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Two Steam Deck findings from a field report of "the decoder was not found, it fell back to H.264 — but sometimes HEVC worked". **The 4:4:4 advertisement was a promise nothing checked.** `VIDEO_CAP_444` rode the "Full chroma" setting alone. That was safe while a software HEVC decoder existed underneath it; M8 removed one (there is no permissively licensed HEVC CPU decoder, so `software_decodable_codecs()` is H.264|AV1). The host grants 4:4:4 on HEVC ONLY, and answers the resolved chroma in the Welcome before the client builds a decoder — so on a device with no 4:4:4 decode the toggle did not cost crispness, it cost the entire codec: the Vulkan rung refuses the shape at construction, VAAPI refuses it too, there is no CPU rung, and the session reconnects on H.264. AMD has no HEVC 4:4:4 decode on any silicon, so every Deck with that switch on lost HEVC. It is per-profile and default-off, which is exactly why it looked intermittent — a "Work" profile lost HEVC where "Game" kept it, same box, same host. Gated on `hevc_444_hardware_decodable`, which asks the driver through the SAME code the rung uses at construction (`VkH265Decoder::probe_stream_support`), so the advertisement and the rung that must honour it cannot disagree. Both depths are required, not either: with HDR on the host may resolve 4:4:4 10-bit, and a device offering YUV444_8 but not YUV444_10 would land in the same hole. Answering from the Vulkan rung alone is exact rather than approximate — it is the only rung in this build that implements 4:4:4 at all (`pf_vaadec::profile_for` errors on chroma_format_idc 3, pf-dxvadec refuses anything but 4:2:0, the CPU rung is 8-bit 4:2:0). Deliberately NOT extended to VIDEO_CAP_10BIT/HDR: all three rungs implement 10-bit 4:2:0, so a Vulkan-only probe there would withdraw HDR from boxes whose VAAPI/DXVA rung decodes it perfectly — a real regression against a case never observed. The bit arithmetic moves into `video::video_caps_for` so the part that was wrong is testable without a GPU, a host or a Hello; the test is verified non-vacuous against the planted original defect. **`--probe-decode` described a different device from the one that streams.** The RADV video-decode opt-in sat AFTER the --list-adapters/--probe-decode/--list-audio /--pair early exits, so the triage tool never had it. Measured on a Deck (canary e22af40f), same binary back to back: bare `--probe-decode` printed "vulkan video decode: no", "driver decode ops: none (0x0)", "no queue family advertises VIDEO_DECODE"; with RADV_PERFTEST=video_decode in the environment, "YES" and "H.264, H.265, AV1, VP9". Any Deck triage that consulted it reached the opposite of the truth. Hoisted to the top of `run`, ahead of every early exit — nothing touches Vulkan before it (`main` calls `run` directly). Gates, in the Linux container: fmt, plain `cargo build` (not only --all-targets), `clippy --all-targets -D warnings`, and 185 tests.