OpenVoiceOS TTS Server

Turn any OVOS TTS plugin into a microservice — a small, stateless FastAPI app that exposes text-to-speech over HTTP.

• 🔌 Plugin-agnostic — serve Piper, Coqui, Azure, or any OVOS TTS plugin behind one consistent HTTP API.
• 🧩 Drop-in cloud-API compatibility — speak the ElevenLabs, OpenAI, Coqui, Google, Amazon Polly, Azure, MaryTTS, Cartesia, Deepgram Aura, and PlayHT APIs so existing clients and SDKs work unmodified (see API compatibility).
• 🪶 Stateless & tiny — each request loads nothing extra; ideal for containers and horizontal scaling.
• 🎛️ Format conversion — return WAV out of the box, or mp3/ogg/flac/… with the optional [audio] extra.

Install

pip install ovos-tts-server

# Optional: enable non-WAV output (mp3, ogg, flac, …) via pydub
pip install "ovos-tts-server[audio]"

You also need at least one TTS plugin, e.g. Piper:

pip install ovos-tts-plugin-piper

Quickstart

# Start the server with the Piper plugin
ovos-tts-server --engine ovos-tts-plugin-piper

# Synthesize over HTTP
curl "http://localhost:9666/v2/synthesize?utterance=hello%20world" -o hello.wav

Command line

ovos-tts-server [-h] [--engine ENGINE] [--port PORT] [--host HOST] [--cache] [--lang LANG]

Configuration

The plugin is configured exactly as it would be inside the assistant — through mycroft.conf:

{
  "tts": {
    "module": "ovos-tts-plugin-piper",
    "ovos-tts-plugin-piper": {
      "model": "alan-low"
    }
  }
}

See docs/configuration.md for how voice and language flow from a request to the plugin.

HTTP API

The native OVOS endpoints:

Any extra query parameters on the synthesis endpoints are forwarded to the plugin as synthesis options. CORS is enabled for all origins.

curl http://localhost:9666/status
# {"status": "ok", "plugin": "ovos-tts-plugin-piper", "langs": ["en-us"], ...}

Third-party API compatibility

The server can additionally expose the same plugin behind drop-in compatibility endpoints for popular cloud TTS APIs. Each vendor lives under its own URL prefix, so every compat layer is active at once with no path collisions. Auth tokens / API keys are accepted and silently ignored — put real auth in a reverse proxy if you need it.

Kokoro / kokoro-fastapi clients are OpenAI-compatible and need no dedicated prefix — point them at /openai/v1/audio/speech.

Most official SDKs accept a custom base URL; point them at http://<host>:9666/<prefix> and they work unmodified. See docs/api-compatibility.md for per-vendor endpoints, parameters, SDK snippets, and curl examples.

Python API

from ovos_tts_server import start_tts_server

app, engine = start_tts_server("ovos-tts-plugin-piper", cache=False)
# `app` is a FastAPI instance — mount it, test it, or run it with uvicorn

create_app(tts_engine) is also available if you want to build and inject the TTSEngineWrapper yourself.

Docker

Build a small image that serves any plugin:

FROM python:3.11-slim

RUN pip install --no-cache-dir "ovos-tts-server[audio]" {PLUGIN_HERE}

ENTRYPOINT ["ovos-tts-server", "--engine", "{PLUGIN_HERE}", "--cache"]

docker build . -t my_ovos_tts_plugin
docker run -p 8080:9666 my_ovos_tts_plugin
curl "http://localhost:8080/v2/synthesize?utterance=hello" -o hello.wav

Each plugin can ship its own Dockerfile in its repository using ovos-tts-server.

Companion plugin

Consume this server from a voice assistant via the companion TTS plugin.

Development

pip install -e ".[audio,test]"
pytest test/ -v

Documentation

• API compatibility reference — every vendor prefix, endpoint, and SDK snippet
• Voice & language configuration — how requests map to the plugin
• Audio format conversion — convert_audio() and the [audio] extra
• Architecture overview — classes, entry points, request flow

Agent Integration

UTCP — Universal Tool Calling Protocol

The server exposes a UTCP manual at GET /utcp. Any UTCP-aware agent (e.g. ovos-tool-adapters UTCPToolBox) can point at this URL and auto-discover every synthesis endpoint without additional configuration.

No extra dependencies are needed — GET /utcp is always available.

Example response (abbreviated):

{
  "utcp_version": "1.0.1",
  "manual_version": "1.0.0",
  "tools": [
    {
      "name": "tts_synthesize_v2",
      "description": "Synthesize speech from text (OVOS v2 endpoint)...",
      "inputs": {
        "type": "object",
        "properties": {
          "utterance": {"type": "string"},
          "voice":     {"type": "string"},
          "lang":      {"type": "string"}
        },
        "required": ["utterance"]
      },
      "tool_call_template": {
        "call_template_type": "http",
        "url": "http://localhost:9666/v2/synthesize",
        "http_method": "GET"
      }
    }
  ]
}

ovos-tool-adapters config example:

{
  "utcp_config": {
    "providers": [
      {
        "provider_type": "http",
        "name": "ovos-tts",
        "url": "http://localhost:9666/utcp"
      }
    ]
  }
}

MCP — Model Context Protocol

The server can optionally expose a FastMCP server mounted at /mcp, providing a synthesize tool callable by any MCP-compatible agent (Claude Desktop, Claude Code, etc.).

Install the extra:

pip install "ovos-tts-server[mcp]"

Start with MCP enabled:

ovos-tts-server --engine ovos-tts-plugin-piper --mcp

Or from Python:

from ovos_tts_server import start_tts_server
app, engine = start_tts_server("ovos-tts-plugin-piper", enable_mcp=True)

The MCP server uses streamable HTTP transport (SSE-compatible) and is mounted alongside the existing FastAPI app — no separate process needed.

Claude Desktop claude_desktop_config.json example:

{
  "mcpServers": {
    "ovos-tts": {
      "transport": "http",
      "url": "http://localhost:9666/mcp"
    }
  }
}

synthesize tool:

Returns a JSON object:

{
  "mime_type": "audio/wav",
  "data": "<base64-encoded WAV>",
  "path": "/tmp/ovos_synth_abc123.wav",
  "phonemes": null
}

Credits

Developed by TigreGótico for OpenVoiceOS.

This project was funded through the NGI0 Commons Fund, a fund established by NLnet with financial support from the European Commission's Next Generation Internet programme, under the aegis of DG Communications Networks, Content and Technology under grant agreement No 101135429.