Streaming AudioConversational STT for Voice Agents (Flux)

Understanding the Flux State Machine

Traditional STT+VAD requires you to build complex interruption logic. Flux handles this natively.

Emitted events adhere to the below state machine for managing turns:

  1. Update messages are sent for approximately every 0.25 seconds of transcribed audio, regardless of transcript updates, unless a state change has occurred.
  2. An EagerEndOfTurn message always contains a nonempty transcript.
  3. A TurnResumed message can only follow a preceding EagerEndOfTurn message.
  4. The EndOfTurn transcript will always match the immediately preceding EagerEndOfTurn transcript. If the transcript changes after an EagerEndOfTurn, a TurnResumed event will occur first.
  5. The turn_index increments immediately following an EndOfTurn message.

Configuring Event Behavior: The EagerEndOfTurn and TurnResumed events are only triggered when you set the eager_eot_threshold parameter. The EndOfTurn event behavior is controlled by eot_threshold and eot_timeout_ms parameters. See the End-of-Turn Configuration for details on tuning these thresholds for your use case.

Turn Lifecycle Example

Here’s how Flux processes a customer calling support saying “Hi I need to cancel my subscription please.”

Notice how confidence builds up and how the EagerEndOfTurn event fires before the final EndOfTurn. With EagerEndOfTurn, your voice agent can begin preparing a response before the user has fully finished speaking. This allows you to send a synchronous request with early context, creating the effect of a faster, more natural reply.

1{
2  "event": "Update",
3  "turn_index": 0,
4  "audio_window_start": 0.0,
5  "audio_window_end": 0.2,
6  "transcript": "",
7  "words": [],
8  "end_of_turn_confidence": 0.1
9}
10
11{
12  "event": "Update",
13  "turn_index": 0,
14  "audio_window_start": 0.0,
15  "audio_window_end": 0.5,
16  "transcript": "",
17  "words": [],
18  "end_of_turn_confidence": 0.1
19}
20
21{
22  "event": "StartOfTurn",
23  "turn_index": 0,
24  "audio_window_start": 0.0,
25  "audio_window_end": 0.6,
26  "transcript": "Hi I",
27  "words": [
28    {
29      "word": "Hi",
30      "confidence": 0.95
31    },
32    {
33      "word": "I",
34      "confidence": 0.92
35    }
36  ],
37  "end_of_turn_confidence": 0.1
38}
39
40{
41  "event": "Update",
42  "turn_index": 0,
43  "audio_window_start": 0.0,
44  "audio_window_end": 0.8,
45  "transcript": "Hi I need to",
46  "words": [...],
47  "end_of_turn_confidence": 0.1
48}
49
50{
51  "event": "Update",
52  "turn_index": 0,
53  "audio_window_start": 0.0,
54  "audio_window_end": 1.0,
55  "transcript": "Hi I need to cancel my subscription.",
56  "words": [...],
57  "end_of_turn_confidence": 0.3
58}
59
60{
61  "event": "EagerEndOfTurn",
62  "turn_index": 0,
63  "audio_window_start": 0.0,
64  "audio_window_end": 1.1,
65  "transcript": "Hi I need to cancel my subscription.",
66  "words": [...],
67  "end_of_turn_confidence": 0.3
68}
69
70{
71  "event": "TurnResumed",
72  "turn_index": 0,
73  "audio_window_start": 0.0,
74  "audio_window_end": 1.2,
75  "transcript": "Hi I need to cancel my subscription please",
76  "words": [...],
77  "end_of_turn_confidence": 0.1
78}
79
80{
81  "event": "Update",
82  "turn_index": 0,
83  "audio_window_start": 0.0,
84  "audio_window_end": 1.4,
85  "transcript": "Hi I need to cancel my subscription please.",
86  "words": [...],
87  "end_of_turn_confidence": 0.3
88}
89
90{
91  "event": "EagerEndOfTurn",
92  "turn_index": 0,
93  "audio_window_start": 0.0,
94  "audio_window_end": 1.5,
95  "transcript": "Hi I need to cancel my subscription please.",
96  "words": [...],
97  "end_of_turn_confidence": 0.3
98}
99
100{
101  "event": "Update",
102  "turn_index": 0,
103  "audio_window_start": 0.1,
104  "audio_window_end": 1.6,
105  "transcript": "Hi I need to cancel my subscription please.",
106  "words": [...],
107  "end_of_turn_confidence": 0.5
108}
109
110{
111  "event": "EndOfTurn",
112  "turn_index": 0,
113  "audio_window_start": 0.0,
114  "audio_window_end": 1.7,
115  "transcript": "Hi I need to cancel my subscription please.",
116  "words": [...],
117  "end_of_turn_confidence": 0.7
118}