🐞Disable AV1 setting ignored on initial playback, video plays AV1 until page refresh

[WhitespaceQ]

Bug Report:

• Description: When clicking on a new video with Disable AV1 codec settings activated, the video will still plays AV1 codec. Refreshing the page will then switch the codec to any other than AV1
• Steps to reproduce this: With Disable AV1 codec active, open YouTube and click on a video then show Stats for nerds, it will show that the video is encoded using AV1. Refresh the page and play again, the video will then switch to any other codec
• Since when?: Not relevant, I just installed this extension
• Browser: Firefox 144.0.2 (64-bit)
• Are any errors shown in the Browser-Console?: None
• Does the bug not happen when you log out of YouTube?: Yes, all on (1) logged-in, (2) logged-out but not on private window, only sometimes. I cannot reliably reproduce the issue on Incognito. I'll provide a diff from yt.config_.EXPERIMENT_FLAGS

diff --git a/logged-in normal browser.txt b/logged-out private window.txt
index a4e42fa..52dbb0a 100644
--- a/logged-in normal browser.txt	
+++ b/logged-out private window.txt	
@@ -1,12 +1,10 @@
 {
   "PremiumClientSharedConfig__enable_att_context_processor": true,
-  "PremiumClientSharedConfig__enable_att_for_get_download_action_on_web_client": true,
-  "PremiumClientSharedConfig__enable_att_for_get_premium_on_web_client": true,
+  "ab_det_apb_b": true,
   "ab_det_apm": true,
   "ab_det_el_h": true,
   "ab_det_em_inj": true,
   "ab_det_pp_ov": true,
-  "ab_fk_sk_cl": true,
   "ab_l_sig_st": true,
   "ab_l_sig_st_e": true,
   "ab_sa_ef": true,
@@ -21,7 +19,6 @@
   "browse_next_continuations_migration_playlist": true,
   "c3_watch_page_component": true,
   "cancel_pending_navs": true,
-  "channel_details_shelf_resize_observer": true,
   "check_user_lact_at_prompt_shown_time_on_web": true,
   "clean_up_manual_attribution_header": true,
   "clear_user_partitioned_ls": true,
@@ -102,6 +99,7 @@
   "enable_creator_details_shelf_m2": true,
   "enable_dai_sdf_h5_preroll": true,
   "enable_datasync_id_header_in_web_vss_pings": true,
+  "enable_default_mono_cta_migration_web_client": true,
   "enable_description_content_list_support": true,
   "enable_desktop_amsterdam_info_panels": true,
   "enable_desktop_fountain_reaction_item_data_consumption": true,
@@ -112,6 +110,7 @@
   "enable_entity_store_from_dependency_injection": true,
   "enable_fix_two_tab_short_dr": true,
   "enable_flow_logging_p4e": true,
+  "enable_force_imp_autoplay_on_desktop_search": true,
   "enable_full_length_inline_playback_desktop_search_shorts": true,
   "enable_fully_reactive_badge_shape": true,
   "enable_fully_reactive_chip_shape": true,
@@ -132,6 +131,7 @@
   "enable_inline_shorts_on_wn": true,
   "enable_installment_wave2_view_agreement": true,
   "enable_interstitial_entity_check": true,
+  "enable_is_extended_monitoring": true,
   "enable_is_mini_app_page_active_bugfix": true,
   "enable_lcr_emoji_fountain": true,
   "enable_live_overlay_feed_in_live_chat": true,
@@ -182,7 +182,6 @@
   "enable_post_scheduling": true,
   "enable_premium_voluntary_pause": true,
   "enable_primitive_dialog_aria_hide_siblings": true,
-  "enable_profile_cards_on_comments": true,
   "enable_programmed_playlist_color_sample": true,
   "enable_programmed_playlist_redesign": true,
   "enable_purchase_activity_in_paid_memberships": true,
@@ -231,6 +230,7 @@
   "enable_topsoil_wta_for_halftime_live_infra": true,
   "enable_track_defined_via_mutable_signal": true,
   "enable_unavailable_videos_watch_page": true,
+  "enable_unified_cancellation_for_premium_immediate_refunds": true,
   "enable_variable_timeout_web": true,
   "enable_video_display_compact_button_group_for_desktop_search": true,
   "enable_view_i18n_pronouns": true,
@@ -357,6 +357,7 @@
   "kevlar_player_load_player_no_op": true,
   "kevlar_player_new_bootstrap_adoption": true,
   "kevlar_player_playlist_use_local_index": true,
+  "kevlar_player_unified_player_loading": true,
   "kevlar_player_watch_endpoint_navigation": true,
   "kevlar_playlist_drag_handles": true,
   "kevlar_playlist_use_x_close_button": true,
@@ -383,6 +384,7 @@
   "kevlar_typography_spacing_update": true,
   "kevlar_typography_update": true,
   "kevlar_unified_errors_init": true,
+  "kevlar_unified_player": true,
   "kevlar_use_response_ttl_to_invalidate_cache": true,
   "kevlar_use_vimio_behavior": true,
   "kevlar_use_wil_icons": true,
@@ -541,7 +543,6 @@
   "use_new_in_memory_storage": true,
   "use_player_abuse_bg_library": true,
   "use_request_time_ms_header": true,
-  "use_rta_manager_for_async": true,
   "use_session_based_sampling": true,
   "use_ts_visibilitylogger": true,
   "use_unified_flag_accessor": true,
@@ -577,6 +578,7 @@
   "web_cinematic_theater_mode": true,
   "web_collab_playlist_thumbnail_size": true,
   "web_collect_offline_state": true,
+  "web_context_menu_a11y_opacity_visibility": true,
   "web_continuation_response_processing": true,
   "web_cow_button_renderer_wiz_token": true,
   "web_csi_action_sampling_enabled": true,
@@ -614,8 +616,6 @@
   "web_enable_merch_item_rounding_fix": true,
   "web_enable_player_creator_endscreen_manager": true,
   "web_enable_rich_item_on_focus_effect": true,
-  "web_enable_rich_item_on_focus_effect_monet": true,
-  "web_enable_rich_item_on_focus_effect_monet_additive": true,
   "web_enable_shorts_new_carousel": true,
   "web_enable_shorts_new_carousel_single_instance": true,
   "web_enable_shorts_remix_handoff_ep": true,
@@ -773,6 +773,7 @@
   "web_watch_fullerscreen_deprecation_ui": true,
   "web_watch_get_updated_metadata_manager": true,
   "web_watch_get_updated_metadata_response_processing": true,
+  "web_watch_move_summary_to_sd": true,
   "web_watch_panel_content_match_theme": true,
   "web_watch_provide_undefined_comments": true,
   "web_watch_rounded_player_large": true,
@@ -897,10 +898,10 @@
   "prefetch_comments_ms_after_video": 0,
   "prefetch_coordinator_command_timeout_ms": 60000,
   "prefetch_coordinator_max_inflight_requests": 1,
-  "rich_grid_default_max_items_per_row": 6,
+  "rich_grid_default_max_items_per_row": 3,
   "rich_grid_dynamic_offset_anchor_width": 1200,
-  "rich_grid_dynamic_offset_lower_bound": -40,
-  "rich_grid_dynamic_offset_offset_per_100": 20,
+  "rich_grid_dynamic_offset_lower_bound": -100,
+  "rich_grid_dynamic_offset_offset_per_100": 30,
   "rich_grid_max_item_width": 700,
   "rich_grid_min_item_width": 310,
   "send_config_hash_timer": 0,

• ImprovedTube Version: 4.1350
• OS / Device: Windows 11 Pro Version 23H2 OS build 22631.6060 Windows Feature Experience Pack 1000.22700.1106.0
• ImprovedTube Settings:

{
    "ads": "block_all",
    "ambient_lighting": false,
    "below_player_keyscene": false,
    "below_player_loop": false,
    "below_player_pip": false,
    "below_player_screenshot": false,
    "block_av1": true,
    "block_h264": false,
    "block_vp9": true,
    "lastDarkTheme": "dark",
    "only_one_player_instance_playing": false,
    "player_SDR": true,
    "player_h264": true,
    "player_quality": "disabled",
    "theme": "dark"
}

Behavior when logged-in, see that in the first play codec is AV1 then after a refresh it's changed to vp9

Youtube.Mozilla.Firefox.2025-10-30.18-07-35-1.1.mp4

[zzamn]

Yeah. This is a weird bug.

You've blocked both AV1 and VP9
(Your settings show:
"block_av1": true,
"block_h264": false,
"block_vp9": true,
)
For some reason that makes the code wack out and randomly allow AV1 and VP9.

However, I noticed that when ONLY blocking AV1, the code is pretty consistent, and almost always blocks AV1, even when you click on the video for the first time

I'm assuming that you're okay with using VP9, and AV1 is your only problem.

If that is the case, I think unblocking VP9 will help. Let me know if it doesn't.

IF you don't want AV1 OR VP9, just let me know, and I'll see what I can do.

Cheers

[WhitespaceQ]

Yeah, I’m enabling both VP9 and H.264, and now the player defaults to VP9 on first load. My speculation is that YouTube may no longer provide H.264 streams in some cases, so when AV1 is blocked and there’s no H.264 fallback, the player ends up defaulting to VP9.

This works fine on my side since my device supports VP9 hardware decoding so I still avoid the expensive AV1 software decoding.

That said, the toggle still doesn’t behave as expected, so I think this remains a bug worth investigating.

[zzamn]

Gotcha. Good theory, either way (if yt does or doesn't support h.246) we'll have to tweak the extension's code i guess. I'll check it out 🫡

[zzamn]

Hello.

avc1 is the same as h264. It does NOT indicate av1. sorry 😅

If av1 was active, you'd get 'av01.blahblah'. like so:

You getting this after blocking av1 and vp9 means that the code is working as intended initially, and that h264 (symbolized with avc1) is the codec that's active

So in conclusion, you can go back to av1 and vp9. Your video player was using h264 at the start

So in that case the only bug happening in the video you sent - would be the video player using vp09 after reload lol

But for me, even after having this setting on

and reloading the yt page, i consistently get avc1 (i.e.: h264) instead of vp09. So, i dunno. I'm not using firefox though.

You can close this issue if you want, but if it's still open I'll let you know what happens when I use firefox with your original settings and reload the webpage :)

[WhitespaceQ]

Huh that's weird for YouTube to use technically correct but confusing terminology but that's a fault on my part too for not checking.

I'm sorry for raising a false alarm.