With "--gpu-api=vulkan", MPV complains that it cannot find SPIRV, despite spirv-tools being installed

Please rebuild the package and to the problem!

This happens because your mpv is compiled without support for libshaderc, which is the only SPIR-V backend that mpv supports for now (apart from the deprecated nvidia-only VK_NV_glsl_shader which we will be dropping support for eventually). To solve the issue you will need to install libshaderc and make sure mpv is built against it.

Note: It wouldn't be too difficult to support using glslang directly instead of shaderc. I have already written a C wrapper for glslang for this purpose for a different project; if you want to try merging it to mpv I could offer some pointers. That said, using shaderc is still preferred to glslang since it means you (i.e. we) get better diagnostics.

ProblemType: Bug
DistroRelease: Ubuntu 18.10
Package: mpv 0.29.0-1ubuntu2
Uname: Linux 4.20.0-042000-generic x86_64
ApportVersion: 2.20.10-0ubuntu13.1
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Wed Jan 2 10:46:23 2019
InstallationDate: Installed on 2018-12-10 (22 days ago)
InstallationMedia: Ubuntu 18.10 "Cosmic Cuttlefish" - Release amd64 (20181017.3)
SourcePackage: mpv
UpgradeStatus: No upgrade log present (probably fresh install)