton/TileLang | `import regex as re` and `pybase64`; do not grow the allowlist without review |
| `check-torch-cuda-call` | New `torch.cuda.*` call site | Use `current_omni_platform`; do not grow `ALLOWED_FILES` without review |
| `check-tts-adapter-migration` | New `_tts_model_type` branch in `serving_speech.py` | Put logic in `tts_adapters/`; lower `MAX_MODEL_TYPE_BRANCHES` when removing branches |
| `check-test-ci-coverage` | Test file missing level or hardware mark | Add `core_model`/… plus `cpu`/`cuda`/`hardware_test(` |
| `shellcheck` | Native `shellcheck` missing, or script warning | Install via apt/dnf/brew (or `shellcheck.exe` on Windows). See [Linting](../README.md#linting) |
| `mypy-3.10` / `markdownlint-cli2` / `check-buildkite` | Types, docs markdown, or Buildkite YAML | See [Linting](../README.md#linting) |

### DCO sign-off

Every commit must carry a `Signed-off-by` trailer. Use the `-s` flag when committing:

```bash
git commit -s -m "feat(your-model): add YourModel TTS support"
```

Or configure git to add it automatically:

```bash
git config format.signOff true
```

To fix a missing sign-off on the most recent commit:

```bash
git commit --amend -s --no-edit
git push origin your-branch --force-with-lease
```

> The DCO check verifies that the commit author email matches the `Signed-off-by` email.
> Make sure `git config user.email` is set to the address associated with your GitHub
> account before committing.

## Adding a Model Recipe

After implementing and testing your model, add a model recipe to the
[vllm-project/recipes](https://github.com/vllm-project/recipes) repository so users can
get started quickly. See [Adding an Omni-Modality Model](./adding_omni_model.md#adding-a-model-recipe)
for the expected format.

## Summary

Adding a TTS model to vLLM-Omni involves:

1. **Create model directory** with AR stage, decoder stage, and unified class (two-stage)
   or a single unified class with generator-based streaming (single-stage)
2. **AR stage** - use vLLM's native decoder layers with fused QKV; do not wrap HF directly
3. **Decoder stage** - thin wrapper around your audio decoder; implement `chunked_decode_streaming()`
4. **Unified class** - dispatches on `model_stage`; same structure as `Qwen3TTSModelForGeneration`
5. **Register** all stage classes in `registry.py`
6. **YAML configs** - provide both batch and `async_chunk` variants (two-stage), or a single-stage AR config
7. **Stage input processor** - buffer Stage 0 outputs and forward in chunks of 25 (two-stage only)
8. **Online serving** - add all 5 integration points to `serving_speech.py` in one commit
9. **Tests** - cover single request, batching, and streaming
10. **Pre-commit + DCO** - run `pre-commit` before pushing; sign every commit with `git commit -s`
11. **Model recipe** - add to [vllm-project/recipes](https://github.com/vllm-project/recipes)
12. **Invariants** - re-check I1–I5 (streaming contract, consumer hygiene, hot-loop discipline, validation pyramid, per-request state) at the end of every phase

### Qwen3-TTS Reference Files

| File | Purpose |
| ------ | --------- |
| `models/qwen3_tts/qwen3_tts.py` | Unified model class |
| `models/qwen3_tts/qwen3_tts_code_predictor_vllm.py` | AR stage with vLLM fused ops |
| `models/qwen3_tts/qwen3_tts_code2wav.py` | Decoder stage with `chunked_decode_streaming()` |
| `models/qwen3_tts/pipeline.py` | Frozen pipeline topology (registered at import time) |
| `deploy/qwen3_tts.yaml` | Deploy config (user-editable, async_chunk + SharedMemoryConnector) |
| `stage_input_processors/qwen3_tts.py` | Stage transition processors |

For more information, see:

- [Architecture Overview](../../design/architecture_overview.md)
- [Async Chunk Design](../../design/feature/async_chunk.md)
- [Pipeline and Deploy Configuration Guide](../../configuration/stage_configs.md)