From 1b6ec8fc2589a1733a0937270faa4230ce6b1ca5 Mon Sep 17 00:00:00 2001
From: bashonly <88596187+bashonly@users.noreply.github.com>
Date: Tue, 17 Mar 2026 17:44:30 -0500
Subject: [PATCH] [ie/youtube] Fix `--live-from-start` support (#16254)

Closes #16237
Authored by: bashonly
---
 yt_dlp/extractor/youtube/_video.py | 24 ++++++++++++++----------
 1 file changed, 14 insertions(+), 10 deletions(-)

diff --git a/yt_dlp/extractor/youtube/_video.py b/yt_dlp/extractor/youtube/_video.py
index c9b1e40404..174be95471 100644
--- a/yt_dlp/extractor/youtube/_video.py
+++ b/yt_dlp/extractor/youtube/_video.py
@@ -1939,7 +1939,7 @@ class YoutubeIE(YoutubeBaseInfoExtractor):
         start_time = time.time()
         formats = [f for f in formats if f.get('is_from_start')]
 
-        def refetch_manifest(format_id, delay):
+        def refetch_manifest(itag, client_name, delay):
             nonlocal formats, start_time, is_live
             if time.time() <= start_time + delay:
                 return
@@ -1954,20 +1954,20 @@ class YoutubeIE(YoutubeBaseInfoExtractor):
             is_live = live_status == 'is_live'
             start_time = time.time()
 
-        def mpd_feed(format_id, delay):
+        def mpd_feed(itag, client_name, delay):
             """
             @returns (manifest_url, manifest_stream_number, is_live) or None
             """
             for retry in self.RetryManager(fatal=False):
                 with lock:
-                    refetch_manifest(format_id, delay)
+                    refetch_manifest(itag, client_name, delay)
 
-                f = next((f for f in formats if f['format_id'] == format_id), None)
+                f = next((f for f in formats if f.get('_itag') == itag and f.get('_client') == client_name), None)
                 if not f:
                     if not is_live:
                         retry.error = f'{video_id}: Video is no longer live'
                     else:
-                        retry.error = f'Cannot find refreshed manifest for format {format_id}{bug_reports_message()}'
+                        retry.error = f'Cannot find refreshed manifest for format {itag}{bug_reports_message()}'
                     continue
 
                 # Formats from ended premieres will be missing a manifest_url
@@ -1980,7 +1980,7 @@ class YoutubeIE(YoutubeBaseInfoExtractor):
 
         for f in formats:
             f['is_live'] = is_live
-            gen = functools.partial(self._live_dash_fragments, video_id, f['format_id'],
+            gen = functools.partial(self._live_dash_fragments, video_id, f['_itag'], f['_client'],
                                     live_start_time, mpd_feed, not is_live and f.copy())
             if is_live:
                 f['fragments'] = gen
@@ -1989,7 +1989,7 @@ class YoutubeIE(YoutubeBaseInfoExtractor):
                 f['fragments'] = LazyList(gen({}))
                 del f['is_from_start']
 
-    def _live_dash_fragments(self, video_id, format_id, live_start_time, mpd_feed, manifestless_orig_fmt, ctx):
+    def _live_dash_fragments(self, video_id, itag, client_name, live_start_time, mpd_feed, manifestless_orig_fmt, ctx):
         FETCH_SPAN, MAX_DURATION = 5, 432000
 
         mpd_url, stream_number, is_live = None, None, True
@@ -2013,7 +2013,7 @@ class YoutubeIE(YoutubeBaseInfoExtractor):
             old_mpd_url = mpd_url
             last_error = ctx.pop('last_error', None)
             expire_fast = immediate or (last_error and isinstance(last_error, HTTPError) and last_error.status == 403)
-            mpd_url, stream_number, is_live = (mpd_feed(format_id, 5 if expire_fast else 18000)
+            mpd_url, stream_number, is_live = (mpd_feed(itag, client_name, 5 if expire_fast else 18000)
                                                or (mpd_url, stream_number, False))
             if not refresh_sequence:
                 if expire_fast and not is_live:
@@ -2039,7 +2039,7 @@ class YoutubeIE(YoutubeBaseInfoExtractor):
             _last_seq = int(re.search(r'(?:/|^)sq/(\d+)', fragments[-1]['path']).group(1))
             return True, _last_seq
 
-        self.write_debug(f'[{video_id}] Generating fragments for format {format_id}')
+        self.write_debug(f'[{video_id}] Generating fragments for format {itag}')
         while is_live:
             fetch_time = time.time()
             if no_fragment_score > 30:
@@ -3747,11 +3747,15 @@ class YoutubeIE(YoutubeBaseInfoExtractor):
                         sub[STREAMING_DATA_CLIENT_NAME] = client_name
                     subtitles = self._merge_subtitles(subs, subtitles)  # Prioritize HLS subs over DASH
                     for f in formats:
-                        if process_manifest_format(f, 'dash', client_name, f['format_id'], require_po_token and not po_token):
+                        # Save original itag value as format_id because process_manifest_format mutates f
+                        format_id = f['format_id']
+                        if process_manifest_format(f, 'dash', client_name, format_id, require_po_token and not po_token):
                             f['filesize'] = int_or_none(self._search_regex(
                                 r'/clen/(\d+)', f.get('fragment_base_url') or f['url'], 'file size', default=None))
                             if needs_live_processing:
                                 f['is_from_start'] = True
+                                f['_itag'] = format_id
+                                f['_client'] = client_name
                             yield f
         yield subtitles