diff --git a/python/tests/test_diarize.py b/python/tests/test_diarize.py
index 3ceccc6..377e297 100644
--- a/python/tests/test_diarize.py
+++ b/python/tests/test_diarize.py
@@ -1,7 +1,13 @@
 """Tests for diarization service data structures and payload conversion."""
 
+import time
+from unittest.mock import MagicMock, patch
+
+import pytest
+
 from voice_to_notes.services.diarize import (
     DiarizationResult,
+    DiarizeService,
     SpeakerSegment,
     diarization_to_payload,
 )
@@ -31,3 +37,73 @@ def test_diarization_to_payload_empty():
     assert payload["num_speakers"] == 0
     assert payload["speaker_segments"] == []
     assert payload["speakers"] == []
+
+
+def test_diarize_threading_progress(monkeypatch):
+    """Test that diarization emits progress while running in background thread."""
+    # Track written messages
+    written_messages = []
+    def mock_write(msg):
+        written_messages.append(msg)
+
+    # Mock pipeline that takes ~5 seconds
+    def slow_pipeline(file_path, **kwargs):
+        time.sleep(5)
+        # Return a mock diarization result
+        mock_result = MagicMock()
+        mock_track = MagicMock()
+        mock_track.start = 0.0
+        mock_track.end = 5.0
+        mock_result.itertracks = MagicMock(return_value=[(mock_track, None, "SPEAKER_00")])
+        return mock_result
+
+    mock_pipeline_obj = MagicMock()
+    mock_pipeline_obj.side_effect = slow_pipeline
+
+    service = DiarizeService()
+    service._pipeline = mock_pipeline_obj
+
+    with patch("voice_to_notes.services.diarize.write_message", mock_write):
+        result = service.diarize(
+            request_id="req-1",
+            file_path="/fake/audio.wav",
+            audio_duration_sec=60.0,
+        )
+
+    # Filter for diarizing progress messages (not loading_diarization or done)
+    diarizing_msgs = [
+        m for m in written_messages
+        if m.type == "progress" and m.payload.get("stage") == "diarizing"
+        and "elapsed" in m.payload.get("message", "")
+    ]
+
+    # Should have at least 1 progress message (5s sleep / 2s interval = ~2 messages)
+    assert len(diarizing_msgs) >= 1, (
+        f"Expected at least 1 diarizing progress message, got {len(diarizing_msgs)}"
+    )
+
+    # Progress percent should be between 20 and 85
+    for msg in diarizing_msgs:
+        pct = msg.payload["percent"]
+        assert 20 <= pct <= 85, f"Progress {pct} out of expected range 20-85"
+
+    # Result should be valid
+    assert result.num_speakers == 1
+    assert result.speakers == ["SPEAKER_00"]
+
+
+def test_diarize_threading_error_propagation(monkeypatch):
+    """Test that errors from the background thread are properly raised."""
+    mock_pipeline_obj = MagicMock()
+    mock_pipeline_obj.side_effect = RuntimeError("Pipeline crashed")
+
+    service = DiarizeService()
+    service._pipeline = mock_pipeline_obj
+
+    with patch("voice_to_notes.services.diarize.write_message", lambda m: None):
+        with pytest.raises(RuntimeError, match="Pipeline crashed"):
+            service.diarize(
+                request_id="req-1",
+                file_path="/fake/audio.wav",
+                audio_duration_sec=30.0,
+            )
diff --git a/python/voice_to_notes/services/diarize.py b/python/voice_to_notes/services/diarize.py
index 692b32f..b55ff65 100644
--- a/python/voice_to_notes/services/diarize.py
+++ b/python/voice_to_notes/services/diarize.py
@@ -6,6 +6,7 @@ import os
 import subprocess
 import sys
 import tempfile
+import threading
 import time
 from dataclasses import dataclass, field
 from pathlib import Path
@@ -141,6 +142,7 @@ class DiarizeService:
         min_speakers: int | None = None,
         max_speakers: int | None = None,
         hf_token: str | None = None,
+        audio_duration_sec: float | None = None,
     ) -> DiarizationResult:
         """Run speaker diarization on an audio file.
 
@@ -184,12 +186,40 @@ class DiarizeService:
             flush=True,
         )
 
-        # Run diarization
-        try:
-            raw_result = pipeline(audio_path, **kwargs)
-        finally:
-            if temp_wav:
-                os.unlink(temp_wav)
+        # Run diarization in background thread for progress reporting
+        result_holder: list = [None]
+        error_holder: list[Exception | None] = [None]
+        done_event = threading.Event()
+
+        def _run():
+            try:
+                result_holder[0] = pipeline(audio_path, **kwargs)
+            except Exception as e:
+                error_holder[0] = e
+            finally:
+                done_event.set()
+
+        thread = threading.Thread(target=_run, daemon=True)
+        thread.start()
+
+        elapsed = 0.0
+        estimated_total = max(audio_duration_sec * 0.5, 30.0) if audio_duration_sec else 120.0
+        while not done_event.wait(timeout=2.0):
+            elapsed += 2.0
+            pct = min(20 + int((elapsed / estimated_total) * 65), 85)
+            write_message(progress_message(
+                request_id, pct, "diarizing",
+                f"Analyzing speakers ({int(elapsed)}s elapsed)..."))
+
+        thread.join()
+
+        # Clean up temp file
+        if temp_wav:
+            os.unlink(temp_wav)
+
+        if error_holder[0] is not None:
+            raise error_holder[0]
+        raw_result = result_holder[0]
 
         # pyannote 4.0+ returns DiarizeOutput; older versions return Annotation directly
         if hasattr(raw_result, "speaker_diarization"):
diff --git a/python/voice_to_notes/services/pipeline.py b/python/voice_to_notes/services/pipeline.py
index 3d84a81..281db16 100644
--- a/python/voice_to_notes/services/pipeline.py
+++ b/python/voice_to_notes/services/pipeline.py
@@ -139,6 +139,7 @@ class PipelineService:
                 min_speakers=min_speakers,
                 max_speakers=max_speakers,
                 hf_token=hf_token,
+                audio_duration_sec=transcription.duration_ms / 1000.0,
             )
         except Exception as e:
             import traceback