ext-apps/examples/say-server

Say Server - Streaming TTS MCP App

A real-time text-to-speech MCP App with karaoke-style text highlighting, powered by Kyutai's Pocket TTS.

MCP Client Configuration

Add to your MCP client configuration (stdio transport):

{
  "mcpServers": {
    "say": {
      "command": "uv",
      "args": [
        "run",
        "--default-index",
        "https://pypi.org/simple",
        "https://raw.githubusercontent.com/modelcontextprotocol/ext-apps/refs/heads/main/examples/say-server/server.py",
        "--stdio"
      ]
    }
  }
}

MCP App Features Demonstrated

This example showcases several MCP App capabilities:

• Single-file executable: Python server with embedded React UI - no build step required
• Partial tool inputs (ontoolinputpartial): The view receives streaming text as it's being generated
• Queue-based streaming: Demonstrates how to stream text out and audio in via a polling tool (adds text to an input queue, retrieves audio chunks from an output queue)
• Model context updates: The view updates the LLM with playback progress ("Playing: ...snippet...")
• Native theming: Uses CSS variables for automatic dark/light mode adaptation
• Fullscreen mode: Toggle fullscreen via requestDisplayMode() API, press Escape to exit
• Multi-view speak lock: Coordinates multiple TTS views via localStorage so only one plays at a time
• Hidden tools (visibility: ["app"]): Private tools only accessible to the view, not the model
• External links (openLink): Attribution popup uses app.openLink() to open external URLs
• CSP metadata: Resource declares required domains (esm.sh) for in-browser transpilation

Features

• Streaming TTS: Audio starts playing as text is being generated
• Karaoke highlighting: Words are highlighted in sync with speech
• Interactive controls: Click to pause/resume, double-click to restart
• Low latency: Uses a polling-based queue for minimal delay

Prerequisites

• uv - fast Python package manager

Quick Start

The server is a single self-contained Python file that can be run directly from GitHub:

# Run directly from GitHub (uv auto-installs dependencies)
uv run https://raw.githubusercontent.com/modelcontextprotocol/ext-apps/main/examples/say-server/server.py

The server will be available at http://localhost:3109/mcp.

Running with Docker

Run directly from GitHub using the official uv Docker image:

docker run --rm -it \
  -p 3109:3109 \
  -v ~/.cache/huggingface-docker-say-server:/root/.cache/huggingface \
  ghcr.io/astral-sh/uv:debian \
  uv run https://raw.githubusercontent.com/modelcontextprotocol/ext-apps/main/examples/say-server/server.py

Usage

With Claude Desktop

Add to your Claude Desktop config (~/Library/Application Support/Claude/claude_desktop_config.json):

{
  "mcpServers": {
    "say": {
      "command": "uv",
      "args": [
        "run",
        "https://raw.githubusercontent.com/modelcontextprotocol/ext-apps/main/examples/say-server/server.py",
        "--stdio"
      ]
    }
  }
}

With MCP Clients

Connect to http://localhost:3109/mcp and call the say tool:

{
  "name": "say",
  "arguments": {
    "text": "Hello, world! This is a streaming TTS demo."
  }
}

Available Voices

The default voice is cosette. Use the list_voices tool or pass a voice parameter to say:

Predefined Voices

• alba, marius, javert, jean - from alba-mackenna (CC BY 4.0)
• cosette, eponine, azelma, fantine - from VCTK dataset (CC BY 4.0)

Custom Voices

You can also use HuggingFace URLs or local file paths:

{"text": "Hello!", "voice": "hf://kyutai/tts-voices/voice-donations/alice.wav"}
{"text": "Hello!", "voice": "/path/to/my-voice.wav"}

See the kyutai/tts-voices repository for more voice collections

Architecture

The entire server is contained in a single server.py file:

1. say tool: Public tool that triggers the view with text to speak
2. Private tools (create_tts_queue, add_tts_text, poll_tts_audio, etc.): Hidden from the model, only callable by the view
3. Embedded React view: Uses Babel standalone for in-browser JSX transpilation - no build step needed
4. TTS backend: Manages per-request audio queues using Pocket TTS

The view communicates with the server via MCP tool calls:

• Receives streaming text via ontoolinputpartial callback
• Incrementally sends new text to the server as it arrives (via add_tts_text)
• Polls for generated audio chunks while TTS runs in parallel
• Plays audio via Web Audio API with synchronized text highlighting

Multi-view Speak Lock

When multiple TTS views exist in the same browser (e.g., multiple chat messages each with their own say view), they coordinate via localStorage to ensure only one plays at a time:

1. Unique view IDs: Each view receives a UUID via toolResult._meta.viewUUID
2. Announce on Play: When starting, a view writes {uuid, timestamp} to localStorage["mcp-tts-playing"]
3. Poll for Conflicts: Every 200ms, playing views check if another view took the lock
4. Yield Gracefully: If another view started playing, pause and yield
5. Clean Up: On pause/finish, clear the lock (only if owned)

This "last writer wins" protocol ensures a seamless experience: clicking play on any view immediately pauses others, without requiring cross-iframe postMessage coordination.

TODO

• Persist caret position in localStorage (resume from where you left off)
• Click anywhere in text to move the cursor/playback position

Credits

This project uses Pocket TTS by Kyutai - a fantastic open-source text-to-speech model. Thank you to the Kyutai team for making this technology available!

The server includes modified Pocket TTS code to support streaming text input (text can be fed incrementally while audio generation runs in parallel). A PR contributing this functionality back to the original repo is planned.

License

This example is MIT licensed.

Third-Party Licenses

This project uses the following open-source components:

Component	License	Link
pocket-tts	MIT	Python TTS library
Kyutai TTS model	CC-BY 4.0	Text-to-speech model weights
kyutai/tts-voices	Mixed (see below)	Voice prompt files

Voice Collection Licenses

The predefined voices in this example use CC-BY 4.0 licensed collections:

Collection	License	Commercial Use
alba-mackenna	CC-BY 4.0	✅ Yes (with attribution)
vctk	CC-BY 4.0	✅ Yes (with attribution)
cml-tts/fr	CC-BY 4.0	✅ Yes (with attribution)
voice-donations	CC0 (Public Domain)	✅ Yes
expresso	CC-BY-NC 4.0	❌ Non-commercial only
ears	CC-BY-NC 4.0	❌ Non-commercial only

⚠️ Note: If you use voices from the expresso/ or ears/ collections, your use is restricted to non-commercial purposes.