ElevenLabs

The ElevenLabs provider integrates multiple AI audio capabilities for comprehensive voice AI testing and evaluation.

tip

For a comprehensive step-by-step tutorial, see the Evaluating ElevenLabs voice AI guide.

Quick Start

Get started with ElevenLabs in 3 steps:

Install and authenticate:

npm install -g promptfoo
export ELEVENLABS_API_KEY=your_api_key_here

Create a config file (promptfooconfig.yaml):

prompts:
  - 'Welcome to our customer service. How can I help you today?'

providers:
  - id: elevenlabs:tts:21m00Tcm4TlvDq8ikWAM

tests:
  - description: Generate welcome message
    assert:
      - type: cost
        threshold: 0.01
      - type: latency
        threshold: 2000

Run your first eval:
```
promptfoo eval
```
View results with promptfoo view or in the web UI.

Setup

Set your ElevenLabs API key as an environment variable:

export ELEVENLABS_API_KEY=your_api_key_here

Alternatively, specify the API key directly in your configuration:

providers:
  - id: elevenlabs:tts
    config:
      apiKey: your_api_key_here

tip

Get your API key from ElevenLabs Settings. Free tier includes 10,000 characters/month.

Capabilities

The ElevenLabs provider supports multiple capabilities:

Text-to-Speech (TTS)

Generate high-quality voice synthesis with multiple models and voices:

elevenlabs:tts:<voice_id> - TTS with specified voice ID (e.g., elevenlabs:tts:21m00Tcm4TlvDq8ikWAM for Rachel)
elevenlabs:tts - TTS with default voice

Use a voice ID from your ElevenLabs voice list, not a display name such as rachel. Set it in the provider ID or config.voiceId; an explicit voiceId takes precedence. Voice availability depends on your account.

Models available:

eleven_flash_v2_5 - Fastest, lowest latency (~200ms)
eleven_turbo_v2_5 - Deprecated low-latency model; prefer Flash v2.5 for new configurations
eleven_multilingual_v2 - Default model, suited to consistent long-form speech

ElevenLabs scheduled monolingual and multilingual v1 for removal on July 9, 2026. Legacy type values remain for compatible endpoints; use a current model for the native API.

Example:

providers:
  - id: elevenlabs:tts:21m00Tcm4TlvDq8ikWAM
    config:
      modelId: eleven_flash_v2_5
      voiceSettings:
        stability: 0.5
        similarity_boost: 0.75
        speed: 1.0

Speech-to-Text (STT)

Transcribe audio with speaker diarization and accuracy metrics. The default is scribe_v2; ElevenLabs scheduled Scribe v1 for removal on July 9, 2026. The legacy modelId: scribe_v1 remains configurable for compatible endpoints.

elevenlabs:stt - Speech-to-text transcription

Features:

Speaker diarization (identify multiple speakers)
Word Error Rate (WER) calculation
Multiple language support

Example:

providers:
  - id: elevenlabs:stt
    config:
      modelId: scribe_v2
      diarization: true
      maxSpeakers: 3

Conversational Agents

Test voice AI agents with LLM backends and evaluation criteria:

elevenlabs:agents - Voice AI agent testing

Features:

Multi-turn conversation simulation
Automated evaluation criteria
Tool calling and mocking
LLM cascading for cost optimization
Custom LLM endpoints
Multi-voice conversations
Phone integration (Twilio, SIP)

Example:

providers:
  - id: elevenlabs:agents
    config:
      agentConfig:
        name: Customer Support Agent
        prompt: You are a helpful support agent
        voiceId: 21m00Tcm4TlvDq8ikWAM
        llmModel: gpt-4o
      evaluationCriteria:
        - name: helpfulness
          description: Agent provides helpful responses
          weight: 1.0
          passingThreshold: 0.8

Supporting APIs

Additional audio processing capabilities:

elevenlabs:history - Retrieve agent conversation history
elevenlabs:isolation - Remove background noise from audio
elevenlabs:alignment - Generate time-aligned subtitles

Configuration Parameters

All providers support these common parameters:

Parameter	Description
`apiKey`	Your ElevenLabs API key
`apiKeyEnvar`	Environment variable containing the API key
`baseUrl`	Custom base URL for API (default: ElevenLabs API)
`timeout`	Request timeout in milliseconds
`cache`	Enable response caching
`cacheTTL`	Cache time-to-live in seconds
`enableLogging`	Enable debug logging
`retries`	Number of retry attempts for failed requests

TTS-Specific Parameters

Parameter	Description
`modelId`	TTS model (e.g., `eleven_flash_v2_5`)
`voiceId`	Voice ID (e.g., `21m00Tcm4TlvDq8ikWAM`)
`voiceSettings`	Voice customization (stability, similarity, style, speed)
`outputFormat`	Audio format (e.g., `mp3_44100_128`, `pcm_44100`)
`seed`	Best-effort repeatability for HTTP and WebSocket TTS; deterministic output is not guaranteed
`streaming`	Enable WebSocket streaming for low latency
`pronunciationRules`	Custom pronunciation rules (creates a dictionary at init)
`pronunciationDictionaryId`	Apply an existing pronunciation dictionary by ID
`voiceDesign`	Generate voice from text description
`voiceRemix`	Modify voice characteristics (gender, accent, age)

ulaw_8000 returns raw 8 kHz μ-law audio with media type audio/basic. Saved files use the .ulaw extension.

STT-Specific Parameters

Parameter	Description
`modelId`	STT model (default: `scribe_v2`)
`language`	ISO 639-1 language code (e.g., `en`, `es`)
`diarization`	Enable speaker diarization
`maxSpeakers`	Expected number of speakers (hint)
`audioFormat`	Input audio format

Agent-Specific Parameters

Parameter	Description
`agentId`	Use existing agent ID
`agentConfig`	Ephemeral agent configuration
`simulatedUser`	Automated user simulation settings
`evaluationCriteria`	Evaluation criteria for agent performance
`toolMockConfig`	Mock tool responses for testing
`maxTurns`	Maximum conversation turns (default: 10)
`llmCascade`	LLM fallback configuration
`customLLM`	Custom LLM endpoint configuration
`mcpConfig`	Model Context Protocol integration
`multiVoice`	Multi-voice conversation configuration
`postCallWebhook`	Webhook notification after conversation
`phoneConfig`	Twilio or SIP phone integration

Examples

Text-to-Speech: Voice Comparison

prompts:
  - 'Welcome to ElevenLabs. Our AI voice technology delivers natural-sounding speech.'

providers:
  - id: elevenlabs:tts:21m00Tcm4TlvDq8ikWAM
    config:
      modelId: eleven_flash_v2_5

  - id: elevenlabs:tts:2EiwWnXFnvU5JabPnv8n
    config:
      modelId: eleven_turbo_v2_5

tests:
  - description: Audio generation succeeds
    assert:
      - type: cost
        threshold: 0.01
      - type: latency
        threshold: 5000

Speech-to-Text: Accuracy Testing

Use the transcription accuracy guide to provide a recording and reference transcript, then assert on the word error rate in context.providerResponse.metadata.wer.

Conversational Agents: Evaluation

The provider returns a text summary in output and structured evaluation results in context.providerResponse.metadata.evaluationResults. Use explicit criterion IDs to check that every required result is present and passed.

prompts:
  - |
    User: I need help with my order
    Agent: I'd be happy to help! What's your order number?
    User: ORDER-12345

providers:
  - id: elevenlabs:agents
    config:
      agentConfig:
        prompt: You are a helpful customer support agent
        llmModel: gpt-4o
      evaluationCriteria:
        - id: greeting
          name: greeting
          weight: 0.8
          passingThreshold: 0.8
        - id: understanding
          name: understanding
          weight: 1.0
          passingThreshold: 0.9

tests:
  - description: Agent meets evaluation criteria
    assert:
      - type: javascript
        value: |
          const results = context.providerResponse.metadata?.evaluationResults;
          const required = ['greeting', 'understanding'];
          return Array.isArray(results) && required.every(id =>
            results.some(result => result.criterion === id && result.passed === true)
          );

Audio Processing: Pipeline

Run each stage as a separate eval and pass the saved results to the next stage:

Use elevenlabs:isolation with the original audio path in vars.audioFile. Save the returned audio to a file such as audio/cleaned.mp3.
Use elevenlabs:stt with prompts: ['{{audioFile}}'] and vars.audioFile: audio/cleaned.mp3. Save the transcription text from the output.
Use elevenlabs:alignment with the cleaned audio path in vars.audioFile and the transcription in vars.transcript to generate time-aligned output.

Advanced Features

Pronunciation Dictionaries

Customize pronunciation for technical terms:

providers:
  - id: elevenlabs:tts:21m00Tcm4TlvDq8ikWAM
    config:
      pronunciationRules:
        - word: 'API'
          pronunciation: 'A P I'
        - word: 'OAuth'
          phoneme: 'əʊɔːθ'
          alphabet: 'ipa'

Voice Design

Generate custom voices from descriptions:

providers:
  - id: elevenlabs:tts
    config:
      voiceDesign:
        name: Custom Voice
        description: A middle-aged American male with a deep, authoritative tone
        gender: male
        age: middle_aged
        accent: american

LLM Cascading

Optimize costs with automatic fallback:

providers:
  - id: elevenlabs:agents
    config:
      llmCascade:
        primary: gpt-4o
        fallback:
          - gpt-4o-mini
          - gpt-3.5-turbo
        cascadeOnError: true
        cascadeOnLatency:
          enabled: true
          maxLatencyMs: 5000

Multi-voice Conversations

Different voices for different characters:

providers:
  - id: elevenlabs:agents
    config:
      multiVoice:
        characters:
          - name: Agent
            voiceId: 21m00Tcm4TlvDq8ikWAM
            role: Customer support representative
          - name: Customer
            voiceId: 2EiwWnXFnvU5JabPnv8n
            role: Customer seeking help

Phone Integration

Test agents with real phone calls:

providers:
  - id: elevenlabs:agents
    config:
      phoneConfig:
        provider: twilio
        twilioAccountSid: ${TWILIO_ACCOUNT_SID}
        twilioAuthToken: ${TWILIO_AUTH_TOKEN}
        twilioPhoneNumber: +1234567890

Cost Tracking

Promptfoo reports estimated costs when the provider has enough usage information. Estimates use built-in rates and may differ from your bill; consult ElevenLabs pricing for current plans and rates.

STT estimates require a known audio duration in the API response's duration_ms field. Responses without duration omit cost; they do not report free transcription. A cost assertion requires a reported cost, so omit it for native Scribe responses without duration.

For responses that include a cost estimate, apply a budget assertion:

tests:
  - assert:
      - type: cost
        threshold: 0.50 # Max $0.50 per test

Popular Voices

Common voice IDs and names:

Name	ID	Description
Rachel	21m00Tcm4TlvDq8ikWAM	Calm, clear female
Clyde	2EiwWnXFnvU5JabPnv8n	Warm male
Drew	29vD33N1CtxCmqQRPOHJ	Well-rounded male
Paul	5Q0t7uMcjvnagumLfvZi	Casual male
Domi	AZnzlk1XvdvUeBnXmlld	Energetic female
Bella	EXAVITQu4vr4xnSDxMaL	Expressive female
Antoni	ErXwobaYiN019PkySvjV	Deep male
Elli	MF3mGyEYCl7XYWbV9V6O	Young female

Common Workflows

Voice Quality Testing

Compare voice quality across models and voices:

prompts:
  - 'The quick brown fox jumps over the lazy dog. This sentence contains every letter of the alphabet.'

providers:
  - id: elevenlabs:tts:21m00Tcm4TlvDq8ikWAM
    label: Flash Model (Fastest)
    config:
      modelId: eleven_flash_v2_5

  - id: elevenlabs:tts:21m00Tcm4TlvDq8ikWAM
    label: Turbo Model (Legacy Comparison)
    config:
      modelId: eleven_turbo_v2_5

tests:
  - description: Audio generation completes quickly
    assert:
      - type: latency
        threshold: 1000

  - description: Audio generation stays within the cost budget
    assert:
      - type: cost
        threshold: 0.01

Transcription Accuracy Pipeline

Generate and save speech in a TTS eval, then pass the saved audio file path and config.referenceText to a separate STT eval. The transcription pipeline guide shows both configs and an assertion using context.providerResponse.metadata.wer.

Agent Regression Testing

Ensure agent improvements don't degrade performance:

prompts:
  - |
    User: I need to cancel my subscription
    User: Yes, I'm sure
    User: Account email is user@example.com

providers:
  - id: elevenlabs:agents
    config:
      agentConfig:
        prompt: You are a customer service agent. Always confirm cancellations.
        llmModel: gpt-4o
      evaluationCriteria:
        - id: confirmation_requested
          name: confirmation_requested
          description: Agent asks for confirmation before canceling
          weight: 1.0
          passingThreshold: 0.9
        - id: professional_tone
          name: professional_tone
          description: Agent maintains professional tone
          weight: 0.8
          passingThreshold: 0.8

tests:
  - description: Agent handles cancellation properly
    assert:
      - type: javascript
        value: |
          const results = context.providerResponse.metadata?.evaluationResults;
          const required = ['confirmation_requested', 'professional_tone'];
          return Array.isArray(results) && required.every(id =>
            results.some(result => result.criterion === id && result.passed === true)
          );

Best Practices

1. Choose the Right Model

Flash v2.5: Use for real-time applications, live streaming, or when latency is critical (<200ms)
Turbo v2.5: Keep for legacy comparisons; ElevenLabs recommends Flash for new configurations
Multilingual v2: Use for non-English languages or when switching between languages

2. Optimize Voice Settings

For natural conversation:

voiceSettings:
  stability: 0.5 # More variation
  similarity_boost: 0.75
  speed: 1.0

For consistent narration:

voiceSettings:
  stability: 0.8 # Less variation
  similarity_boost: 0.85
  speed: 0.95

For expressiveness:

voiceSettings:
  stability: 0.3 # High variation
  similarity_boost: 0.5
  style: 0.8 # Amplify style
  speed: 1.1

3. Cost Optimization

Use caching for repeated phrases:

providers:
  - id: elevenlabs:tts:21m00Tcm4TlvDq8ikWAM
    config:
      cache: true
      cacheTTL: 86400 # 24 hours

Implement LLM cascading for agents:

providers:
  - id: elevenlabs:agents
    config:
      llmCascade:
        primary: gpt-4o-mini # Cheaper first
        fallback:
          - gpt-4o # Better fallback
        cascadeOnError: true

Test with shorter prompts during development:

providers:
  - id: elevenlabs:tts:21m00Tcm4TlvDq8ikWAM
tests:
  - vars:
      shortPrompt: 'Test' # Use during dev
      fullPrompt: 'Full production message'

4. Agent Testing Strategy

Start simple, add complexity incrementally:

# Phase 1: Basic functionality
evaluationCriteria:
  - name: responds
    description: Agent responds to user
    weight: 1.0

# Phase 2: Add quality checks
evaluationCriteria:
  - name: responds
    weight: 0.8
  - name: accurate
    description: Response is factually correct
    weight: 1.0

# Phase 3: Add conversation flow
evaluationCriteria:
  - name: responds
    weight: 0.6
  - name: accurate
    weight: 1.0
  - name: natural_flow
    description: Conversation feels natural
    weight: 0.8

5. Audio Quality Assurance

Always test on target platforms:

providers:
  - id: elevenlabs:tts:21m00Tcm4TlvDq8ikWAM
    config:
      outputFormat: mp3_44100_128 # Good for web
      # outputFormat: pcm_44100      # Better for phone systems
      # outputFormat: mp3_22050_32   # Smaller files for mobile

Test with diverse content:

prompts:
  # Numbers and dates
  - 'Your appointment is on March 15th at 3:30 PM. Confirmation number: 4829.'

  # Technical terms
  - 'The API returns a JSON response with OAuth2 authentication tokens.'

  # Multi-language
  - 'Bonjour! Welcome to our multilingual support.'

  # Edge cases
  - 'Hello... um... can you hear me? Testing, 1, 2, 3.'

6. Monitoring and Observability

Check TTS latency, estimated cost, and audio metadata:

prompts:
  - 'Check the generated audio format and size.'

providers:
  - id: elevenlabs:tts:21m00Tcm4TlvDq8ikWAM
    config:
      modelId: eleven_flash_v2_5
      outputFormat: pcm_16000

tests:
  - assert:
      # Latency thresholds
      - type: latency
        threshold: 2000

      # Cost budgets
      - type: cost
        threshold: 0.50

      # Audio metadata
      - type: javascript
        value: |
          const response = context.providerResponse;
          return response.audio?.format === 'pcm'
            && Number.isFinite(response.metadata?.audioSize)
            && response.metadata.audioSize > 0;

Use labels for organized results:

providers:
  - label: v1-baseline
    id: elevenlabs:tts:21m00Tcm4TlvDq8ikWAM
    config:
      modelId: eleven_flash_v2_5

  - label: v2-improved
    id: elevenlabs:tts:21m00Tcm4TlvDq8ikWAM
    config:
      modelId: eleven_flash_v2_5
      voiceSettings:
        stability: 0.6 # Tweaked setting

Troubleshooting

API Key Issues

Error: ELEVENLABS_API_KEY environment variable is not set

Solution: Ensure your API key is properly set:

# Check if key is set
echo $ELEVENLABS_API_KEY

# Set it if missing
export ELEVENLABS_API_KEY=your_key_here

# Or add to your shell profile
echo 'export ELEVENLABS_API_KEY=your_key' >> ~/.zshrc
source ~/.zshrc

Authentication Errors

Error: 401 Unauthorized

Solution: Verify your API key is valid:

# Test API key directly
curl -H "xi-api-key: $ELEVENLABS_API_KEY" https://api.elevenlabs.io/v1/voices

If this fails, regenerate your API key at ElevenLabs Settings.

Rate Limiting

Error: 429 Too Many Requests

Solution: Add retry logic and respect rate limits:

providers:
  - id: elevenlabs:tts:21m00Tcm4TlvDq8ikWAM
    config:
      retries: 3 # Retry failed requests
      timeout: 30000 # Allow time for retries

For high-volume testing, consider:

Spreading tests over time
Upgrading to a paid plan
Using caching to avoid redundant requests

Audio File Issues

Error: Failed to read audio file or Unsupported audio format

Solution: Ensure audio files are accessible and in supported formats:

providers:
  - id: elevenlabs:stt
    config:
      audioFormat: mp3 # Supported: mp3, wav, flac, ogg, webm, m4a

Verify file exists:

ls -lh /path/to/audio.mp3
file /path/to/audio.mp3

Agent Conversation Timeouts

Error: Conversation timeout after X turns

Solution: Adjust conversation limits:

providers:
  - id: elevenlabs:agents
    config:
      maxTurns: 20 # Increase if needed
      timeout: 120000 # 2 minutes

Memory Issues with Large Evals

Error: JavaScript heap out of memory

Solution: Increase Node.js memory:

export NODE_OPTIONS="--max-old-space-size=4096"
promptfoo eval

Or run fewer concurrent tests:

promptfoo eval --max-concurrency 2

Voice Not Found

Error: Voice ID not found

Solution: Use a voice ID (voice names are not resolved):

providers:
  # Use the official voice ID from your voice library
  - id: elevenlabs:tts:21m00Tcm4TlvDq8ikWAM

List available voices:

curl -H "xi-api-key: $ELEVENLABS_API_KEY" https://api.elevenlabs.io/v1/voices

Cost Tracking Inaccuracies

Issue: Cost estimates don't match billing

Solution: Cost tracking is estimated based on:

TTS: Character count × model rate
STT: Known audio duration × per-minute rate; omitted when duration is unavailable
Agents: Conversation duration × LLM rates

For exact costs, check your ElevenLabs billing dashboard.

Examples

Complete working examples:

TTS Basic - Simple voice generation
TTS Advanced - Voice design, streaming, pronunciation
STT - Transcription with diarization
Agents Basic - Simple agent testing

Learn More

Promptfoo Resources

Evaluating ElevenLabs voice AI - Step-by-step tutorial

ElevenLabs Resources

ElevenLabs API Documentation
Voice Library - Browse and preview voices
Conversational AI Docs - Agent setup guide
Pricing - Plan comparison
Status Page - API status and incidents

Quick Start​

Setup​

Capabilities​

Text-to-Speech (TTS)​

Speech-to-Text (STT)​

Conversational Agents​

Supporting APIs​

Configuration Parameters​

TTS-Specific Parameters​

STT-Specific Parameters​

Agent-Specific Parameters​

Examples​

Text-to-Speech: Voice Comparison​

Speech-to-Text: Accuracy Testing​

Conversational Agents: Evaluation​

Audio Processing: Pipeline​

Advanced Features​

Pronunciation Dictionaries​

Voice Design​

LLM Cascading​

Multi-voice Conversations​

Phone Integration​

Cost Tracking​

Popular Voices​

Common Workflows​

Voice Quality Testing​

Transcription Accuracy Pipeline​

Agent Regression Testing​

Best Practices​

1. Choose the Right Model​

2. Optimize Voice Settings​

3. Cost Optimization​

4. Agent Testing Strategy​

5. Audio Quality Assurance​

6. Monitoring and Observability​

Troubleshooting​

API Key Issues​

Authentication Errors​

Rate Limiting​

Audio File Issues​

Agent Conversation Timeouts​

Memory Issues with Large Evals​

Voice Not Found​

Cost Tracking Inaccuracies​

Examples​

Learn More​

Promptfoo Resources​

ElevenLabs Resources​

Quick Start

Setup

Capabilities

Text-to-Speech (TTS)

Speech-to-Text (STT)

Conversational Agents

Supporting APIs

Configuration Parameters

TTS-Specific Parameters

STT-Specific Parameters

Agent-Specific Parameters

Examples

Text-to-Speech: Voice Comparison

Speech-to-Text: Accuracy Testing

Conversational Agents: Evaluation

Audio Processing: Pipeline

Advanced Features

Pronunciation Dictionaries

Voice Design

LLM Cascading

Multi-voice Conversations

Phone Integration

Cost Tracking

Popular Voices

Common Workflows

Voice Quality Testing

Transcription Accuracy Pipeline

Agent Regression Testing

Best Practices

1. Choose the Right Model

2. Optimize Voice Settings

3. Cost Optimization

4. Agent Testing Strategy

5. Audio Quality Assurance

6. Monitoring and Observability

Troubleshooting

API Key Issues

Authentication Errors

Rate Limiting

Audio File Issues

Agent Conversation Timeouts

Memory Issues with Large Evals

Voice Not Found

Cost Tracking Inaccuracies

Examples

Learn More

Promptfoo Resources

ElevenLabs Resources