Flush syntax

Overview

The flush syntax is a VAPI audio control token that forces immediate transmission of LLM output to voice providers, eliminating buffering delays for real-time voice interactions.

When to use flush syntax:

• Acknowledge user requests immediately during processing
• Provide feedback during long-running tool executions
• Create natural conversation pauses
• Support custom LLM integrations with processing delays

How it works

The flush syntax bypasses normal buffering to provide immediate audio feedback:

1. Detection: VAPI scans LLM output for flush syntax using regex pattern
2. Split: Text is divided at the flush position
3. Immediate Send: Content before flush is sent instantly to voice provider
4. Continue: Remaining text follows normal buffering

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const { sendToTTS, flush, remainingBuffer } = ttsBuffer(buffer, voice);
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if (sendToTTS.length > 0) {
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  pushBuffer(sendToTTS, flush); // flush=true triggers immediate send
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  buffer = remainingBuffer;
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}

Syntax formats

VAPI supports three flush formats with case-insensitive matching:

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<flush />
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``` ```html title="Opening tag"
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<flush>``` ```html title="Closing tag"</flush>

Configuration requirements

Flush syntax requires proper voice configuration:

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{
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  "voice": {
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    "chunkPlan": {
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      "enabled": true  // Required for flush to work
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    }
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  }
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}

Usage examples

Basic acknowledgment

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"I'm processing your request... <flush /> Let me check that for you.";

Tool processing feedback

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"Looking up that information... <flush /> This may take a moment.";

Conversation flow

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"That's a great question. <flush /> Based on the data I have...";

Custom LLM integration

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"Here's your answer: 42. <flush /> Would you like an explanation?";

Best practices

When to use flush

When to avoid flush

• Every response - Causes audio fragmentation
• Mid-sentence - Breaks natural speech flow
• Short responses - Normal buffering is sufficient
• Multiple per response - Can create choppy audio

Implementation guidelines

1. Place at natural boundaries - Use between complete thoughts or sentences
2. Test with your voice provider - Effectiveness varies by provider
3. Monitor conversation quality - Ensure audio remains smooth and natural
4. Document usage - Include in code comments for team understanding

Advanced usage

Dynamic insertion

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const acknowledgment = "I understand your request";
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const detailedResponse = await processRequest(userInput);
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const responseWithFlush = `${acknowledgment} <flush /> ${detailedResponse}`;

System prompt integration

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const systemPrompt = `When providing lengthy responses, use <flush /> after acknowledging the user's request to provide immediate feedback.`;

Nested handling

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"Starting process... <flush> Step 1 complete </flush> Moving to step 2...";

Troubleshooting

Technical considerations

Provider compatibility

• Effectiveness varies by voice provider
• Test thoroughly with your chosen provider
• Monitor performance impact on response times

Cost implications

• Increased API calls to voice provider
• Higher usage on usage-based pricing
• Monitor billing if using flush frequently

VAPI-only feature

• Platform exclusive - not available on other voice platforms
• Configuration dependent - requires chunking enabled
• Version specific - ensure using compatible VAPI version

Next steps

Now that you understand flush syntax:

• Voice formatting plan: Control voice output formatting and timing
• Background messages: Send messages during conversations
• Custom tools: Build tools that benefit from flush syntax feedback